SC4520SETRT Semtech, SC4520SETRT Datasheet - Page 6

• Manufacturer Part Number: SC4520SETRT
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,SOP,8PIN,PLASTIC
• Manufacturer: Semtech

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 = 3
C
the part enters a normal startup cycle. C
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 = 120
sustained overcurrent can be approximately calculated
as:
Figure 1 shows the voltage on COMP during a sustained
overcurrent condition.
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 150
mode, the switch is turned off. The SC4520 is turned on
if the EN pin is pulled high.
POWER MANAGEMENT
Application Information
comp
2006 Semtech Corp.
. Once the COMP voltage has fallen below 250mV,
Figure 1. Voltage on COMP for Startup and OCP
µ
A/C
T
comp
retry
. Therefore, the retry time for a
C
comp
120uA
µ
A. During the shut down
2V
C
comp
comp
is the total
3
2
uA
V
µ
A/
6
Oscillator
The external resistor connected to the FSET pin sets the
PWM frequency from 100kHz to 600kHz.
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4520. As
V
V
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:
f
V
D = duty ratio, V
V
S
IN
IN
O
IN
= output voltage and
= switching frequency,
= input voltage.
increases during power up, the internal circuit senses
and keeps the power transistor off until V
700
600
500
400
300
200
100
0
0
50
100
I
O
OMAX
150
/ V
PEAK
200
IN
Frequency vs R
in the switch is internally limited.
3
250
300
V
R
osc
O
2
350
- kOhm
1 (
L
OSC
400
f
s
D
450
)
500
550
SC4520
www.semtech.com
600
650
IN
reaches
700
OMAX