7

Applications Information: continued

normal off time will ensue. The PWM comparator termi-

cycle as required to ensure the regulator output voltage

tracks the output of the error amplifier.

The soft start and COMP capacitors will charge to their

final levels, providing a controlled turn on of the regulator

output. Regulator turn on time is determined by the COMP

capacitor charging to its final value. Its voltage is limited

by the soft start COMP clamp and the voltage on the soft

start pin (see Figures 2 and 3).

Figure 2: CS5157 demonstration board startup in response to increasing

12V and 5V input voltages. Extended off time is followed by normal off

time operation when output voltage achieves regulation to the error

amplifier output.

Figure 3: CS5157 demonstration board startup waveforms.

If the input voltage rises quickly, or the regulator output is

enabled externally, output voltage will increase to the level

set by the error amplifier output more rapidly, usually

within a couple of cycles (see Figure 4).

Figure 4: CS5157 demonstration board enable startup waveforms.

Normal Operation

During normal operation, switch off time is constant and

changes in regulator switching frequency, duty cycle, and

output ripple in response to changes in load and line.

Output voltage ripple will be determined by inductor rip-

ple current working into the ESR of the output capacitors

(see Figures 5 and 6).

Trace 1 - Regulator Output Voltage (10mV/div.)

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

Trace 1 - Regulator Output Voltage (5V/div.)

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

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

Trace 3 - COMP Pin (error amplifier output) (1V/div.)

Trace 4 - Soft Start Pin (2V/div.)

Ch1 High

2.80V

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

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

Trace 4 - 5V Input (1V/div.)