SC4521EVB-2 Semtech, SC4521EVB-2 Datasheet - Page 6

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SC4521EVB-2

Manufacturer Part Number
SC4521EVB-2
Description
EVALUATION BOARD
Manufacturer
Semtech
Datasheet

Specifications of SC4521EVB-2

Silicon Manufacturer
Semtech
Application Sub Type
Step Down Switching Regulator
Kit Application Type
Power Management - Voltage Regulator
Silicon Core Number
SC4521
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
The SC4521 is a current mode buck converter regulator.
The SC4521 uses two feedback loops that control the
duty cycle of the internal power switch. The error amplifier
functions like that of the voltage mode converter. The
output of the error amplifier works as a switch current
reference. This technique effectively removes one of the
double poles in the voltage mode system. With this, it is
much simpler to compensate a current mode converter
to have better performance.
Soft Start
Internally, connected to the SS pin is a 100K pull-up
resistor from an internal 1.6V regulator and the collector
of an NPN pull-down transistor from SS to GND. The NPN
transistor is “on” when the Enable pin is low or when a
fault is detected (Input UVLO, Over-Current, or Over
Temp). When the SC4521 is disabled or when a fault is
detected, the NPN transistor pulls the SS pin low. The
SS charging time is controlled by the internal 100k
resistor and external soft-start cap. This is a closed-loop
soft-start which effectively “ramps the reference”. The
SS process completes when the SS pin voltage is
exceeds the internal reference voltage, 0.8V. The SS pin
continues to charge up to 1.6V, well above the reference
voltage to ensure it does not interfere with normal
operation. The governing SS equation is:
For example, a 22nF SS cap would give a SS time of
approximately 1.5mS.
However, when V
requires a pull-up resistor from the SS pin to V I for normal
operation. The softstart time can be estimated as:
Where:
Rss = pull-up resistance from SS pin to V
Css = capacitance from SS pin to GND, and
V
Overcurrent Protection
The current sense amplifier in the SC4521 monitors the
switch current during each cycle. Overcurrent protection
(OCP) is triggered when the current limit exceeds the upper
limit of 3A, detected by a voltage on COMP greater than
POWER MANAGEMENT
Application Information
I
2007 Semtech Corp.
= input voltage.
T
SS
I
is higher than about 13V, the SC4521
T
R
SS
SS
C
69000
SS
[
Ln
C
1 (
SS
0
V
8 .
I
I
)]
,
6
about 2V. When an OCP fault is detected, the switch is
turned off and the external COMP capacitor is gradually
discharged at the rate of dv/dt=3
the total capacitance value attached to COMP. At the
same time, the soft start cap C
Once the COMP voltage has fallen below 250mV, the
part enters a normal startup cycle. In the case of
sustained overcurrent or dead-short, the part will
continually cycle through the retry sequence as described
above.
Enable
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4521 which reduces the input
supply current to 250µA. During the shut down mode,
the switch is turned off. The SC4521 is turned on if the
EN pin is pulled high.
Oscillator
Its internal free running oscillator sets the PWM frequency
at 600kHz for the SC4521 without any external
components to program the frequency.
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4521. As
VIN increases during power up, the internal circuit senses
VIN and keeps the power transistor off until VIN reaches
4.4V.
Load Current
The peak current I
For a specific application, the allowed load current I
will change if the input voltage drifts away from the original
design as given for continuous current mode:
Where:
fs = switching frequency,
Vo = output voltage and
D = duty ratio, V
V
I
= input voltage.
I
O
OMAX
/V
PEAK
I
in the switch is internally limited.
3
V
O
2
1 (
L
SS
f
s
D
is quickly discharged.
µA/Ccomp
)
www.semtech.com
SC4521
. Ccomp is
OMAX

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