CS5155 datasheet(7/14 Pages) CHERRY

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Applications Information: continued

When regulator output voltage achieves the 1V level pre-

sent at the COMP pin, regulation has been achieved and

normal off time will ensue. The PWM comparator termi-

nates the switch on time, with off time set by the COFF

capacitor. The V2ª control loop will adjust switch duty

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: CS5155 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: CS5155 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: CS5155 demonstration board enable startup waveforms.

Normal Operation

During normal operation, switch off time is constant and

set by the COFF capacitor. Switch on time is adjusted by the

V2ª control loop to maintain regulation. This results in

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).

Figure 5: Peak-to-peak ripple on VOUT = 2.8V, IOUT = 0.5A (light load).

Trace 1 - Regulator Output Voltage (10V/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.)

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

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

Trace 3 - 12V input (VCC1 and VCC2) (5V/div.)

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

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