SC4603 Datasheet, PDF (8/16 Pages) Semtech Corporation – Very Low Input, MHz Operation, High Efficiency Synchronous Buck

SC4603
POWER MANAGEMENT
Applications Information - (Cont.)
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 SC4603 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:
T_{SOFT _ START}
=
1020
f_S
As can be seen here, the soft start time is switching
frequency dependant. For example, if f_s = 600kHz,
T _ _{SOFT START} = 1020/600k = 1.7ms. But if f_s = 1MHz,
T _ _{SOFT START} = 1020/1M = 1.02ms.
The SC4603 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.
I_MAX and the internal 50µA pull down current available on
the I_SET pin based on the following expression:
R _SET
=
I_MAX • R_DS(ON)
50µA
Kelvin sensing connections should be used at the drain
and source of P-MOSFET.
The R_DS(ON) sensing used in the SC4603 has an addi-
tional feature that enhances the performance of the over
current protection. Because the R_DS(ON) has a positive
temperature coefficient, the 50µA current source has a
positive coefficient of about 0.3%/C° providing first or-
der correction for current sensing vs temperature. This
compensation depends on the high amount of thermal
transferring that typically exists between the high side P-
MOSFET and the SC4603 due to the compact layout of
the power supply.
When the converter detects an over current condition
(I > I_MAX) as shown in Figure 1, the first action the SC4603
takes is to enter cycle by cycle protection mode (Point B
to Point C), which responds to minor over current cases.
Then the output voltage is monitored. If the over current
and low output voltage (set at 70% of nominal output
voltage) occur at the same time, the Hiccup mode op-
eration (Point C to Point D) of the SC4603 is invoked
and the internal soft start capacitor is discharged. This is
like a typical soft start cycle.
Over Current Protection
Over current protection for the SC4603 is implemented
by detecting the voltage drop of the high side P-MOSFET
during conduction, also known as high side R_DS(ON) detec-
tion. This loss-less detection eliminates the sense resis-
tor and its loss. The overall efficiency is improved and
the number of components and cost of the converter
are reduced. R_DS(ON) sensing is by default inaccurate 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 R_DS(ON) of P-MOSFET varies. Even for the same
unit, the over current trigger point will vary as the junc-
tion temperature of P-MOSFET varies. The SC4603 pro-
vides a built-in 50µA current source, which is combined
with R_SET (connected between V_CC and I_SET) to determine
the current limit threshold. The value of R_SET can be prop-
erly selected according to the desired current limit point
A
V_{O −nom}
B
0.6807.57⋅V_O−nom
C
V_O
D
I_MA
I_O
Figure 1. Over current protection characteristic of
SC4603
 2004 Semtech Corp.
8
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