sc4508a Semtech Corporation, sc4508a Datasheet - Page 7

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

I
V
V
V
f
L - inductor
Error Amplifier
The error amplifier in the SC4508A is a transconductance
error amplifier, which is easily configured as a type two
compensator by connecting compensation network from
the COMP pin to AGND. The output voltage of the error
amplifier is compared to the sensed current signal
(amplified by gain 8) plus internal 500mV ramp to generate
duty cycle.
Both the non-inverting and the inverting inputs of the
error amplifier are brought out as device pins so that a
converter can be configured as either buck or buck-boost
converter.
Soft-Start and Overload Protection
The undervoltage lockout circuit discharges the SS/EN
capacitors. After V
capacitors are slowly charged by internal 10uA current
source. As the SS/EN capacitor continues to be charged,
the VREF and the COMP voltage follows. The converter
gradually delivers increasing power to the output. The
inductor current follows the COMP voltage envelope until
the output goes into regulation.
After the SS/EN capacitor is charged above 1.4V (high
enough for the error amplifier to provide full load current),
the overload detection circuit is activated. If the CS pin
senses 32 consective switching cycles of over current,
the SC4508A will shut down and hold off the MOSFET
while discharging the soft-start capacitor. The SS/EN
capacitor is discharged with an internal 12mA current
sink. The overload latch is reset when the SS/EN capacitor
is discharged below 0.5V. The SS/EN capacitor is then
recharged with the 10µA current source and the converter
Application Information (Cont.)
O
POWER MANAGEMENT
S
O
IN
D
- full load current
- switching frequency
2007 Semtech Corp.
- output voltage
- diode forward voltage drop
- input voltage
( I
for
L
pk
Buck
)
I
O
V
Boost
IN
V
V
IN
DD
O
rises above 2.5V, the SS/EN
V
D
2
1
f
s
V
IN
L
(
V
IN
V
O
V
O
V
D
V
D
)
7
undergoes soft-start. If overload persists, the SC4508A
will undergo repetitive auto-shutdown-restart hiccup
mode.
In normal operation, the VREF voltage follows the SS/
EN voltage from 0 to 0.5V as the SS/EN voltage ramps
up from 1.4V to 1.9V. After the SS/EN voltage rises
above 1.9V, it settles at final value depending upon VDD.
If VDD higher than 7V, it is clamped around 6.8V. In the
worst case, the clamped voltage is about 10V. Therefore,
the external soft-start capacitor should be rated at least
16V.
The SS/EN pin can also be used as the enable input. The
MOSFET will be turned off if the SS/EN pin is pulled below
0.5V.
Converter Specifications
Buck or buck-boost converter design includes the
following specifications:
Input voltage range:
Input voltage ripple (peak-to-peak): DV
Output voltage: V
Output voltage accuracy: e
Output voltage ripple (peak-to-peak): DV
Nominal output (load) current: I
Maximum output current limit: I
Output (load) current transient slew rate: dI
Circuit efficiency: h
Selection criteria and design procedures for the
following are described.
1) output inductor (L) type and value
2) output capacitor (C
3) input capacitor (C
4) power MOSFETs
5) current sensing and limiting circuit
6) voltage sensing circuit
7) loop compensation network
Inductor (L) and Ripple Current
The output inductor selection/design is based on the
output DC and transient requirements. Both output
current and voltage ripples are reduced with larger
inductors but it takes longer to change the inductor
current during load transients. Conversely smaller
inductors results in lower DC copper losses but the AC
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in
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SC4508A
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