CS5165
Application Information: continued
threshold, the GateH output is activated, and the Soft Start
Capacitor begins charging. The GateH output will remain
on, enabling the NFET switch, until terminated by either
the PWM Comparator, or the Maximum On-Time Timer.
If the Maximum On-Time is exceeded before the regulator
output voltage achieves the 1V level, the pulse is terminat-
ed. The GateH pin drives low, and the GateL pin drives
high for the duration of the Extended Off-Time. This time
is set by the Time-out Timer and is approximately equal to
the Maximum On-Time, resulting in a 50% Duty Cycle. The
GateL Pin will then drive low, the GateH pin will drive
high, and the cycle repeats.
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 C
OFF
Capacitor. The V
2
™ control loop will adjust switch Duty
Cycle as required to ensure the regulator output voltage
tracks the output of the Error Amp.
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.
Power Supply Sequencing
The CS5165 offers inherent protection from undefined
start-up conditions, regardless of the 12V and 5V supply
power-up sequencing. The turn-on slew rates of the 12V
and 5V power supplies can be varied over wide ranges
without affecting the output voltage or causing detrimental
effects to the buck regulator.
Trace 1 Soft Start pin (2V/div)
Trace 2 COMP pin (error amplifier output) (1V/div)
Trace 4 Regulator output voltage (1V/div)
Figure 3: Demonstration board start up waveforms.
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 4: Demonstration board enable start up waveforms.
Normal Operation
During Normal operation, Switch Off-Time is constant and
set by the C
OFF
capacitor. Switch On-Time is adjusted by
the V
2
™ 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 and the ESR of the output capacitors (see fig-
ures 5 & 6 ).
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Trace 3 - 12V input (V
CC
) (5V/div.)
Trace 4 - 5V Input (1V/div.)
Figure 2: Demonstration board start up 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.
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
