POWER MANAGEMENT
Description
COMBI-SYNC Secondary Side
Synchronous Rectifier and Regulator
Features
Single controller performs synchronous rectification
and post regulation
Independent regulation of multiple outputs from a
common secondary winding
Turn on shoot through inherently eliminated
Primary switch turns ON and OFF with zero current
No synchronising or any other signals needed across
the isolation boundary
Eliminates the need for a secondary bias supply
Independent soft start, ON/OFF, remote sense and
current limit for each output
Resistive or inductive current sensing with 2V current
signal and 2.5V reference for external applications.
4.5 to 18V operation
Max operating frequency to 1 MHz
Low profile 16 pin TSSOP package
Based on Semtech’s patented Combi Sync
concept
SC4901
SC4901 is a unique secondary side regulator designed
for implementing Semtech’s proprietary Combi-Sync
topology for isolated convertors with multiple outputs. The
Combi-Sync is a true all-MOSFET topology that allows
synchronous rectification and post regulation of
transformer isolated outputs, resulting in higher efficiency
and low component count. Details of this unique topology
are described in the Application Information section.
Multiple outputs can be synchronously rectified and
independently regulated by individual SC4901s. Each
output has its own ON/OFF control, soft start, remote sense
and current limits. They can be turned on or off without
affecting the primary or other outputs. The secondary side
control makes it much easier to design tight control loops
and implement load current sharing and hot swap
features. All devices are synchronised to the transformer
winding. The current sense can be either from the output
inductor for high efficiency or a resistor for better accuracy.
An amplified current signal is provided for external use.
Each device is capable of driving high side MOSFETs with
2A current. The forward drive is configured for direct
connection to a 1:1 pulse transformer. SC4901 has an
undervoltage lockout with typical turn-on threshold of 4.5V
and is available in a low cost TSSOP-16 package
Applications
Telecom isolated DC to DC converters with multiple
low voltage outputs
High density brick and sub brick modules with
independently regulated multiple outputs
Distributed power architectures
Isolated VRMs
VO UT
R1
R2
R3
Typical Application Circuit
QS
QF
R4
R5
R8
R9
T2
T1
••
SIN GLE
EN DED
FORW ARD
CONVERTOR
L1
QR
R6
• •
C1
R7
C2
R10
C3
R TN
C6
8
7
6
5
C4
4
C5
3
2
1
D1
D2
XF RA
O UTB
CS-
PGND
SSEN
CS+
VC C
C AO
R11
R12
RZ
RT
R13
D3
R14
AG N D
U1
O UTA
P VC C
RE F
S C 4901
VE A-
ZC D
RC T
VE A
CAO
9
10
11
12
13
14
15
D4
CT
C9
C8
C7
C10
R15
Revision: May, 2005
United States Patent 6,788,554
1
16
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
