MPQ8616-12 Datasheet, PDF (20/29 Pages) Monolithic Power Systems – High Efficiency, 6A/12A, 6V Synchronous Step-Down Converter

MPQ8616, 6A/12A, 6V, SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
PWM Operation
The MPQ8616 is fully integrated synchronous
rectified step-down switch mode converter.
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 (V_FB) is below the reference
voltage (V_REF), which indicates insufficient output
voltage. The ON period is determined by the
input voltage and the frequency-set resistor as
follows:
t_ON (ns)

4.8  R_FREQ(k)
V_IN(V)  0.49
(1)
After the ON period elapses, the HS-FET is
turned off, or becomes OFF state. It is turned ON
again when V_FB drops below V_REF. By repeating
operation 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, or LS-FET off and HS-FET on.
Figure 2—PWM Operation
MPQ8616 always operating in continuous-
conduction-mode (CCM), which means the
inductor current can go negative at light load.
The CCM mode operation is shown in Figure2.
When V_FB is below V_REF, HS-MOSFET is turned
on for a fixed interval which is determined by
one- shot on-timer as equation 1 shown. When
the HS-MOSFET is turned off, the LS-MOSFET
is turned on until next period.
For the MPQ8616 is operated in CCM, the
switching frequency is fairly constant and it is
called PWM mode.
Switching Frequency
The selection of switching frequency is a tradeoff
between efficiency and component size. Low
frequency operation increases efficiency by
reducing MOSFET switching losses, but requires
larger inductance and capacitance to maintain
low output voltage ripple.
For MPQ8616 ， the on time can be set using
FREQ pin, then the frequency is set in steady
state operation at CCM mode.
Adaptive constant-on-time (COT) control is used
in MPQ8616 and there is no dedicated oscillator
in the IC. Connect FREQ pin to IN pin through
resistor R_FREQ and the input voltage is feed-
forwarded to the one-shot on-time timer through
the resistor R_FREQ. When in steady state
operation at CCM, the duty ratio is kept as
V_OUT/V_IN. Hence the switching frequency is fairly
constant over the input voltage range. The
switching frequency can be set as follows:
f_SW (kHz)

10⁶
4.8  R_FREQ (k) _ V_IN(V)
V_IN(V)  0.49 V_OUT (V)
 t_DELAY (ns)
(2)
Where t_DELAY is the comparator delay. It’s about
40ns.
Generally, the MPQ8616 is set for 300kHz to
1MHz application. It is optimized to operate at
high switching frequency 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 shows jitter occurring in PWM mode.
When there is noise in the V_FB downward slope,
the ON time of HS-FET deviates from its
intended location and produces jitter. It is
necessary to understand that there is a
relationship between a system’s stability and the
steepness of the V_FB ripple’s downward slope.
The slope steepness of the V_FB ripple dominates
MPQ8616 Rev. 1.01
www.MonolithicPower.com
20
8/4/2013
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