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PDF SC4525C Data sheet ( Hoja de datos )
| Número de pieza | SC4525C | |
| Descripción | 28V 3A Step-Down Switching Regulator | |
| Fabricantes | Semtech Corporation | |
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SC4525C
28V 3A Step-Down Switching Regulator
POWER MANAGEMENT
Features
Wide input range: 3V to 28V
3A Output Current
200kHz to 2MHz Programmable Frequency
Precision 1V Feedback Voltage
Peak Current-Mode Control
Cycle-by-Cycle Current Limiting
Hiccup Overload Protection with Frequency Foldback
Soft-Start and Enable
Thermal Shutdown
Thermally Enhanced 8-pin SOIC Package
Fully RoHS and WEEE compliant
Applications
XDSL and Cable Modems
Set Top Boxes
Point of Load Applications
CPE Equipment
DSP Power Supplies
LCD and Plasma TVs
Automotive Car Audio
Description
The SC4525C is a constant frequency peak current-mode
step-down switching regulator capable of producing 3A
output current from an input ranging from 3V to 28V. The
switching frequency of the SC4525C can be programmed
up to 2MHz for component miniaturization or it can be
set at lower frequencies to accommodate high step-down
ratios. The SC4525C is suitable for next generation XDSL
modems, high-definition TVs and various point of load
applications.
Peak current-mode PWM control employed in the
SC4525C achieves fast transient response with simple loop
compensation. Cycle-by-cycle current limiting and hiccup
overload protection reduces power dissipation during
output overload. Soft-start function reduces input start-
up current and prevents the output from overshooting
during power-up.
The SC4525C is available in SOIC-8 EDP package.
Typical Application Circuit
SC4525A
VIN 10V– 28V
C4
4.7mF
IN
BST
SW
SS/EN SC4525C
COMP
ROSC
FB
GND
C7
22nF
C8
22pF
R7
12.7k
C5
2.2nF
R5
15.8k
D1
1N4148
C1
0.33mF
L1
5.2mH
R4
33.2k
OUT
5V/3A
D2
20BQ030
R6
8.25k
C2
10mFX3
L1: Coiltronics CD1- 5R2
C2: Murata GRM31CR60J106K
C4: Murata GRM32ER71H475K
Efficiency
90
80
VIN = 12V
VIN = 24V
70
60
50
40
0.0
0.5 1.0 1.5 2.0 2.5
Load Current (A)
3.0
Figure 1. 1MHz 10V -28V to 5V/3A Step-down Converter
Revision 5.1
© 2011 Semtech Corporation
Efficiency of the 1MHz 10V-28V to 5V/3A Step-Down Conve
1 page  
SC4525C
Pin Descriptions
SO-8
1
2
3
4
5
6
7
8
9
Pin Name Pin Function
SW
Emitter of the internal NPN power transistor. Connect this pin to the inductor, the freewheeling diode and the
bootstrap capacitor.
IN
ROSC
GND
SS/EN
COMP
Power supply to the regulator. It is also the collector of the internal NPN power transistor. It must be closely by-
passed to the ground plane.
An external resistor from this pin to ground sets the oscillator frequency.
Ground pin
Soft-start and regulator enable pin. A capacitor from this pin to ground provides soft-start and overload hiccup
functions. Hiccup can be disabled by overcoming the internal soft-start discharging current with an external pull-
up resistor connected between the SS/EN and the IN pins. Pulling the SS/EN pin below 0.2V completely shuts off
the regulator to low current state.
The output of the internal error amplifier. The voltage at this pin controls the peak switch current. A RC compensa-
tion network at this pin stabilizes the regulator.
FB
The inverting input of the error amplifier. If VFB falls below 0.8V, then the switching frequency will be reduced to
improve short-circuit robustness (see Applications Information for details).
BST
Exposed Pad
Supply pin to the power transistor driver. Tie to an external diode-capacitor bootstrap circuit to generate drive
voltage higher than VIN in order to fully enhance the internal NPN power transistor.
The exposed pad serves as a thermal contact to the circuit board. It is to be soldered to the ground plane of the
PC board.
5 Page 
SC4525C
Applications Information (Cont.)
To allow for transient headroom, the minimum operating
switch on time should be at least 20% to 30% higher than
the worst-case minimum on time.
200
An inductor ripple current between 20% to 50% of the
maximum load current, IO, gives a good compromise
among efficiency, cost and size. Re-arranging the previous
equation and assuming 35% inductor ripple current, the
inductor is given by
180 VO =1.5V, IO =1A, 1MHz
L
=
(VO + VD )x( − D)
35 % xIO xFSW
160
140
120
100
-50 -25
0 25 50 75
Temperature (OC)
100 125
Figure 4 — Variation of Minimum On Time
with Ambient Temperature
If the input voltage varies over a wide range, then choose
L1 based on the nominal input voltage. Always verify
converter operation at the input voltage extremes.
The peak current limit of SC4525C power transistor is at
least 3.9A. The maximum deliverable load current for the
SC4525C is 3.9A minus one half of the inductor ripple
current.
Input Decoupling Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a buck converter. This value is given by
IRMS_ CIN = IO x Dx ( − D)
Minimum Off Time Limitation
The PWM latch in Figure 2 is reset every cycle by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
switching frequency. The measured minimum off time is
100ns typically. If the required duty cycle is higher than
the attainable maximum, then the output voltage will not
be able to reach its set value in continuous-conduction
mode.
Inductor Selection
The inductor ripple current for a non-synchronous step-
down converter in continuous-conduction mode is
DIL
= (VO
+ VD )x( − D)
FSW x L
where FSW is the switching frequency and L1 is the
inductance.
The input capacitance must also be high enough to keep
input ripple voltage within specification. This is important
in reducing the conductive EMI from the regulator. The
input capacitance can be estimated from
CIN >
IO
4 x DVIN x FSW
where DVIN is the allowable input ripple voltage.
Multi-layer ceramic capacitors, which have very low ESR
(a few mW) and can easily handle high RMS ripple current,
are the ideal choice for input filtering. A single 4.7µF
X5R ceramic capacitor is adequate for 500kHz or higher
switching frequency applications, and 10µF is adequate
for 200kHz to 500kHz switching frequency. For high
voltage applications, a small ceramic (1µF or 2.2µF) can be
placed in parallel with a low ESR electrolytic capacitor to
satisfy both the ESR and bulk capacitance requirements.
11
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