CS5166
Application Information: continued
The first range is 2.125V to 3.525V in 100mV steps, the sec-
ond is 1.325V to 2.075V in 50mV steps, depending on the
digital input code. If all five bits are left open, the CS5166
enters adjust mode. In adjust mode, the designer can
choose any output voltage by using resistor divider feed-
back to the V
FB
pin, as in traditional controllers. The
CS5166 is specifically designed to meet or exceed Intel’s
Pentium
®
II specifications.
Start-up
Until the voltage on the V
CC
Supply pin exceeds the 3.95V
monitor threshold, the Soft Start and Gate pins are held
low. The Fault latch is Reset (no Fault condition). The out-
put of the Error Amp (COMP) is pulled up to 1V by the
Comp Clamp. When the V
CC
pin exceeds the monitor
threshold, the GATE(H) output is activated, and the Soft
Start Capacitor begins charging. The GATE(H) 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 GATE(H) pin drives low, and the GATE(L) 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 GATE(L) Pin will then drive low, the GATE(H)
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
TM
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.
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - COMP Pin (error amplifier output) (1V/div.)
Trace 4 - Soft Start Pin (2V/div.)
Figure 4: Demonstration board startup waveforms.
Trace 1 - Regulator Output Voltage (5V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 5: Demonstration board enable startup 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
TM
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
Figures 6 and 7).
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 3: 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.
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
