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
2
™
capacitor. The V
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).
Trace 1 - Regulator Output Voltage (5V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 4: CS5155H 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
2
™
V
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).
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Trace 3 - 12V input (V
and V
) (5V/div.)
CC1
CC2
Trace 4 - 5V Input (1V/div.)
Figure 2: CS5155H demonstration board startup in response to increas-
ing 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.
Trace 1 - Regulator Output Voltage (10V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 5: Peak-to-peak ripple on VOUT = 2.8V, IOUT = 0.5A (light load).
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - COMP Pin (error amplifier output) (1V/div.)
Trace 4 - Soft Start Pin (2V/div.)
Figure 3: CS5155H 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).
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
