Datasheet

LT1074/LT1076
10
1074fd
Figure 5. Shutdown Circuitry
Figure 6. Undervoltage Lockout
Figure 7. Adding Hysteresis
DESCRIPTIO S
U
PI
U
Undervoltage Lockout
Undervoltage lockout point is set by R1 and R2 in Figure␣ 6.
To avoid errors due to the 10µA shutdown pin current, R2
is usually set at 5k, and R1 is found from:
RR
VV
V
TP SH
SH
12=
−
()
V
TP
= Desired undervoltage lockout voltage
V
SH
= Threshold for lockout on the
shutdown pin = 2.45V
If quiescent supply current is critical, R2 may be increased
up to 15kΩ, but the denominator in the formula for R2
should replace V
SH
with V
SH
– (10µA)(R2).
–
+
C1
10 Aµ
300 Aµ
–
+
SHUTDOWN
PIN
C2
V
IN
I
PIN
6V
Q1
TO TOTAL
REGULATOR
SHUTDOWN
2.3V
0.3V
LT1074•PD07
LIM
EXTERNAL
C
LIM
Hysteresis in undervoltage lockout may be accomplished
by connecting a resistor (R3) from the I
LIM
pin to the
shutdown pin as shown in Figure 7. D1 prevents the
shutdown divider from altering current limit.
LT1074•PD08
R1
R2
5k
V
SHUT
GND
IN
LT1074
LT1074•PD09
R1
R2
R3
V
SHUT
I
LIM
IN
LT1074
OPTIONAL CURRENT
LIMIT RESISTOR
D1*
*1N4148
Trip Po V V
R
R
TP
int .== +






235 1
1
2
If R3 is added, the lower trip point (V
IN
descending) will be
the same. The upper trip point (V
UTP
) will be:
VV
R
R
R
R
V
R
R
UTP SH
=+






−






1
1
2
1
3
08
1
3
.
If R1 and R2 are chosen, R3 is given by:
R
VVR
VV
R
R
SH
UTP SH
3
08 1
1
1
2
=
−
()()
−+






.
Example: An undervoltage lockout is required such that
the output will not start until V
IN
= 20V, but will continue
to operate until V
IN
drops to 15V. Let R2 = 2.32k.
Rk
VV
V
k
Rk
1234
15 2 35
235
12 5
3
235 08 125
20 2 35 1
12 5
232
39
=
()
−
()
=
=
−
()()
−+






=
.
.
.
.
...
.
.
.
.