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CS5159 Datasheet(PDF) 7 Page - ON Semiconductor

Part No. CS5159
Description  CPU 5−Bit Synchronous Buck Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5159 Datasheet(HTML) 7 Page - ON Semiconductor

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CS5159
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7
The V2 control method is illustrated in Figure 3. 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 V2
control scheme to compensate the duty cycle. Since the
change in inductor current modifies the ramp signal, as in
current mode control, the V2 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 V2 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 CS5159 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 COFF capacitor.
To maintain regulation, the V2 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 25 μs timer,
minimizing stress to the power components.
Programmable Output
The CS5159 is designed to provide two methods for
programming 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. The
first range is 2.10 V to 3.50 V in 100 mV steps, the second
is 1.30 V to 2.05 V in 50 mV steps, depending on the digital
input code. If all five bits are left open, the CS5159 enters
adjust mode. In adjust mode, the designer can choose any
output voltage by using resistor divider feedback to the VFB
and VFFB pins, as in traditional controllers.
Start Up
Until the voltage on the VCC1 supply pin exceeds the 3.9 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 VCC1 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 1.0 V level, the pulse is
terminated. 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 1.0 V level
present at the COMP pin, regulation has been achieved and
normal off time will ensue. The PWM comparator
terminates the switch on time, with off time set by the COFF
capacitor. The V2 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


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