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70mΩ Protected Load Management Switch – Design Note 117
Mitchell Lee and Tim Skovmand
In an effort to conserve energy, simple shutdown schemes
are incorporated into many battery-operated circuits. Not
all circuits lend themselves to direct control, however, and
instead the supply must be turned off by a switch. The
LTC ®1477 high side switch is designed for this purpose
and includes short-circuit current limit and thermal shutdown to guard against faulty loads. Figure 1 shows a
simplified block diagram of the LTC1477.
VIN
V+
D1
D2
D3
CHARGE
PUMP
ENABLE
LTC1477
S2
VOUT
Figure 2 shows the LTC1477 and LTC699 conjoined in an
undervoltage disconnect application. The LTC699 microprocessor supervisor disables the LTC1477 and hence the
load whenever the input voltage falls below 4.65V. An
external logic signal applied to the gate of Q1 can also
disable the LTC1477. When enabled, the LTC1477 output
ramps over a period of approximately 1ms, thereby limiting
the peak current in the load capacitor to 500mA. This
prevents glitches on the 5V source line that might otherwise affect adjacent loads.
An LT ®1301 is used in Figure 3 to boost a 3.3V or 5V input
to 12V, such as VPP for Flash memory. Although the
LT1301 features a shutdown control, the input supply can
still feed through to the output through L1 and D1. Similarly, a short circuit on the output could drag down the input
supply. With the addition of the LTC1477 the circuit furnishes 100% load shutdown and output short-circuit protection.
THERMAL SHUTDOWN/
SHORT-CIRCUIT
CURRENT LIMIT
S1
At the heart of the LTC1477 is a 70mΩ N-channel MOSFET.
The split drains allow for selection of 0.85A, 1.5A or 2A
current limit. While enabled, the LTC1477 draws about
100µA quiescent current, dropping to 10nA in its disabled
state.
DN117 • F01
Figure 1. LTC1477 Block Diagram
, LTC and LT are registered trademarks of Linear Technology Corporation.
5V
1
C1
1µF
2
R1
510k
NC
6
7
LTC699CS8
DISABLE
4
5
8
VOUT
VOUT
VIN1
VIN2
ISC = 2A
CLOAD
100µF
LTC1477CS8
Q1
2N7002
3
+
VINS
EN
VIN3
GND
DN117 • F02
Figure 2. Switched 5V Line with Undervoltage Lockout and Current Limiting
11/95/117
Low voltage cutoff is desirable in battery-operated systems
to prevent deep discharge damage to the battery. The
LT1304 micropower boost regulator (Figure 4) contains a
low-battery detector which is active even when the regulator is shut down. The output of the detector controls both
the LTC1477 and the LT1304 5V boost regulator. In this
application the LTC1477 serves to protect against short
circuits (850mA limit selected) and completely disconnects the load under a low-battery condition. In shutdown,
the circuit draws less than 25µA from the battery.
A dual version, the LTC1478 is available in a 16-pin narrow
body SO package. This device is well suited for dual voltage
(5V/3.3V) switching applications.
D1
MBRS130LT3
L1*
10µH
+
3.3V
OR 5V
0.1µF
VOUT
VOUT
VIN1
VIN2
100µF
10V
2
6
7
VIN
SW
SEL
SENSE
VIN3
EN
ON/OFF
4
+
LT1301
LTC1477
VINS
12V
120mA
3
SHDN
ILIM
5
NC
GND
PGND
GND
47µF
16V
TANT
0.1µF
8
1
*COILCRAFT DO1608-103
DN117 • F03
Figure 3. Short-Circuit Protection and 100% Shutdown for a Micropower Boost Regulator
L1*
22µH
(2.7V TO 4.2V)
+
100µF
10V
562k
1%
220k
NC
SINGLE**
Li-Ion
CELL
6
1
432k
1%
7
MBRS130LT3
(5V)
3
4
VIN
SW
ILIM
SENSE
LT1304CS8-5
+
VOUT
8
+
LBI
SHDN
GND
LBO
2
100µF
16V
TANT
1µF
VOUT
VIN1
5V
300mA
VIN2
NC
VIN3
NC
LTC1477
VINS
EN
5
GND
DN117 • F04
*SUMIDA CD54-220
**PRIMARY Li-Ion BATTERY PROTECTION MUST BE PROVIDED BY AN INDEPENDENT CIRCUIT
Figure 4. Situated Downstream, the LTC1477 Is Controlled by the LT1304’s Low-Battery Detector
For literature on our High Side Switches,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 525
Linear Technology Corporation
LT/GP 1095 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1995