Applications Information: continued

8

If the maximum on time is exceeded while responding to a

sudden increase in load current, a normal off time occurs to

prevent saturation of the output inductor.

Figure 7: CS5151 demonstration board response to a 0.5 to 13A load

pulse (output set for 2.8V).

Figure 8: CS5151 demonstration board response to 13A load turn on

trol loop immediately connects the inductor to the input voltage, pro-

viding 100% duty cycle. Regulation is achieved in less than 20µs.

Figure 9: CS5151 demonstration board response to 13A load turn off

inductor to ground, providing 0% duty cycle. Regulation is achieved in

less than 10µs.

To maintain predictable startup and shutdown characteris-

part from operating below 3.75V minimum startup. The

tees a 3.70V minimum shutdown threshold.

Short Circuit Protection

A lossless hiccup mode short circuit protection feature is

provided, requiring only the soft start capacitor to imple-

low comparator sets the FAULT latch. This causes the MOS-

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 current.

If the short circuit condition persists, the regulator output

before the soft start capacitor is charged to its upper 2.5V

threshold. If this happens the cycle will repeat itself until the

short is removed. The soft start charge/discharge current

ratio sets the duty cycle for the pulses (2µA/60µA = 3.3%),

while actual duty cycle is half that due to the extended off

time mode (1.65%).

This protection feature results in less stress to the regulator

Protection and Monitoring Features

Trace1 - Regulator Output Voltage (1V/div.)

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

Trace 3 - Output Current (13 to 0.5 Amps) (20V/div.)

Trace 1 - Regulator Output Voltage (1V/div.)

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

Trace 3 - Output Current (0.5 to 13 Amps) (20V/div.)

Trace 1 - Regulator Output Voltage (1V/div.)

Trace 3 - Regulator Output Current (20V/div.)