MP1492 – 2A, 4.2V-16V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
PWM Operation
The MP1492 is a fully integrated synchronous
rectified step-down switch converter. Adaptive
constant-on-time (COT) control is employed to
provide fast transient response and easy loop
stabilization. At the beginning of each cycle, the
high-side MOSFET (HS-FET) is turned ON when
the feedback voltage (FB) is below the reference
voltage (REF) which indicates insufficient output
voltage. The ON period is determined by the
input voltage and the frequency-set resistor as
follows:
9.3×R7(kΩ)
Figure 2
(1)
TON(ns) =
+ 40ns
V (V)− 0.4
Floating Driver and Bootstrap Charging
IN
The floating power MOSFET driver is powered by
an external bootstrap capacitor. This floating
driver has its own UVLO protection. This UVLO’s
rising threshold is 2.2V with a hysteresis of
150mV. The bootstrap capacitor voltage is
charged from VCC through N1 (Figure 3). N1
turns on when LS switches turns on and turns off
when LS switch turns off.
After the ON period elapses, the HS-FET is
turned off. It is turned ON again when FB drops
below REF. By repeating operation in this way,
the converter regulates the output voltage. The
integrated low-side MOSFET (LS-FET) is turned
on when the HS-FET is in its OFF state to
minimize the conduction loss. There will be a
dead short between input and GND if both HS-
FET and LS-FET are turned on at the same time.
It’s called shoot-through. In order to avoid shoot-
through, a dead-time (DT) is internally generated
between HS-FET off and LS-FET on.
Switching Frequency
Adaptive constant-on-time (COT) control is used
in MP1492 and there is no dedicated oscillator in
the IC. The input voltage is feed-forwarded to the
on-time one-shot timer through the resistor R7.
The duty ratio is kept as VOUT/VIN. Hence the
switching frequency is fairly constant over the
input voltage range. The switching frequency
can be set as follows:
When the output current is high, the HS-FET and
LS-FET repeat on/off as described above. In this
operation, the inductor current will never go to
zero. It’s called continuous-conduction-mode
(CCM) operation. In CCM operation, the
switching frequency (Fs) is fairly constant.
106
(2)
F (kHz) =
S
Light-Load Operation
9.3×R7(kΩ) V (V)
IN
×
+ TDELAY (ns)
When the load current decreases, MP1492
reduces the switching frequency automatically to
maintain high efficiency. As the output current
reduces from heavy-load condition, the inductor
current decreases as well, and eventually comes
close to zero current. The LS-FET driver turns
into tri-state (high Z) whenever the inductor
current reaches zero level. The current modulator
takes over the control of LS-FET and limits the
inductor current to less than -1mA. Hence,
efficiency at light-load condition is optimized.
V (V)− 0.4 VOUT (V)
IN
Where TDELAY is the comparator delay, it’s
about 40ns.
MP1492 is optimized to operate at high switching
frequency but with high efficiency. High switching
frequency makes it possible to utilize small sized
LC filter components to save system PCB space.
Jitter and FB Ramp Slope
Figure 3 and Figure 4 show jitter occurring in
both PWM mode and skip mode. When there is
noise in the VFB downward slope, the ON time of
MP1492 Rev. 1.1
3/13/2012
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