LT1074 Linear Technology, LT1074 Datasheet - Page 10

• Manufacturer Part Number: LT1074
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology

SHUTDOWN
LT1074/LT1076
PI
Undervoltage Lockout
Undervoltage lockout point is set by R1 and R2 in Figure 6.
To avoid errors due to the 10 A shutdown pin current, R2
is usually set at 5k, and R1 is found from:
If quiescent supply current is critical, R2 may be increased
up to 15k , but the denominator in the formula for R2
should replace V
10
V
V
R
U
TP
SH
1
PIN
DESCRIPTIO S
= Desired undervoltage lockout voltage
10 A
= Threshold for lockout on the
R
shutdown pin = 2.45V
2
V
TP
2.3V
0.3V
Figure 6. Undervoltage Lockout
V
Figure 5. Shutdown Circuitry
R1
R2
SH
SH
5k
V
SH
with V
–
+
–
+
C1
C2
SH
U
– (10 A)(R2).
300 A
SHUT
LT1074
GND
V
IN
Q1
TO TOTAL
REGULATOR
SHUTDOWN
LT1074•PD08
V
LT1074•PD07
IN
I
PIN
6V
LIM
EXTERNAL
C
LIM
Hysteresis in undervoltage lockout may be accomplished
by connecting a resistor (R3) from the I
shutdown pin as shown in Figure 7. D1 prevents the
shutdown divider from altering current limit.
If R3 is added, the lower trip point (V
the same. The upper trip point (V
If R1 and R2 are chosen, R3 is given by:
Example: An undervoltage lockout is required such that
the output will not start until V
to operate until V
R
R
R
Trip Po
V
UTP
3
1
3
V
20 2 35 1
2 34
UTP
.
*1N4148
int
2 35 0 8 12 5
V
V
SH
.
SH
k
R1
R2
.
1
V
Figure 7. Adding Hysteresis
V
R3
15
R
IN
TP
R
SH
0 8
2
.
1
.
drops to 15V. Let R2 = 2.32k.
V
2 35
V R
1
.
D1*
. 2 35
R
R
12 5
2 32
2 35
3
.
1
R
.
R
V
.
1
.
2
1
V
SHUT
I
V
LIM
0 8
1
IN
.
LT1074
OPTIONAL CURRENT
LIMIT RESISTOR
3 9
= 20V, but will continue
V
UTP
.
12 5
V
R
R
IN
2
IN
1
k
R
R
.
) will be:
descending) will be
3
1
k
LT1074•PD09
LIM
pin to the