SC4612 Ver la hoja de datos (PDF) - Semtech Corporation
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SC4612 Datasheet PDF : 24 Pages
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SC4612
POWER MANAGEMENT
Applications Information
INTRODUCTION
START UP SEQUENCE
The SC4612 is a versatile voltage mode synchronous Start up is inhibited until VDD input reaches its UVLO
rectified buck PWM convertor, with an input supply (VIN) threshold. The UVLO limit is 4.5V (TYP).
ranging from 4.5V to 28V designed to control and drive Meanwhile, the high side and low side gate drivers DH,
N-channel MOSFETs.
and DL, are kept low. Once VDD exceeds the UVLO
The power dissipation is controlled using a novel low volt- threshold, the external soft-start capacitor starts to be
age supply technique, allowing high speed and integra- charged by a 25µA current source. If an over current
tion with the high drive currents to ensure low MOSFET condition occurs, the SS/EN pin will discharge to 500mV
switching loss. The synchronous buck configuration also by an internal switch. During this time, both DH and DL
allows converter sinking current from load without losing will be turned off.
output regulation.
When the SS pin reaches 0.8V, the converter will start
The internal reference is trimmed to 500mV with ± 1% switching. The reference input of the error amplifier is
accuracy, and the output voltage can be adjusted by two ramped up with the soft-start signal. Initially only the high
external resistors.
side driver is enabled. Keeping the low side MOSFET off
A fixed oscillator frequency (up to 1.2MHz) can be
programmed by an external capacitor for an optimized
design.
During the Asynchronous start up, the SC4612 provides a
during start up is useful where multiple convertors are
operating in parallel. It prevents forward conduction in the
freewheeling MOSFET which might otherwise cause a dip
in the common output bus.
top MOSFET shut down over current protection, while under In case of over current condition which is longer than 10
normal operating conditions a low side MOSFET RDS-ON cycles during the asynchronous start up, SC4612 will turn
current sensing with hiccup mode over current protection, off the high side MOSFET gate drive, and the soft-start
minimizes power dissipation and provides further sequence will repeat.
protection.
When the SS pin reaches 1.3V, the low side MOSFET will
Other features of the SC4612 include:
begin to switch and the convertor is fully operational in
Wide input power voltage range (from 4.5V to 28V), low
output voltages down to 500mV, externally programmable
soft-start, hiccup over current protection, wide duty cycle
range, thermal shutdown, asynchronous start-up
the synchronous mode. The soft-start duration is
controlled by the value of the SS cap. If the SS pin is pulled
below 0.5V, the SC4612 is disabled and draws a typical
quiescent current of 5mA.
protection, and a -40 to 125°C junction operating
temperature range.
Bias Generation
A 4.5V to 10V (MAX) supply voltage is required to power
THEORY OF OPERATION
SUPPLIES
up the SC4612. This voltage could be provided by an ex-
ternal power supply or derived from VDD (VDD >10V)
through an internal pass transistor.
Two pins (VDD and DRV) are used to power up the SC4612.
If input supply (VDD) is less than 10V (MAX), tie DRV and
VDD together.
The internal pass transistor will regulate the DRV from an
external supply >10V connected to VDD to produce 7.8V
(TYP) at the DRV pin.
This supply should be bypassed with a low ESR 2.2uF (or
greater) ceramic capacitor directly at the DRV to GND pins
of the SC4612.
The DRV supply also provides the bias for the low and the
high side MOSFET gate drive.
The maximum rating for DRV supply is 10V and for
applications where input supply is below 10V, it may be
connected directly to VDD.
Soft start / Shut down
An external capacitor at the SS/EN pin is used to set up
the soft-start duration. The capacitor value in conjunction
with the internal current source, controls the duration of
soft-start time. If the SS/EN pin is pulled down to GND,
the SC4612 is disabled. The soft-start pin is charged by a
25µA current source and discharged by an internal switch.
When SS/EN is released it charges up to 0.5V as the con-
trol circuit starts up.
2007 Semtech Corp.
9
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