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CS5165GDWR16

Fast, Precise 5-Bit Synchronous Buck Controller for the Next Generation Low Voltage Pentium® II Processors

Cherry semiconductor 

Application Information: continued

threshold. If this happens the cycle will repeat itself until

the short is removed. The Soft Start charge/discharge cur-

rent ratio sets the duty cycle for the pulses (2µA/60µA =

3.3%), while actual duty cycle is half that due to the extend-

ed off time mode (1.65%).

This protection feature results in less stress to the regulator

components, input power supply, and PC board traces

than occurs with constant current limit protection (see

Figures 10 and 11).

If the short circuit condition is removed, output voltage

will rise above the 1V level, preventing the FAULT latch

from being set, allowing normal operation to resume.

Trace 1 - GATE(H) (10V/div)

Trace 2 - Inductor Switching Node (5V/div)

Trace 3 -Load Current (5A/div)

Trace 4 - VOUT (100mV/div)

Figure 8: Output Voltage Transient Response to a 14A load step, VOUT =

+2.84V(DAC = 10111).

Trace 1 - GATE(H) (10V/div)

Trace 2 - Inductor Switching Node (5V/div)

Trace 3 -Load Current (5A/div)

Trace 4 - VOUT(100mV/div)

Figure 9: Output Voltage Transient Response to a 14A load turn-off,

VOUT = +2.84V (DAC = 10111).

Trace 4 - 5V Supply Voltage (2V/div.)

Trace 3 - Soft Start Timing Capacitor (1V/div.)

Trace 2 - Inductor Switching Node (2V/div.)

Figure 10: Demonstration board hiccup mode short circuit protection.

Gate pulses are delivered while the Soft Start capacitor charges, and

cease during discharge.

Protection and Monitoring Features

Short Circuit Protection

A lossless hiccup mode short circuit protection feature is

provided, requiring only the Soft Start capacitor to imple-

ment. If a short circuit condition occurs the VFB low com-

parator sets the FAULT latch. This causes the top FET to

shut off, disconnecting the regulator from its input voltage.

The Soft Start capacitor is then slowly discharged by a 2µA

current source until it reaches its lower 0.7V threshold. The

regulator will then attempt to restart normally, operating

in its extended off time mode with a 50% duty cycle, while

the Soft Start capacitor is charged with a 60µA charge cur-

rent.

If the short circuit condition persists, the regulator output

will not achieve the 1V low VFB comparator threshold

before the Soft Start capacitor is charged to its upper 2.5V

Trace 4 = 5V from PC Power Supply (2V/div.)

Trace 2 = Inductor Switching Node (2V/div.)

Figure 11: Demonstration board Start up with regulator output shorted

to ground.

10

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