SC4602A データシートの表示（PDF） - Semtech Corporation
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SC4602A Datasheet PDF : 18 Pages
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SC4602A/B
POWER MANAGEMENT
Applications Information
Enable
Pulling and holding the SYNC/SLEEP pin below 0.8V ini-
tializes the SLEEP mode of the SC4602A/B with its typi-
cal SLEEP mode supply current of 10uA. During the SLEEP
mode, the high side and low side MOSFETs are turned
off and the internal soft start voltage is held low.
Oscillator
during start up and over current to set the hiccup time.
The soft start time can be calculated by:
TSOFT _ START
=
720
fs
As can be seen here, the soft start time is switching fre-
quency dependant. For example, if fs = 300kHz, T _ SOFT START
= 720/300k = 2.4ms. But if fs = 600kHz, T _ SOFT START =
720/600k = 1.2ms.
The SC4602A/B is a constant frequency, voltage mode,
and synchronous step down controller ideal for low volt-
age, high efficiency, precisely regulated output DC/DC
converters. Its internal free running oscillator sets the
PWM frequency at 300kHz for the SC4602A and 550kHz
for the SC4602B without any external components to
set the frequency. A 100% maximum duty cycle allows
the SC4602A/B to operate as a low dropout regulator in
the event of a low battery condition. An external clock
connected to SYNC/SLEEP activates its synchronous
mode and the frequency of the clock can be up to
450kHz for the SC4602A and 700kH for the SC4602B.
UVLO
When the SYNC/SLEEP pin is pulled and held above 2V,
the voltage on the Vcc pin determines the operation of
the SC4602A/B. As Vcc increases during start up, the
UVLO block senses Vcc and keeps the high side and low
side MOSFETs off and the internal soft start voltage low
until Vcc reaches 2.75V. If no faults are present, the
SC4602A/B will initiate a soft start when Vcc exceeds
2.75V. A hysteresis (150mV) in the UVLO comparator
provides noise immunity during its start up.
Soft Start
The SC4602A/B implements its soft start by ramping up
the error amplifier reference voltage providing a con-
trolled slew rate of the output voltage, then preventing
overshoot and limiting inrush current during its start up.
Over Current Protection
Over current protection for the SC4602A/B is imple-
mented by detecting the voltage drop of the high side P-
MOSFET during conduction, also known as high side RDS(ON)
detection. This loss-less detection eliminates the sense
resistor and its loss. The overall efficiency is improved,
and the number of components and cost of the con-
verter are reduced. RDS(ON) sensing is by default inaccu-
rate and is mainly used to protect the power supply dur-
ing a fault case. The over current trigger point will vary
from unit to unit as the RDS(ON) of P-MOSFET varies. Even
for the same unit, the over current trigger point will vary
as the junction temperature of P-MOSFET varies. The
SC4602A/B provides a built-in 300mV voltage source.
The over current trigger point can be determined based
on the internal 300mV voltage source and the RDS(ON) of
P-MOSFET as follows:
Itrigger
=
300mV
RDS(ON)
The soft start function is required for step down control- Kelvin sensing connections should be used at the drain
lers to prevent excess inrush current through the DC bus and source of P-MOSFET.
during start up. Generally this can be done by sourcing a
controlled current into a timing capacitor and then using
The R sensing used in the SC4602A/B has an addi-
DS(ON)
the voltage across this capacitor to slowly ramp up the tional feature that enhances the performance of the over
error amp reference. The closed loop creates narrow current protection. Because the RDS(ON) has a positive
width driver pulses while the output voltage is low and temperature coefficient, the 300mV voltage source has
allows these pulses to increase to their steady state duty a positive coefficient of about 0.4%/C° providing first
cycle as the output voltage reaches its regulated value. order correction for current sensing vs temperature. This
With this, the inrush current from the input side is con- compensation depends on the high amount of thermal
trolled. The duration of the soft start in the SC4602A/B transferring that typically exists between the high side P-
is controlled by an internal timing circuit which is used MOSFET and the SC4602A/B due to the compact layout
of the power supply.
2006 Semtech Corp.
9
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