CS5151
Applications Information: continued
ing 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 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 sig-
nal for a deviation in load. The V
2
™ 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 CS5151 uses a con-
stant 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 tran-
sients. PWM slope compensation to avoid sub-harmonic
oscillations at high duty cycles is avoided.
Switch on time is limited by an internal 30µs timer, mini-
mizing stress to the power components.
Programmable Output
The CS5151 is designed to provide two methods for pro-
gramming the output voltage of the power supply. A four
bit on board digital to analog converter (DAC) is used to
program the output voltage from 2.14V to 3.54V in 100mV
steps, depending on the digital input code. If all four bits
are left open, the CS5151 enters adjust mode. In adjust
mode, the designer can choose any output voltage by using
resistor divider feedback to the V
FB
and V
FFB
pins, as in tra-
ditional controllers. The CS5151 is specifically designed to
be upwards compatible with the CS5156, which uses a five
bit DAC code.
Start Up
Until the voltage on the V
CC1
supply pin exceeds the 3.9V
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 1V by the
comparator clamp. When the V
CC1
pin exceeds the monitor
threshold, the Gate output is activated, and the soft start
capacitor begins charging. The Gate 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 pin drives low for the duration of the extend-
ed off time. This time is set by the time out timer and is
6
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Trace 3 - 12V input (V
CC1
and V
CC2
) (5V/div.)
Trace 4 - 5V Input (1V/div.)
approximately equal to the maximum on time, resulting in
a 50% duty cycle. Then, the Gate 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 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).
Figure 2: CS5151 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.
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: CS5151 demonstration board startup waveforms.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
