SC4605 データシートの表示（PDF） - Semtech Corporation
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SC4605 Datasheet PDF : 19 Pages
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SC4605
POWER MANAGEMENT
Applications Information
Enable
Soft Start
Pulling and holding the FS/SYNC pin below 0.2V initial- The soft start function is required for step down control-
izes the SLEEP mode of the SC4605 with its typical SLEEP lers to prevent excess inrush current through the DC bus
mode supply current of 10µA. During the SLEEP mode, during start up. Generally this can be done by sourcing a
the high side and low side MOSFETs are turned off and controlled current into a timing capacitor and then using
the internal soft start voltage is held low.
the voltage across this capacitor to slowly ramp up the
error amp reference. The closed loop creates narrow
Oscillator
width driver pulses while the output voltage is low and
allows these pulses to increase to their steady state duty
The oscillator uses an external capacitor between FS and cycle as the output voltage reaches its regulated value.
GND to set the oscillation frequency. The ramp wave- With this, the inrush current from the input side is con-
form is a triangle at the PWM frequency with a peak volt- trolled. The duration of the soft start in the SC4605 is
age of 2V and a valley voltage of 0.5V. The PWM duty controlled by an internal timing circuit which is used dur-
ratio is limited to a maximum of 97%, which allows the ing start up and over current to set the hiccup time. The
bootstrap capacitor to be charged during each cycle. The soft start time can be calculated by:
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:
fs
=
1.55 ⋅10−4
CT
TSOFT _ START
=
720
fs
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.
In its synchronous mode, a low value resistor needs to
be connected between ground and the frequency set- The SC4605 implements its soft start by ramping up the
ting capacitor, CT. Then an external clock connects to error amplifier reference voltage providing a controlled
the junction of the resistor and the capacitor to activate slew rate of the output voltage, then preventing over-
its synchronous mode. The frequency of the clock can shoot and limiting inrush current during its start up.
be used up to 700kHz. This external clock signal should
have a duty cycle from 5% to 10% and the peak voltage Over Current Protection
at the junction from the clock signal should be about
0.2V.
Over current protection for the SC4605 is implemented
by detecting the voltage drop of the high side N-MOSFET
UVLO
during its conduction, also known as high side RDS(ON)
detection. This loss-less detection eliminates the sense
When the FS/SYNC pin is not pulled and held below 0.2V, resistor and its loss. The overall efficiency is improved
the voltage on the Vcc pin determines the operation of and the number of components and cost of the con-
the SC4605. As Vcc increases during start up, the UVLO verter are reduced. RDS(ON) sensing is by default inac-
block senses Vcc and keeps the high side and low side curate and is mainly used to protect the power supply
MOSFETs off and the internal soft start voltage low until during a fault case. The over current trigger point will
Vcc reaches 2.8V. If no faults are present, the SC4605 vary from unit to unit as the RDS(ON) of N-MOSFET var-
will initiate a soft start when Vcc exceeds 2.8V. A hyster- ies. Even for the same unit, the over current trigger point
esis (150mV) in the UVLO comparator provides noise will vary as the junction temperature of N-MOSFET var-
immunity during its start up.
ies. The SC4605 provides a built-in 50µA 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 50µA
pull down current available on the ISET pin based on the
following expression:
2004 Semtech Corp.
6
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