_CC) and the negative supply voltage (V_EE). These primary voltages are distributed through the head disk assembly (HDA) connector 204 to the flex circuit 206.

The flex circuit 206 delivers these voltages to the pre-amplifier 208, which become the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}). While the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) are derived from the positive supply voltage (V_CC) and the negative supply voltage (V_EE) respectively, they are considered separate supply nodes due to the parasitic elements introduced by the flex circuit 206.

In embodiments, the head disk assembly (HDA) connector 204 is an electrical connector that provides a reliable, high-density, and compact connection between the head disk assembly and the flex circuit 206. It is responsible for carrying electrical signals and power between these components. Head disk assembly (HDA) connector 204 typically includes connections for read/write signals, servo signals, motor control signals, and power supply voltages.

The head disk assembly (HDA) connector 204 ensures proper signal and power transmission while minimizing signal integrity issues, such as crosstalk and electromagnetic interference (EMI). The head disk assembly (HDA) connector 204 delivers the positive supply voltage (V_CC) and the negative supply voltage (V_EE) from the power supply circuit 202 to the flex circuit 206.

In addition to the disturbances during the transition from read to write at the writer circuit 100, the flex circuit 206 can adversely affect the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) supply. The transfer function of the flex circuit 206 affects the supply voltages provided to the pre-amplifier 208. When the writer circuit 100 in the pre-amplifier 208 is switched on, the writer circuit 100 writes data to the magnetic disk within approximately ten nanoseconds. During this data transition, the current absorbed from the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) supply changes rapidly.

The interaction of the absorbed currents with the positive supply voltage (V_CC) and the negative supply voltage (V_EE) distribution paths on the flex circuit 206 can cause disturbances in the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) before they can recover.

As a result of the supply voltage disturbances, the initial zero-crossing points of the output current (I_OUT) of the writer circuit 100 may no longer be aligned with the input data pattern, leading to phase-shifting. This phase-shifting can adversely affect the ability of the writer circuit 100 to accurately write data to the magnetic disk, potentially increasing the system's bit error rate.

Supply voltage management systems can include bypass capacitors to help stabilize the supply voltages provided to the pre-amplifier 208. However, space constraints in the hard disk drive assembly require the bypass capacitors to be placed at a distance from the pre-amplifier 208. At high operating frequencies, such as 2.5 GHZ, this physical separation reduces the effectiveness of the bypass capacitors in maintaining stable supply voltages. As a result, the bypass capacitors cannot adequately filter noise and fluctuations from the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}).

FIG. 3 illustrates a block diagram of an embodiment ballast system 300. FIG. 4 illustrates an embodiment timing diagram 400 of the operation of the ballast system 300. The ballast system 300 includes a ballast circuit 302 and the writer circuit 100, which may (or may not) be arranged as shown. Ballast system 300 may include additional components not shown. The timing diagram 400 shows the timing between the ballast current (I_BAL) 402, the write current (I_WR) 404, and the output current (I_OUT) 406.

The ballast circuit 302 generates its ballast currents using the positive supply voltage (V_CC) and the negative supply voltage (V_EE) as sources but injects these currents to stabilize the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) at the pre-amplifier 208. This arrangement allows the ballast circuit 302 to compensate for the voltage drops and disturbances along the supply path through the flex circuit 206.

The bypass capacitors in the supply voltage management system include parasitic elements that affect their performance at high frequencies. These parasitic elements include equivalent series inductance (ESL) and equivalent series resistance (ESR). The equivalent series inductance represents the inherent inductance of the capacitor's internal structure and connections. In contrast, the equivalent series resistance represents the total effective resistance in series with the ideal capacitor.

When the writer circuit 100 demands a sudden change in current during a read-to-write transition, these parasitic elements limit the bypass capacitors' ability to respond quickly. The equivalent series inductance requires a certain amount of time to allow current changes, creating an inductive voltage drop during rapid current transitions. Similarly, the equivalent series resistance causes voltage drops proportional to the current flowing through the capacitor. These effects, combined with the physical separation between the bypass capacitors and the pre-amplifier 208, reduce the effectiveness of the bypass capacitors in maintaining stable supply voltages at high frequencies. The ballast circuit 302 helps compensate for these limitations by pre-charging the parasitic inductances and managing the voltage drops before the writer circuit 100 begins its operation.

The ballast circuit 302 reduces the initial phase shifting in the output current (I_OUT) 406 of the writer circuit 100 by stabilizing the supply voltages during the switching period. Phase shifting occurs when the writer circuit 100 transitions from a read mode to a write mode, causing a sudden increase in current drawn from the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}). The rapid change in current interacts with the parasitic inductances and capacitances in the supply network, such as those associated with the flex circuit 206 and bypass capacitors, leading to supply voltage fluctuations that can affect the timing of the output current (I_OUT) 406.

To mitigate this issue, the ballast circuit 302 is coupled in parallel with the writer circuit 100. The ballast circuit 302 is coupled between the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}), which, in embodiments, can be provided by the flex circuit 206. The ballast circuit 302 generates timed current pulses that intentionally disturb the supply voltages just before the writer circuit 100 begins drawing the large write current. By doing so, the ballast circuit 302 pre-charges the parasitic inductances and capacitances in the supply network, reducing voltage fluctuations when the writer circuit 100 switches on.

The ballast circuit 302 generates precisely timed current pulses anticipating the write current transitions during read-to-write operations. At time T₀, the ballast circuit 302 begins exciting the flex circuit 206 with the ballast current (I_BAL) 402. The duration between T₀and T₁and the amplitude of the ballast current (I_BAL) 402 are calibrated to match the characteristics of the writer circuit 100. This timing ensures that when the writer circuit 100 begins delivering current to the load at time T₁, the supply voltages have been pre-conditioned to a stable state, reducing disturbances in the positive write current (I_{VCC_WR}) and the negative write current (I_{VEE_WR}).

Further, the ballast circuit 302 pre-charges the parasitic inductances in the supply network before the writer circuit 100 begins drawing current. Since inductors require time to store energy and reach their steady-state condition, the ballast current (I_BAL) 402 is initiated before the write operation. Accordingly, the supply network is better prepared when the writer circuit 100 draws current by pre-charging these parasitic inductances to their required energy levels. This pre-charging mechanism helps minimize voltage drops and allows faster recovery of the supply voltages during the critical read-to-write transition.

The ballast circuit 302 is configured to generate the timed current pulses that are synchronized in time with the pulse transitions of the positive write current (I_{VCC_WR}) and the negative write current (I_{VEE_WR}) of the write current (I_WR) 404 during the read-to-write transition and the current consumption by the load at the writer circuit 100. To do this, the flex circuit 206 is excited in advance at time T₀, considering the current consumption by the load such that the duration (Δt) corresponding to the difference between time T₁and time T₀, and the amplitude of the ballast current (I_BAL) 402, such that the writer circuit 100 at time T₁starts delivering current with a stable voltage supply.

The ballast circuit 302 generates the ballast current (I_BAL) 402, which includes a positive ballast current (I_{VCC_BAL}) drawn from the positive pre-amplifier supply voltage (V_{CC_PA}) and a negative ballast current (I_{VEE_BAL}) drawn from the negative pre-amplifier supply voltage (V_{EE_PA}). The ballast current (I_BAL) 402 represents the positive ballast current (I_{VCC_BAL}) and the negative ballast current (I_{VEE_BAL}), which transition from zero to the positive ballast current (I_BAL) at time T₀and fall from the positive ballast current (I_BAL) to zero at time T₁.

The write current (I_WR) 404 represents the current drawn by the writer circuit 100 from the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) during the write operation. The positive write current (I_{VCC_WR}) and the negative write current (I_{VEE_WR}) have the same general shape as shown by the write current (I_WR) 404. Until Time T₁, the write current (I_WR) 404 is equal to the read current (I_{WR_READ}), which is the current absorbed by the writer circuit 100 during read operations. At time T₁, the write current (I_WR) 404 transitions to the write current (I_{WR_WRITE}), which is the current absorbed by the writer circuit 100 during write operations.

The output current (I_OUT) 406 is the resulting current delivered by the writer circuit 100 to the load, such as the magnetic write head.

The falling edge of the ballast current (I_BAL) 402 is synchronized with the rising edge of the write current (I_WR) 404 at time T₁with minimal gaps and overlap to avoid multiple current sources on the flex circuit 206.

As shown in FIG. 4, the ballast circuit 302 asserts the ballast current (I_BAL) 402 in response to the writer circuit 100 is switched ON at time T₀, which is slightly before the writer circuit 100 begins drawing the write current (I_WR) 404 at time T₁. The ballast current (I_BAL) 402 transitions from zero to the positive ballast current (I_BAL), disturbing the supply voltages in a controlled manner. At time T₁, when the writer circuit 100 starts delivering the output current (I_OUT) 406 to the load, the ballast circuit 302 de-asserts the ballast current (I_BAL) 402, transitioning from the positive ballast current (I_BAL) to zero.

The timing and amplitude of the ballast current (I_BAL) 402 impacts the effectiveness of the ballast circuit 302. The pulse characteristics of the ballast current (I_BAL) 402 are configured to match the timing and amplitude characteristics of the writer circuit 100. By injecting the ballast current pulses at the optimal time and with the appropriate magnitude, the ballast circuit 302 can effectively pre-charge the supply network, minimizing the voltage fluctuations and reducing the initial phase shifting in the output current (I_OUT) 406.

The intentional disturbing of the supply voltages by the ballast circuit 302 results in improved stability for the positive pre-amplifier supply voltage (V_{CC_PA}), the negative pre-amplifier supply voltage (V_{EE_PA}), and the output current (I_OUT). By reducing the supply voltage fluctuations during the switching period, the ballast circuit 302 helps maintain the proper timing of the output current (I_OUT) 406, minimizing phase shifting and improving the overall performance of the writer circuit 100. This, in turn, leads to more accurate data writing and lower bit error rates in the hard disk drive system.

FIG. 5 illustrates a schematic of an embodiment ballast circuit 500, which can be implemented as the ballast circuit 302. The ballast circuit 500 includes an n-channel transistor (Q_N₁) 502, a p-channel transistor (Q_P₁) 504, a resistor (R₁) 506, a configurable current mirror circuit 508, a capacitor (C₁) 518, an optional digital buffer 520, a digital logic 522, and a reference current (I_REF), which may (or may not) be arranged as shown. The ballast circuit 500 may include additional components not shown, such as a controller and memory storage.

The input of the digital logic 522 is coupled to the write pin (WR_N), which is the input pin to the pre-amplifier 208. The propagation delay (Δt₁) is the propagation delay for a signal at the write pin (WR_N) to traverse through the writer circuit 100 and arrive at the driving structure (i.e., the first drive circuit 122, the second drive circuit 124, the third drive circuit 126, and the fourth drive circuit 128).

The output of the digital logic 522 is coupled to the base node (V_BASE) through the capacitor (C₁) 518. The digital logic 522 receives the write enable signal (WR_N) as an input and generates a pulse with a programmable duration (Δt). In embodiments, the programmable duration (Δt) is tunable through selection bits in the digital logic 522, allowing it to match the propagation delay (Δt₁) of the writer circuit 100.

In embodiments, the digital buffer 520 drives the capacitor (C₁) 518 to create a pulse on the output current. In embodiments, the digital buffer 520 is implemented within the digital logic 522.

When the signal at the write pin (WR_N) toggles, indicating the start of a write operation, the digital logic 522 creates a positive pulse at the bottom of the capacitor (C₁) 518. The positive pulse is coupled to the base node (V_BASE), pulling it up and forcing a higher current to flow from the positive supply voltage (V_CC) to the negative supply voltage (V_EE) through the configurable current mirror circuit 508. The increased current flow generates the positive ballast current (I_{VCC_BAL}) and the negative ballast current (I_{VEE_BAL}), which are injected into the supply voltages to disturb them intentionally.

By matching the programmable duration (Δt) of the pulse generated by the digital logic 522 with the propagation delay (Δt₁) of the writer circuit 100, the ballast current pulses are timed correctly to coincide with the switching of the writer circuit 100, effectively pre-charging the supply network and reducing the initial phase shifting in the output current (I_OUT).

In embodiments, the pulse generated by the digital logic 522 at its output (provided to the capacitor (C₁) 518) is timed to transition high (rising edge) when the signal at the write pin (WR_N) toggles, indicating a read-to-write transition. The pulse is then timed to transition low (falling edge) after the passage of a duration equal to the propagation delay (Δt₁) of the writer circuit 100. The pulse is then passed to the base node (V_BASE).

Each of the n-channel transistor (Q_N₁) 502 and the p-channel transistor (Q_P₁) 504 is arranged as a diode-connected transistor. The current path of the n-channel transistor (Q_N₁) 502 is between the reference current (I_REF) 524 and the p-channel transistor (Q_P₁) 504. The current path of the p-channel transistor (Q_P₁) 504 is between the n-channel transistor (Q_N₁) 502 and ground.

This reference current from the reference current (I_REF) source 502 sets the base current level for the configurable current mirror circuit 508, which is then scaled by the transistors of the configurable current mirror circuit 508 to generate the desired ballast current amplitudes. The reference current can be generated using a dedicated current reference circuit or derived from an existing bias current in the system.

In embodiments, the configurable current mirror circuit 508 includes a first configurable n-channel transistor (Q_N₂) 510, a first configurable p-channel transistor (Q_P₂) 512, a second configurable n-channel transistor (Q_N₃) 514, and a second configurable p-channel transistor (Q_P₃) 516.

The current path of the first configurable n-channel transistor (Q_N₂) 510 is between the positive supply voltage (V_CC) and the first configurable p-channel transistor (Q_P₂) 512. The current path of the first configurable p-channel transistor (Q_P₂) 512 is between the first configurable n-channel transistor (Q_N₂) 510 and the negative supply voltage (V_EE).

The control terminal of the first configurable n-channel transistor (Q_N₂) 510 and the second configurable n-channel transistor (Q_N₃) 514 is coupled to the base node (V_BASE). The resistor (R₁) 506 has a first terminal coupled to the control terminal of the n-channel transistor (Q_N₁) 502 and a second terminal coupled to the base node (V_BASE). The control terminals of the p-channel transistor (Q_P₁) 504, the first configurable p-channel transistor (Q_P₂) 512, and the fourth configurable p-channel transistor (Q_P₃) 516 are coupled to a common node.

The configurable current mirror circuit 508 is configured to individually generate the positive ballast current (I_{VCC_BAL}) and the negative ballast current (I_{VEE_BAL}) with adjustable amplitudes. The configurable current mirror circuit 508 allows tuning the current gain (i.e., the overall amount of current) through dedicated selection bits. Accordingly, the overall amount of current flowing from the positive supply voltage (V_CC) to the negative supply voltage (V_EE) can be adjusted to manage any imbalance at the positive ballast current (I_{VCC_BAL}) and the negative ballast current (I_{VEE_BAL}).

By providing a configurable current mirror circuit 508 and a programmable digital logic 522, the ballast circuit 500 offers flexibility in adjusting the amplitude and timing of the ballast current pulses. This allows the ballast circuit 500 to be optimized for different writer circuit designs and operating conditions. This ensures optimal performance in reducing supply voltage fluctuations and minimizing phase shifting in the output current.

The selection bits configuring the configurable current mirror circuit 508 and the digital logic 522 can be stored in registers or memory elements within the ballast circuit 500 or provided by an external control circuit. These selection bits allow dynamic adjustment of the ballast current amplitudes and the operating delay, enabling the system to adapt to changing operating conditions or compensate for process variations.

The values of the capacitor (C₁) 518 and the resistor (R₁) 506 can be chosen to ensure proper pulse shaping and coupling of the digital pulse to the current mirror circuit. The capacitor (C₁) 518 acts as a DC-blocking element, allowing only the AC components of the digital pulse to pass through. At the same time, the resistor (R₁) 506 helps to control the pulse shape and limit current draw.

The ballast circuit 500 can be implemented as an integrated circuit block within the same chip as the writer circuit 100 or as a separate component mounted in close proximity to the writer circuit 100. The implementation choice can depend on factors such as the available chip area, power budget, and the required level of integration between the ballast circuit 302 and the writer circuit 100.

Although not explicitly shown, the ballast circuit 500 or the digital logic 522 can include internal registers or memory elements that store the various selection bits. The selection bits provided or stored in the digital logic 522 can be used to select different tap points in a delay chain or control the pulse-width modulation circuit's duty cycle. By modifying the values of these selection bits, either through hardwired connections or programmable registers, the pulse duration generated by the digital logic 522 can be tuned to match the propagation delay (Δt₁) of the writer circuit 100.

In embodiments, the digital logic 522 has the same circuitry included in the writer circuit 100 up to the driving structure (i.e., the first drive circuit 122, the second drive circuit 124, the third drive circuit 126, or the fourth drive circuit 128). However, implementing the digital logic 522 is non-limiting, and other approaches, such as a delay chain, are contemplated in other embodiments.

In embodiments, the amplitude of the positive ballast current (I_{VCC_BAL}) and the negative ballast current (I_{VEE_BAL}) can be adjusted through the configurable current mirror circuit 508 based on the write operation performance. The performance can be evaluated by measuring the bit error rate of the written data. For example, after writing data to a track, that same track can be read back to determine if the bit error rate meets system specifications. If the bit error rate is higher than desired, the ballast current amplitudes can be adjusted through the configurable current mirror circuit 508 to optimize the write performance.

In embodiments, a ring oscillator can be used to monitor the programmable duration (Δt) of the pulse generated by the digital logic 522, which can then be compared with the propagation delay (Δt₁) of the writer circuit 100. In embodiments, one or more settings for the digital logic 522 can be trimmed during manufacturing.

In embodiments, the ballast circuit 500 can include a ring oscillator to monitor and verify the programmable duration (Δt) of the pulse generated by the digital logic 522. The ring oscillator provides a reference timing signal that can be compared with the propagation delay (Δt₁) of the writer circuit 100. This comparison allows for real-time monitoring of the timing relationship between the ballast circuit 500 and the writer circuit 100. The ring oscillator can be used during initial calibration or during operation to ensure that the programmable duration (Δt) remains properly matched to the propagation delay (Δt₁). If any misalignment is detected, the selection bits controlling the digital logic 522 can be adjusted to maintain optimal timing of the ballast current pulses. This monitoring and adjustment capability can help ensure consistent performance of the ballast circuit 500 across different operating conditions and over time.

FIG. 6 illustrates a timing diagram 600 comparing the operation of the writer circuit 100 with and without the ballast circuit 302. The timing diagram 600 shows the relationship between various currents and voltages in the system over time, demonstrating the effectiveness of the ballast circuit 302 in stabilizing supply voltages during write operations.

The timing diagram 600 highlights three time points in the operation of the system. At time T₀, the write enable signal (WR_N) 602 transitions, indicating the start of a write operation. When the ballast circuit 302 is present, at time T₁it responds to this transition by generating the positive ballast current (I_{VCC_BAL}) 604 and the negative ballast current (I_{VEE_BAL}) 612. These ballast currents begin the pre-conditioning phase of the supply network.

At time T₂, the ballast circuit 302 de-asserts its ballast currents while the writer circuit 100 begins its internal transitions. During this period between T₁and T₂, the writer circuit 100 activates its internal components, but has not yet begun drawing significant current from the supply voltages. This interval allows the supply network to settle from the effects of the ballast currents before the writer circuit 100 begins its operation.

At time T₂, the writer circuit 100 begins drawing the positive write current (I_{VCC_WR}) 606 and the negative write current (I_{VEE_WR}) 614 from the supply voltages. Without the ballast circuit 302, the positive pre-amplifier supply voltage (V_{CC_PA}) 610 experiences a significant drop and the negative pre-amplifier supply voltage (V_{EE_PA}) 618 shows a corresponding rise at time T₂, due to the sudden current demand. However, with the ballast circuit 302, the pre-conditioning performed between T₁and T₂results in the positive pre-amplifier supply voltage (V_{CC_PA}) 608 and negative pre-amplifier supply voltage (V_{EE_PA}) 616 remaining more stable when the writer circuit 100 begins drawing current at time T₂.

The timing relationships between T₀, T₁, and T₂are coordinated to optimize the pre-conditioning effect. The interval between T₀and T₁matches the propagation delay of the writer circuit 100, ensuring the ballast currents have sufficient time to pre-charge the supply network. The interval between T₁and T₂provides a settling period, preventing interference between the ballast currents and the writer circuit's currents.

The write enable signal (WR_N) 602 initiates the write operation at time T₀. In response to this transition, the ballast circuit 302—when available—generates the positive ballast current (I_{VCC_BAL}) 604 and the negative ballast current (I_{VEE_BAL}) 612. These ballast currents are active between time T₁and time T₂, intentionally disturbing the positive supply voltage (V_CC) and the negative supply voltage (V_EE) before the writer circuit 100 begins its operation.

At time T₂, the writer circuit 100 draws the positive write current (I_{VCC_WR}) 606 and the negative write current (I_{VEE_WR}) 614. The timing diagram 600 shows how the supply voltages respond differently with and without the ballast circuit 302. Without the ballast circuit 302, at time T₂, the positive pre-amplifier supply voltage (V_{CC_PA}) 610 experiences a significant drop, while the negative pre-amplifier supply voltage (V_{EE_PA}) 618 shows a corresponding rapid rise due to the sudden current demand.

In contrast, when the ballast circuit 302 is active, the positive pre-amplifier supply voltage (V_{CC_PA}) 608 and the negative pre-amplifier supply voltage (V_{EE_PA}) 616 exhibit much more stable behavior from time T₂. The pre-conditioning of the supply network from time T₁to time T₂by the ballast currents results in smaller voltage fluctuations and faster recovery times when the writer circuit 100 begins drawing current at time T₁.

The timing diagram 600 demonstrates how the ballast circuit 302 effectively manages the supply voltage disturbances by forcing them to occur during a controlled period between T₁and T₂, before the writer circuit 100 becomes active at time T₂. This pre-conditioning ensures that when the writer circuit 100 begins delivering current to the load at time T₂, it operates with more stable supply voltages, reducing the phase shifting in the output current and improving overall system performance.

The improvement in supply voltage stability can be seen by comparing the voltage waveforms with and without the ballast circuit 302. The positive pre-amplifier supply voltage (V_{CC_PA}) 608 with the ballast circuit 302 shows significantly less voltage drop than the positive pre-amplifier supply voltage (V_{CC_PA}) 610 over time (after time T₂) without the ballast circuit 302. Similarly, the negative pre-amplifier supply voltage (V_{EE_PA}) 616 with the ballast circuit 302 exhibits reduced voltage variation over time (after time T₂) compared to the negative pre-amplifier supply voltage (V_{EE_PA}) 618 without the ballast circuit 302.

The effectiveness of the ballast circuit 302 in reducing phase shifting can be quantified through measurements of the output current timing. For example, in one embodiment, without the ballast circuit 302, the phase shifting in the output current can vary significantly, ranging from −6 to 25 picoseconds relative to the ideal crossing points. When the ballast circuit 302 is active, the phase shifting range is substantially reduced, varying only from −5 to 8 picoseconds. This improvement in phase shifting represents a significant enhancement in the writer circuit's timing accuracy, particularly during the critical initial clock writing period. The reduced phase shifting range helps ensure more precise data writing and contributes to lower bit error rates in the hard disk drive system.

Ideally, it would be advantageous to activate the ballast circuit 302 earlier to improve the response of the positive pre-amplifier supply voltage (V_{CC_PA}) 610 and the negative pre-amplifier supply voltage (V_{EE_PA}) 618. However, given the quick transition from read to write in the hard disk drive, the ballast circuit 302 this may not be practical. In these situations, the ballast circuit 302 is activated shortly before time T₂(e.g., 5 to 10 ns), which may not completely compensate for the drop on the supply voltages. Accordingly, theoretically, the response can be improved by an earlier activation of the ballast circuit 302.

FIG. 7 illustrates a flowchart of an embodiment method 700 for operating a hard disk drive pre-amplifier circuit. It is noted that all steps outlined in the method are not necessarily required and can be optional. Further, changes to the arrangement of the steps, removal of one or more steps and path connections, and addition of steps and path connections are similarly contemplated.

At step 702, a pulse is generated having a programmable duration in response to a write enable signal. The programmable duration is selected to match a propagation delay of a writer circuit, such as the writer circuit 100. This propagation delay represents the time required for a signal to travel from the write enable signal input through the writer circuit's logic chains and digital-to-analog converters to reach the driving stage.

This pulse generation can be performed by a digital logic circuit, such as digital logic 522, which receives the write-enabled signal indicating the start of a write operation. The digital logic circuit can generate the pulse with a rising edge coinciding with a transition of the write enable signal and a falling edge after the programmable duration. In embodiments, the digital logic circuit can be implemented with matching circuitry to the writer circuit to match the propagation delay.

At step 704, a first ballast current pulse is generated from a first supply voltage and a second ballast current pulse is generated from a second supply voltage in response to the pulse. The first supply voltage can be a positive supply voltage (V_CC) and the second supply voltage can be a negative supply voltage (V_EE).

In embodiments, the ballast current pulses are generated using a configurable current mirror circuit and are designed to stabilize the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) provided to the writer circuit.

The configurable current mirror circuit scales a reference current to generate the ballast current pulses with adjustable amplitudes. The amplitude of the ballast current pulses can be adjusted by setting the biasing of the transistors using selection bits stored in memory, allowing for optimization of the ballast current levels. In embodiments, these amplitudes can be adjusted based on measured bit error rates of data written by the writer circuit.

At step 706, the first and second ballast current pulses are injected into the positive pre-amplifier supply voltage (V_{CC_PA}) and the negative pre-amplifier supply voltage (V_{EE_PA}) before the writer circuit draws a write current. The injection of the ballast current pulses pre-charges parasitic inductances in the supply network, preparing the supply voltages for the upcoming current demand from the writer circuit.

The injection of ballast current pulses can reduce the phase shifting range of the write current. This is particularly important when the write current is used to write clock data to a magnetic disk during the initial phase of a write operation.

At step 708, the first and second ballast current pulses are de-asserted when the writer circuit begins drawing the write current. This de-assertion is timed to coincide with the writer circuit's current demand, reducing phase shifting in the write current caused by disturbances in the first and second supply voltages.

FIG. 8 illustrates a block diagram of a pre-amplifier 812 that is placed on the disk drive head stack assembly of a hard disk drive. In embodiments, the writer circuit 100 is implemented as the write circuit 816 of the pre-amplifier 812.

The disk drive head stack assembly slides over the disk. The pre-amplifier 812 includes a fly height sensor 810. In embodiments, the fly height sensor 810 includes a biasing circuit and an amplifier (not shown). The fly height sensor 810 is coupled to a resistive sensor 802. The resistive sensor 802 monitors the fly height between the disk drive head and the disk itself.

A write coil 804 is coupled to the write circuit 816 (for writing to the disk), a heater resistor 806 is coupled to the heater circuit 818 (for controlling the fly height spacing), and a read resistor 808 is coupled to the read circuit 820 (for reading from the disk). The fly height sensor 810, write circuit 816, heater circuit 818, and read circuit 820