Datasheet

LT1074/LT1076
10
sn1074 1074fds
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
=
()
−
()
=
=
−
()()
− +
⎛
⎝
⎜
⎞
⎠
⎟
=
.
.
.
.
...
.
.
.
.