NCP5425
http://onsemi.com
8
APPLICATIONS INFORMATION
Theory of Operation
The NCP5425 is a very versatile buck controller using
V
2
t
control method. It can be configured as:
•
Dual output Buck Controller.
• Two phase Buck Controller with current limit.
• Two phase Buck Controller with input power ratio and
current limit.
The fixed−frequency architecture, driven from a common
oscillator, ensures a 180° phase differential between
channels.
V
2
Control Method
The V
2
method of control uses a ramp signal generated by
the ESR (Effective Series Resistance) of the output
capacitors. This ramp is proportional to the AC current
through the main inductor and is offset by 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. The V
2
method differs from traditional techniques such as voltage
mode control, which generates an artificial ramp, and
current mode control, which generates a ramp using the
inductor current.
Figure 3. V
2
Control with Slope Compensation
COMP
+
−
RAMP
PWM
GATE(H)
GATE(L)
Slope
Compensation
Error Signal
+
−
Error
Amplifier
Reference
Voltage
Output
Voltage
V
FB
The V
2
control method is illustrated in Figure 3. The
output voltage generates 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, allowing the control circuit to
drive the main switch to 0% or 100% duty cycle as required.
A variation in line voltage changes the current ramp in the
inductor, which causes the V
2
control scheme to compensate
the duty cycle. Since any variation in inductor current
modifies the ramp signal, as in current mode control, the V
2
control scheme offers the same advantages in line transient
response. A variation in load current will affect the output
voltage, modifying the ramp signal. A load step immediately
changes the state of the comparator output, which controls
the main switch. The comparator response time and the
transition speed of the main switch determine the load
transient response. Unlike traditional control methods, the
reaction time to the output load step is not related to the
crossover frequency of the error signal loop. The error signal
loop can have a low crossover frequency, since the 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 control loops. A voltage mode controller
relies on the 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, the
consequence of which is normally specified as line or load
regulation. A current mode controller maintains a fixed error
signal during line transients, since the slope of the ramp
signal changes in this case. However, regulation of load
transients still requires a change in the error signal. 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.
The stringent load transient requirements of modern
power supplies require the output capacitors to have very
low ESR. The resulting shallow slope in the output ripple
can lead to pulse width jitter and variation caused by both
random and synchronous noise. A ramp waveform
generated in the oscillator is added to the ramp signal from
the output voltage to provide the proper voltage ramp at the
beginning of each switching cycle. This slope compensation
increases the noise immunity, particularly at higher duty
cycle (above 50%).
Startup
The NCP5425 features a programmable soft−start
function, which is implemented through the error amplifier
and external compensation capacitor. This feature reduces
stress to the power components and limits overshoot of the
output voltage, during startup. As power is applied to the
regulator, the NCP5425 Undervoltage Lockout circuit
(UVLO) monitors the IC’s supply voltage (V
CC
). The
UVLO circuit prevents the MOSFET gates from switching
until V
CC
exceeds 4.2 V. Internal UVLO threshold
hysteresis of 200 mV improves noise immunity. During
startup, the external Compensation Capacitor connected to
the COMP pin is charged by an internal 30 mA current
source. When the capacitor voltage exceeds the 0.3 V offset
of the PWM comparator, the PWM control loop will allow
switching to occur. The upper gate driver GATE(H) is now
activated, turning on the upper MOSFET. The output current
then ramps up through the main inductor and linearly
powers the output capacitors and load. When the regulator
