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SC4605 Datasheet(PDF) 6 Page - Semtech Corporation

Part No. SC4605
Description  Low Input, High Efficiency Synchronous, Step Down Controller
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Maker  SEMTECH [Semtech Corporation]
Homepage  http://www.semtech.com
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SC4605 Datasheet(HTML) 6 Page - Semtech Corporation

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 2004 Semtech Corp.
www.semtech.com
SC4605
POWER MANAGEMENT
Applications Information
Enable
Pulling and holding the FS/SYNC pin below 0.2V initial-
izes the SLEEP mode of the SC4605 with its typical SLEEP
mode supply current of 10
µA. During the SLEEP mode,
the high side and low side MOSFETs are turned off and
the internal soft start voltage is held low.
Oscillator
The oscillator uses an external capacitor between FS and
GND to set the oscillation frequency. The ramp wave-
form is a triangle at the PWM frequency with a peak volt-
age of 2V and a valley voltage of 0.5V. The PWM duty
ratio is limited to a maximum of 97%, which allows the
bootstrap capacitor to be charged during each cycle. The
capacitor tolerance adds to the accuracy of the oscilla-
tor frequency. The approximate operating frequency is
determined by the external capacitor connected to the
FS/SYNC pin as shown below:
T
4
s
C
10
55
.
1
f
=
In its synchronous mode, a low value resistor needs to
be connected between ground and the frequency set-
ting capacitor, CT. Then an external clock connects to
the junction of the resistor and the capacitor to activate
its synchronous mode. The frequency of the clock can
be used up to 700kHz. This external clock signal should
have a duty cycle from 5% to 10% and the peak voltage
at the junction from the clock signal should be about
0.2V.
UVLO
When the FS/SYNC pin is not pulled and held below 0.2V,
the voltage on the Vcc pin determines the operation of
the SC4605. 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 2.8V. If no faults are present, the SC4605
will initiate a soft start when Vcc exceeds 2.8V. A hyster-
esis (150mV) in the UVLO comparator provides noise
immunity during its start up.
Soft Start
The soft start function is required for step down control-
lers to prevent excess inrush current through the DC bus
during start up. Generally this can be done by sourcing a
controlled current into a timing capacitor and then using
the voltage across this capacitor to slowly ramp up the
error amp reference. The closed loop creates narrow
width driver pulses while the output voltage is low and
allows these pulses to increase to their steady state duty
cycle as the output voltage reaches its regulated value.
With this, the inrush current from the input side is con-
trolled. The duration of the soft start in the SC4605 is
controlled by an internal timing circuit which is used dur-
ing start up and over current to set the hiccup time. The
soft start time can be calculated by:
s
START
_
SOFT
f
720
T
=
As can be seen here, the soft start time is switching fre-
quency dependant. For example, if fs = 300kHz,
TSOFT_START = 720/300k = 2.4ms. But if fs = 600kHz,
TSOFT_START = 720/600k = 1.2ms.
The SC4605 implements its soft start by ramping up the
error amplifier reference voltage providing a controlled
slew rate of the output voltage, then preventing over-
shoot and limiting inrush current during its start up.
Over Current Protection
Over current protection for the SC4605 is implemented
by detecting the voltage drop of the high side N-MOSFET
during its 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 inac-
curate and is mainly used to protect the power supply
during a fault case. The over current trigger point will
vary from unit to unit as the RDS(ON) of N-MOSFET var-
ies. Even for the same unit, the over current trigger point
will vary as the junction temperature of N-MOSFET var-
ies. The SC4605 provides a built-in 50
µA current source,
which is combined with RSET (connected between VCC
and ISET) to determine the current limit threshold. The
value of RSET can be properly selected according to the
desired current limit point IMAX and the internal 50
µA
pull down current available on the ISET pin based on the
following expression:


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