SC4520 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4520' PDF : 12 Pages View PDF
SC4520
POWER MANAGEMENT
Application Information
The SC4520 is a current mode buck converter regulator.
SC4520 has an internal fixed-frequency clock. The
SC4520 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. The current sense amplifier
in the SC4520 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 about 2V. When an OCP
fault is detected, the switch is turned off and the external
COMP capacitor is discharged at the rate of dv/dt = 3µA/
C . Once the COMP voltage has fallen below 250mV,
comp
the part enters a normal startup cycle. Ccomp is the total
capacitance value attached to COMP. In the case of
sustained overcurrent or dead-short, the part will
continually cycle through the retry sequence as described
above, at a rate dependent on the value of Ccomp. During
start up, the voltage on COMP rises roughly at the rate of
dv/dt = 120µA/Ccomp. Therefore, the retry time for a
sustained overcurrent can be approximately calculated
as:
Tretry
=
Ccomp
•
2V
120uA
+ Ccomp
•
2V
3uA
Figure 1 shows the voltage on COMP during a sustained
overcurrent condition.
Oscillator
The external resistor connected to the FSET pin sets the
PWM frequency from 100kHz to 600kHz.
Frequency vs ROSC
700
600
500
400
300
200
100
0
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
Rosc - kOhm
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4520. As
VIN increases during power up, the internal circuit senses
V and keeps the power transistor off until V reaches
IN
IN
4.4V.
Load Current
The peak current IPEAK in the switch is internally limited.
For a specific application, the allowed load current IOMAX
will change if the input voltage drifts away from the original
design as given for continuous current mode:
IOMAX
=
3−
VO ⋅ (1− D)
2 ⋅ L ⋅ fs
Where:
f = switching frequency,
S
VO= output voltage and
D = duty ratio, VO / VIN
VIN= input voltage.
Figure 1. Voltage on COMP for Startup and OCP
Enable
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4520 which reduces the input
supply current to less than 150µA. During the shut down
mode, the switch is turned off. The SC4520 is turned on
if the EN pin is pulled high.
2006 Semtech Corp.
6
www.semtech.com
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