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MP9447 Datasheet(PDF) 10 Page - Monolithic Power Systems

No. de Pieza. MP9447
Descripción  High-Efficiency, Fast-Transient, 5A, 36V Synchronous, Step-Down Converter
Descarga  19 Pages
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Fabricante  MPS [Monolithic Power Systems]
Página de inicio  http://www.monolithicpower.com
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MP9447 Datasheet(HTML) 10 Page - Monolithic Power Systems

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MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER
MP9447 Rev. 1.2
www.MonolithicPower.com
10
1/5/2016
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
OPERATION
PWM Operation
The MP9447 is a fully-integrated, synchronous,
rectified, step-down, switch-mode converter. 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 ON period is determined by the
input voltage and the frequency-set resistor as:

 
FREQ
ON
DELAY
IN
96 R
k
tns
t
(ns)
V


(1)
After the ON period elapses, the HS-FET turns
OFF. It is turned 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 conduction loss. A dead
short occurs between input and GND if both the
HS-FET and the LS-FET turn on at the same
time (shoot-through). An internal dead-time (DT)
generated between HS-FET OFF and LS-FET
ON, or LS-FET OFF and HS-FET ON prevents
shoot-through.
Heavy-Load Operation
t
ON
Figure 2: Heavy-Load Operation
In continuous-conduction mode (CCM), when the
output current is HIGH, the HS-FET and LS-FET
repeatedly turn ON/OFF as shown in MPS. All
Rights Reserved. The inductor current never
goes to zero. In CCM, the switching frequency
(fSW) is fairly constant.
Light-Load Operation
At light-load or no-load conditions, the output
drops very slowly and the MP9447 reduces the
switching frequency automatically to maintain
high efficiency. Figure 3 shows the light-load
operation. VFB does not reach VREF as the
inductor current approaches zero. The LS-FET
driver enters a tri-state (high Z) whenever the
inductor
current
reaches
zero.
A
current
modulator takes control of the LS-FET and limits
the inductor current to less than -1mA. Hence,
the output capacitors discharge slowly to GND
through the LS-FET to greatly improve the light-
load efficiency. At light loads, the HS-FET does
not turn ON as frequently as at heavy loads. This
is called skip mode.
t
ON
Figure 3: Light-Load Operation
As the output current increases from light-load
condition, the current modulator’s regulatory time
period becomes shorter. The HS-FET turns ON
more frequently, thus increasing the switching
frequency increases. The output current reaches
its critical level when the current modulator time
is zero. The critical output current level is:
IN
OUT
OUT
OUT
SW
IN
(V
V
) V
I
2L F
V


(2)
It enters PWM mode once the output current
exceeds the critical level. After that, the switching
frequency stays fairly constant over the output
current range.
Switching Frequency
The input voltage is feed-forwarded to the on-
time one-shot timer through the resistor, RFREQ.
The duty ratio remains at VOUT/VIN. Hence, the
switching frequency is fairly constant over the
input voltage range. The switching frequency can
be set as:


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