RT9277A Просмотр технического описания (PDF) - Richtek Technology
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RT9277A Datasheet PDF : 12 Pages
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Preliminary
RT9277A/B
EN
COMP
FB
FREQ
LX
VIN
Protection
VFB
1.24V
Error
Amplifier
-
+
Summing
Comparator
-
+
5uA
Oscillator
Slope
Compen-
sation
Clock
4uA
Soft-
Start
Control
and
Driver
N
Logic
Current
Sense
RT9277B
VIN
SS
LX
GND
Operation
The RT9277A/B is a high efficiency step-up Boost converter
with a fixed-frequency, current-mode PWM architecture.
It performs fast transient response and low noise operation
with appropriate component selection. The output voltage
is regulated through a feedback control consisting of an
error amplifier, a summing comparator, and several control
signal generators (as shown in function block diagram).
The feedback reference voltage is 1.24V. The error amplifier
varies the COMP voltage by sensing the FB pin. The slope
compensation signal summed with the current -sense
signal will be compared with the COMP voltage through
the summing comparator to determine the current trip point
and duty cycle. When driving light loads, the RT9277A/B
will perform the pulse-skipping mode to prevent
overcharging the output voltage. In this mode, the switching
frequency will be reduced to perform a higher efficiency.
Soft-Start
The RT9277B provides programmable soft-start function.
When the EN pin is connected to high, a 4μA constant
current is sourced to charge an external capacitor. The
voltage rate of rise on the COMP pin is limited during the
charging period, and so is the peak inductor current.
When the EN pin is connected to GND, the external
capacitor will be discharged to ground for the next time
soft-start.
Current Limitation
The switch current is monitored to limit the value not to
exceed 1.6A typically. When the switch current reaches
1.6A, the output voltage will be pulled down to limit the
total output power to protect the power switch and external
components.
Shutdown
Connect the EN to GND to turn the RT9277A/B off and
reduce the supply current to 0.1μA. In this operation, the
output voltage is the value of VIN to subtract the forward
voltage of catch diode.
Frequency Selection
The switching frequency of RT9277A/B can be selected to
operate at either 640kHz or 1.2MHz. When the FREQ pin
is connected to GND for 640kHz operation, and connected
to VIN for 1.2MHz operation. FREQ is preset to 640kHz
operation for allowing the FREQ pin unconnected.
DS9277A/B-02 March 2007
www.richtek.com
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