MPQ8632 HIGH EFFICIENCY 18V SYNCHRONOUS STEP-DOWN CONVERTER FAMILY FOR 4A TO 20A
OPERATION
PWM Operation
interval determined by the one- shot on-timer as
per equation 1. When the HS-FET turns off, the
LS-FET turns on until the next period.
The MPQ8632 is a fully integrated synchronous
rectified step-down switch mode converter. It
uses Constant-on-time (COT) control to provide a
fast transient response and ease loop
stabilization.
In CCM operation, the switching frequency is
fairly constant and is also called PWM mode.
Light-Load Operation
As the load decreases, the inductor current
decreases too. When the inductor current
touches zero, the operation is transited from
At the beginning of each cycle, the high-side
MOSFET (HS-FET) turns ON when the feedback
voltage (VFB) drops below the reference voltage
(VREF), which indicates an insufficient output
voltage. The input voltage and the frequency-set
resistor determine the ON period as follows:
continuous-conduction-mode
(CCM)
to
discontinuous-conduction-mode (DCM).
Figure 3 shows the light load operation. When
VFB drops below VREF, HS-FET turns on for a
fixed interval determined by the one- shot on-
timer as per equation 1. When the HS-FET turns
off, the LS-FET turns on until the inductor current
reaches zero. In DCM operation, the VFB does
not reach VREF when the inductor current is
approaching zero. The LS-FET driver turns into
tri-state (high Z) whenever the inductor current
reaches zero. A current modulator takes over the
control of LS-FET and limits the inductor current
less than -1mA. Hence, the output capacitors
discharge slowly to GND through LS-FET. As a
result, this mode improves greatly the light load
efficiency. At light load condition, the HS-FET
does not turns ON as frequently as at heavy load
condition. This is called skip mode.
6.1RFREQ(k)
(1)
TON(ns)
V (V) 0.4
IN
After the ON period elapses, the HS-FET turns
off. It turns ON again when VFB drops below VREF
.
By repeating this operation, the converter
regulates the output voltage. The integrated low-
side MOSFET (LS-FET) turns on when the HS-
FET is OFF to minimize the conduction loss.
There is a dead short (or shoot-through)
between input and GND if both HS-FET and LS-
FET turn on at the same time. A dead-time (DT)
internally generated between HS-FET OFF and
LS-FETON, or LS-FET OFF and HS-FET ON
avoids shoot-through.
Heavy-Load Operation
At light load or no load condition, the output
drops very slowly and the MPQ8632 reduces the
switching frequency naturally and then achieves
high efficiency at light load.
Figure 3—Light Load Operation
Figure 2—Heavy Load Operation
When the output current is high and the inductor
current is always above zero amps, it is called
continuous-conduction-mode (CCM). Figure 2
shows the CCM operation. When VFB is below
VREF
,
HS-FET turns on for
a
fixed
MPQ8632 Rev.1.24
8/28/2013
www.MonolithicPower.com
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