700mA SmartLDO
TM
SC1537
PRELIMINARY - July 25, 2000
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
DESCRIPTION
The SC1537 is designed to maintain a glitch-free 3.3V
output when at least one of two inputs, 5V (VIN) and
3.3V (VAUX), is present.
Whenever VIN exceeds a predetermined threshold
value, the internal 3.3V linear regulator is enabled, and
DR is pulled high.
When VIN falls below a lower threshold value, DR is
pulled low and the internal linear regulator is turned off.
DR has been designed to drive the gate of an external
low threshold P-channel MOSFET, which can be used
to connect the 3.3V supply directly to the regulator
output. This ensures an uninterrupted 3.3V output even
if VIN falls out of specification. A maximum R
DS(ON)
of
100m鈩?is recommended.
When both supplies are simultaneously available, the
drive pin (DR) will be pulled High, turning off the exter-
nal PMOS switch.
The internal 5V detector has its upper threshold (for
VIN rising) set to 4.22V (typical) while the lower thresh-
old (for VIN falling) is at 4.05V (typical) giving a hystere-
sis of approximately 170mV.
The SC1537 is available in the popular SO-8 and
5-lead TO-263 surface mount packages.
Q1
FEATURES
鈥?
鈥?
鈥?
鈥?
鈥?
鈥?
鈥?
Glitch-free transition between input sources
Internal logic selects input source
Gate drive for external PMOS bypass switch
5V detector with hysteresis
1% regulated output voltage accuracy
700mA load current capability
SO-8 and TO-263 packages
APPLICATIONS
鈥?
鈥?
鈥?
鈥?
鈥?
Desktop Computers
Network Interface Cards (NICs)
PCMCIA/PCI Interface Cards
Cardbus
TM
Technology
Power supplies with multiple input sources
ORDERING INFORMATION
Part Number
(1)
Package
SO-8
TO-263-5L
SC1537CSTR
SC1537CMTR
Note:
(1) Only available in tape and reel packaging. A reel con-
tains 2500 devices for SO-8 and 800 devices for TO-263.
APPLICATION CIRCUIT
3.3V OUT
U1 SC1537CS
3.3V
5V
1
2
3
4
VAUX
VIN
VO
DR
GND
GND
GND
GND
8
7
6
5
C4
10uF
C5
0.1uF
C1
0.1uF
C2
10uF
C3
0.1uF
Q1
U1
5V
2
VIN
TAB
SC1537CM
5
DR
3.3V
1
VAUX GND
3
VO
4
3.3V OUT
C1
0.1uF
C2
10uF
C3
0.1uF
C4
10uF
C5
0.1uF
NOTE:
(1) External switch (Q1): use a low threshold P-channel MOSFET such as Vishay Si2305DS (R
DS(ON)
< 71m鈩?at
V
GS
= 2.5V) or International Rectifier IRLML6401 (R
DS(ON)
< 85m鈩?at V
GS
= 2.5V), or equivalent.
1
漏 2000 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
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