sc4524c

Manufacturer Part Numbersc4524c
Description28v 2a Step-down Switching Regulator
ManufacturerSemtech Corporation
sc4524c datasheet
 


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Applications Information
Operation
The SC4524C is a constant-frequency, peak current-mode,
step-down switching regulator with an integrated 28V,
2.6A power NPN transistor. Programmable switching
frequency makes the regulator design more flexible. With
the peak current-mode control, the double reactive poles
of the output LC filter are reduced to a single real pole by
the inner current loop. This simplifies loop compensation
and achieves fast transient response with a simple Type-2
compensation network.
As shown in Figure 2, the switch collector current is
sensed with an integrated 5.5mW sense resistor. The
sensed current is summed with a slope-compensating
ramp before it is compared with the transconductance
error amplifier (EA) output. The PWM comparator trip
point determines the switch turn-on pulse width. The
current-limit comparator ILIM turns off the power switch
when the sensed signal exceeds the 18mV current-limit
threshold.
Driving the base of the power transistor above the
input power supply rail minimizes the power transistor
saturation voltage and maximizes efficiency. An external
bootstrap circuit (formed by the capacitor C
diode D
in Figure 1) generates such a voltage at the BST
1
pin for driving the power transistor.
Shutdown and Soft-Start
The SS/EN pin is a multiple-function pin. An external
capacitor (4.7nF to 22nF) connected from the SS pin to
ground sets the soft-start and overload shutoff times of
the regulator (Figure 3). The effect of V
is summarized in Table 1.
Table 1: SS/EN operation modes
SS/EN
SS/EN
Mode
Mode
<0.2V
<0.2V
Shutdown
Shutdown
0.4V to 1.2V
0.4V to 1.2V
Not switching
Not switching
1.2V to 2.15V
1.2V to 2.15V
Switching & hiccup disabled
Switching & hiccup disabled
>2.15V
>2.15V
Switching & hiccup armed
Switching & hiccup armed
Pulling the SS/EN pin below 0.2V shuts off the regulator
and reduces the input supply current to 18µA (V
When the SS/EN pin is released, the soft-start capacitor
is charged with an internal 1.9µA current source (not
shown in Figure 3). As the SS/EN voltage exceeds 0.4V,
the internal bias circuit of the SC4524C turns on and the
SC4524C draws 2mA from V
turns off and the 2.4µA current source I
charges the soft-start capacitor.
The error amplifier EA in Figure 2 has two non-inverting
inputs. The non-inverting input with the lower voltage
predominates. One of the non-inverting inputs is biased
to a precision 1V reference and the other non-inverting
input is tied to the output of the amplifier A
produces an output V
is forced low when V
the effective non-inverting input of EA stays at zero until
the soft-start capacitor is charged above 1.2V. Once V
exceeds 1.2V, COMP is released. The regulator starts to
EN
switch when V
is made sufficiently long, then the FB voltage (hence the
output voltage) will track V
at least 1.83V for the output to achieve regulation. Proper
soft-start prevents output overshoot. Current drawn from
the input supply is also well controlled.
and the
Overload / Short-Circuit Protection
1
Table 2 lists various fault conditions and their
corresponding protection schemes in the SC4524C.
Table 2: Fault conditions and protections
Condition
Condition
IL>ILimit, V
IL>ILimit, V
on the SC4524C
IL>ILimit, V
IL>ILimit, V
SS/EN
VSS/EN Falling
VSS/EN Falling
SS/EN<1.9V
SS/EN<1.9V
Tj>160C
Tj>160C
Supply Current
Supply Current
18uA @ 5Vin
18uA @ 5Vin
As summarized in Table 1, overload shutdown is disabled
2mA
2mA
during soft-start (V
Load dependent
Load dependent
of the overload latch B
voltage is below 2.15V. Once the soft-start capacitor is
charged above 2.15V, the output of the Schmitt trigger
= 5V).
B
goes high, the reset input of B
IN
1
SC4524C
. The 1.9µA charging current
IN
in Figure 3 slowly
C
1
= 2(V
-1.2V). V
is zero and COMP
1
SS/EN
1
is below 1.2V. During start up,
SS/EN
rises above 0.4V. If the soft-start interval
COMP
during start up. V
1
Fault
Fault
Protective Action
Protective Action
Cycle-by-cycle limit at
Cycle-by-cycle limit at
>0.8V
>0.8V
Over current
Over current
FB
FB
programmed frequency
programmed frequency
Cycle-by-cycle limit with
Cycle-by-cycle limit with
<0.8V
<0.8V
Over current
Over current
FB
FB
frequency foldback
frequency foldback
Persistent over current
Persistent over current
Shutdown, then retry
Shutdown, then retry
or short circuit
or short circuit
(Hiccup)
(Hiccup)
Over temperature
Over temperature
Shutdown
Shutdown
<2.15V). In Figure 3, the reset input
SS/EN
will remain high if the SS/EN
2
goes low and hiccup
2
. Amplifier A
1
SS/
must be
SS/EN
9