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MP28248 Datasheet(PDF) 11 Page - Monolithic Power Systems

Part No. MP28248
Description  High-Efficiency, Fast-Transient, 3A, 4.2V-20V Input Synchronous Step-down Converter in a QFN12 (2x3mm) Package
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Maker  MPS [Monolithic Power Systems]
Homepage  http://www.monolithicpower.com
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MP28248 Datasheet(HTML) 11 Page - Monolithic Power Systems

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MP28248 – 3A, 4.2V-20V INPUT, FAST-TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)
MP28248 Rev 1.0
www.MonolithicPower.com
11
1/5/2012
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
PWM Operation
The MP28248 is a fully-integrated, synchronous,
rectified, step-down switch converter. The device
uses constant-on-time (COT) control to provide
fast
transient
response
and
easy
loop
stabilization. At the beginning of each cycle, the
high-side
MOSFET
(HS-FET)
turns
ON
whenever the feedback voltage (VFB) is lower
than the reference voltage (VREF)—a low VFB
indicates insufficient output voltage. The input
voltage and the frequency-set resistor determine
the ON period as follows:


7
ON
IN
9.3 R (k )
t40(ns)
V(V) 0.4
(1)
After the ON period elapses, the HS-FET enters
the OFF state. By cycling HS-FET between the
ON and OFF states, the converter regulates the
output voltage. The integrated low-side MOSFET
(LS-FET) turns on when the HS-FET is in its OFF
state to minimize the conduction loss.
Shoot-through occurs when both the HS-FET
and the LS-FET are turned on at the same time,
causing a dead short between input and GND.
Shoot-through dramatically reduces efficiency,
and the MP28248 avoids this by internally
generating a dead-time (DT) between when HS-
FET turns off and LS-FET turns on, and when
LS-FET turns off and HS-FET turns on.
Heavy-Load Operation
Figure 2: Heavy-Load Operation
During heavy-load operation—when the output
current is high—the MP28248 enters continuous-
conduction mode (CCM) where the HS-FET and
LS-FET repeat the on/off operation described for
PWM operation, the inductor current never goes
to zero, and the switching frequency (fSW) is fairly
constant. Figure 2 shows the timing diagram
during this operation.
Light-Load Operation
During light-load operation—when the output
current is low—the MP28248 reduces the
switching frequency to maintain high efficiency,
and the inductor current drops near zero. When
the inductor current reaches zero, the LS-FET
driver goes into tri-state (high Z). The current
modulator controls the LS-FET and limits the
inductor current to around -1mA as shown in
Figure 3. Hence, the output capacitors discharge
slowly to GND through LS-FET, R1, and R2. This
operation greatly improves device efficiency
when the output current is low.
Figure 3: Light-Load Operation
Light-load operation is also called skip mode
because the HS-FET does not turn on as
frequently as during heavy-load conditions. The
frequency at which the HS-FET turns on is a
function of the output current—as the output
current increases, the time period that the current
modulator regulates becomes shorter, and the
HS-FET turns on more frequently. The switching
frequency increases in turn. The output current
reaches the critical level when the current
modulator time is zero, and can be determined
using the following equation:
IN
OUT
OUT
OUT
SW
IN
(V -V
) V
I=
2L f
V

(2)
The device reverts to PWM mode once the
output current exceeds the critical level. After
that, the switching frequency stays fairly constant
over the output current range.


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