LT1074I Datasheet, PDF (10/16 Pages) Linear Technology – Step-Down Switching Regulator

LT1074/LT1076
PI DESCRIPTIO S
10µ A
SHUTDOWN
PIN
–
C1
2.3V +
–
C2
0.3V +
V_IN
300 µA
I_LIM
PIN
Q1
6V
TO TOTAL
REGULATOR
SHUTDOWN
LT1074•PD07
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.
EXTERNAL
C_LIM
R1
V_IN
SHUT
R3 ^D1*
LT1074
I _LIM
R2
OPTIONAL CURRENT
LIMIT RESISTOR
*1N4148
LT1074•PD09
Figure 7. Adding Hysteresis
Figure 5. Shutdown Circuitry
Undervoltage Lockout
Trip
Po int
=
V_TP
=
⎛
2.35V _⎝⎜1+
R1^⎞
R2⎠⎟
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:
( ) ^VTP ^{− V}SH
R1= R2
V_SH
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).
R1
V_IN
SHUT
LT1074
R2
5k
GND
LT1074•PD08
Figure 6. Undervoltage Lockout
If R3 is added, the lower trip point (V_IN descending) will be
the same. The upper trip point (V_UTP) will be:
V_UTP
=
V_SH^{⎛⎝⎜}1RR21
+
R1^⎞
R3⎠⎟
−
⎛
0.8V_⎝⎜
R1^⎞
R3⎠⎟
If R1 and R2 are chosen, R3 is given by:
( )( ) ^VSH ^{− 0.8V R1}
R3 =
V_UTP
−
⎛
V_{SH⎝⎜}1+
R1^⎞
R2⎠⎟
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.
( ) ^{( ) 15V − 2.35V}
R1= 2.34k
= 12.5k
2.35V
( )( ) ^{2.35 − 0.8 12.5}
R3 =
= 3.9k
20
−
⎛
2.35_⎝⎜1+
12.5^⎞
2.32⎠⎟
10
sn1074 1074fds