CS5165A
http://onsemi.com
8
APPLICATIONS INFORMATION
THEORY OF OPERATION
V
2
Control Method
The V
2
method of control uses a ramp signal that is
generated by the ESR of the output capacitors. This ramp is
proportional to the AC current through the main inductor and
is offset by the value of the DC output voltage. This control
scheme inherently compensates for variation in either line or
load conditions, since the ramp signal is generated from the
output voltage itself. This control scheme differs from
traditional techniques such as voltage mode, which generates
an artificial ramp, and current mode, which generates a ramp
from inductor current.
Figure 9. V
2
Control Diagram
COMP
Reference
Voltage
+
+
PWM
Comparator
Ramp
Signal
Error
Amplifier
Error
Signal
Output
Voltage
Feedback
GATE(L)
E
C
−
−
GATE(H)
The V
2
control method is illustrated in Figure 9. The output
voltage is used to generate both the error signal and the ramp
signal. Since the ramp signal is simply the output voltage, it is
affected by any change in the output regardless of the origin of
that change. The ramp signal also contains the DC portion of
the output voltage, which allows the control circuit to drive the
main switch to 0% or 100% duty cycle as required.
A change in line voltage changes the current ramp in the
inductor, affecting the ramp signal, which causes the V
2
control
scheme to compensate the duty cycle. Since the change in
inductor current modifies the ramp signal, as in current mode
control, the V
2
control scheme has the same advantages in line
transient response.
A change in load current will have an affect on the output
voltage, altering the ramp signal. A load step immediately
changes the state of the comparator output, which controls the
main switch. Load transient response is determined only by the
comparator response time and the transition speed of the main
switch. The reaction time to an output load step has no relation
to the crossover frequency of the error signal loop, as in
traditional control methods.
The error signal loop can have a low crossover frequency,
since transient response is handled by the ramp signal loop.
The main purpose of this ‘slow’ feedback loop is to provide
DC accuracy. Noise immunity is significantly improved, since
the error amplifier bandwidth can be rolled off at a low
frequency. Enhanced noise immunity improves remote sensing
of the output voltage, since the noise associated with long
feedback traces can be effectively filtered.
Line and load regulation are drastically improved because
there are two independent voltage loops. A voltage mode
controller relies on a change in the error signal to compensate
for a deviation in either line or load voltage. This change in the
error signal causes the output voltage to change corresponding
to the gain of the error amplifier, which is normally specified
as line and load regulation. A current mode controller
maintains fixed error signal under deviation in the line voltage,
since the slope of the ramp signal changes, but still relies on a
change in the error signal for a deviation in load. The V
2
method of control maintains a fixed error signal for both line
and load variation, since the ramp signal is affected by both line
and load.
Constant Off Time
To maximize transient response, the CS5165A uses a
constant off time method to control the rate of output pulses.
During normal operation, the off time of the high side switch
is terminated after a fixed period, set by the C
OFF
capacitor.
To maintain regulation, the V
2
control loop varies switch on
time. The PWM comparator monitors the output voltage
ramp, and terminates the switch on time.
Constant off time provides a number of advantages. Switch
duty cycle can be adjusted from 0 to 100% on a pulse by pulse
basis when responding to transient conditions. Both 0% and
100% duty cycle operation can be maintained for extended
periods of time in response to load or line transients. PWM
slope compensation to avoid sub−harmonic oscillations at
high duty cycles is avoided.
Switch on time is limited by an internal 30 ms (typical)
timer, minimizing stress to the power components.
Programmable Output
The CS5165A is designed to provide two methods for
programming the output voltage of the power supply. A 5−bit
on board digital to analog converter (DAC) is used to program
the output voltage within two different ranges. The first range
is 2.14 V to 3.54 V in 100 mV steps, the second is 1.34 V to
2.09 V in 50 mV steps, depending on the digital input code.
If all five bits are left open, the CS5165A enters adjust mode.
In adjust mode, the designer can choose any output voltage by
using resistor divider feedback to the V
FB
pin, as in traditional
controllers. The CS5165A is specifically designed to meet or
exceed Intel’s Pentium II specifications.
Startup
Until the voltage on the V
CC
supply pin exceeds the 3.95 V
monitor threshold, the Soft−Start and GATE pins are held low.
The FAULT latch is reset (no Fault condition). The output of
the error amplifier (COMP) is pulled up to 1.0 V by the
comparator clamp. When the V
CC
pin exceeds the monitor
threshold, the GATE(H) output is activated, and the Soft−Start
