CS5166
Application Information
Theory Of Operation
V
2
TM
Control Method
The V
2
TM
method of control uses a ramp signal that is gen-
erated 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 gen-
erated 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.
PWM
Comparator
+
GATE(H)
C
–
GATE(L)
of the CS5166 single pole feedback loop and demonstrates
the overall stability of the CS5166-based regulator.
.1µF
10K
Open Loop
49.63
BW
62.3 KHz
Phase margin
81.9
Figure 2: Feedback loop Bode Plot.
Ramp Signal
V
FB
Error
Amplifier
COMP
Error
Signal
E
–
+
Reference
Voltage
Figure 1: V
2
2
TM
TM
Control Diagram.
The V control method is illustrated in Figure 1. The out-
put 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 regard-
less of the origin of that change. The ramp signal also con-
tains 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
TM
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
TM
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 effective-
ly filtered.
The Bode plot in Figure 2 shows the gain and phase margin
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 compen-
sate 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
TM
method of control
maintains a fixed error signal for both line and load varia-
tion, since the ramp signal is affected by both line and load.
Constant Off-Time
To maximize transient response, the CS5166 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
TM
Control
Loop varies switch On-Time. The PWM comparator moni-
tors 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 condi-
tions. 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 30µs (typical)
timer, minimizing stress to the Power Components
Programmable Output
The CS5166 is designed to provide two methods for pro-
gramming the output voltage of the power supply. A five
bit on board digital to analog converter (DAC) is used to
program the output voltage within two different ranges.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
