sc4524c Semtech Corporation, sc4524c Datasheet - Page 9

• Manufacturer Part Number: sc4524c
• Description: 28v 2a Step-down Switching Regulator
• Manufacturer: Semtech Corporation

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
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.
Pulling the SS/EN pin below 0.2V shuts off the regulator
and reduces the input supply current to 18µA (V
1.2V to 2.15V
1.2V to 2.15V
0.4V to 1.2V
0.4V to 1.2V
>2.15V
>2.15V
SS/EN
SS/EN
<0.2V
<0.2V
1
Table 1: SS/EN operation modes
in Figure 1) generates such a voltage at the BST
Switching & hiccup disabled
Switching & hiccup disabled
Switching & hiccup armed
Switching & hiccup armed
Not switching
Not switching
Shutdown
Shutdown
Mode
Mode
SS/EN
on the SC4524C
Supply Current
Supply Current
Load dependent
Load dependent
18uA @ 5Vin
18uA @ 5Vin
2mA
2mA
1
and the
IN
= 5V).
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
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.
Overload / Short-Circuit Protection
Table 2 lists various fault conditions and their
corresponding protection schemes in the SC4524C.
As summarized in Table 1, overload shutdown is disabled
during soft-start (V
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
B
1
IL>ILimit, V
IL>ILimit, V
IL>ILimit, V
IL>ILimit, V
exceeds 1.2V, COMP is released. The regulator starts to
goes high, the reset input of B
VSS/EN Falling
VSS/EN Falling
SS/EN<1.9V
SS/EN<1.9V
Condition
Condition
Tj>160C
Tj>160C
Table 2: Fault conditions and protections
FB
FB
FB
FB
>0.8V
>0.8V
<0.8V
<0.8V
COMP
Persistent over current
Persistent over current
rises above 0.4V. If the soft-start interval
SS/EN
Over temperature
Over temperature
or short circuit
or short circuit
1
SS/EN
Over current
Over current
Over current
Over current
= 2(V
<2.15V). In Figure 3, the reset input
Fault
Fault
2
is below 1.2V. During start up,
will remain high if the SS/EN
1
SS/EN
IN
during start up. V
. The 1.9µA charging current
-1.2V). V
2
goes low and hiccup
Cycle-by-cycle limit with
Cycle-by-cycle limit with
programmed frequency
programmed frequency
Cycle-by-cycle limit at
Cycle-by-cycle limit at
Shutdown, then retry
Shutdown, then retry
C
1
frequency foldback
frequency foldback
Protective Action
Protective Action
in Figure 3 slowly
is zero and COMP
SC4524C
Shutdown
Shutdown
(Hiccup)
(Hiccup)
1
. Amplifier A
SS/EN
must be
SS/
9
1