Datasheet LT1074, LT1076 (Analog Devices) - 10

LT1074/LT1076
U U PI DESCRIPTIO S
VIN Hysteresis in undervoltage lockout may be accomplished by connecting a resistor (R3) from the ILIM pin to the 10 A µ 300 A µ shutdown pin as shown in Figure 7. D1 prevents the shutdown divider from altering current limit. SHUTDOWN – PIN ILIM C1 PIN V 2.3V + R1 IN SHUT Q1 D1* LT1074 6V EXTERNAL R3 CLIM I – LIM C2 R2 OPTIONAL CURRENT 0.3V + LIMIT RESISTOR LT1074•PD09 *1N4148 TO TOTAL REGULATOR SHUTDOWN
Figure 7. Adding Hysteresis
LT1074•PD07
Figure 5. Shutdown Circuitry
⎛ R ⎞ 1 Trip Point = V = . 2 3 V TP 5 1+
Undervoltage Lockout
⎝⎜ R2⎠⎟ Undervoltage lockout point is set by R1 and R2 in Figure 6. If R3 is added, the lower trip point (VIN descending) will be To avoid errors due to the 10µA shutdown pin current, R2 the same. The upper trip point (VUTP) will be: is usually set at 5k, and R1 is found from: ⎛ R1 R1⎞ ⎛ R1⎞ V = V 1 + 0. V 8 V ( −V ) UTP SH TP SH ⎝⎜ R2 R3⎠⎟ − ⎝⎜R3⎠⎟ R1= R2 VSH If R1 and R2 are chosen, R3 is given by: VTP = Desired undervoltage lockout voltage ( . )( ) V V − 0 V 8 R1 SH = Threshold for lockout on the SH R3 = shutdown pin = 2.45V ⎛ R1⎞ V − V 1+ If quiescent supply current is critical, R2 may be increased UTP SH⎝⎜ R2⎠⎟ up to 15kΩ, but the denominator in the formula for R2 should replace VSH with VSH – (10µA)(R2).
Example:
An undervoltage lockout is required such that the output will not start until VIN = 20V, but will continue to operate until VIN drops to 15V. Let R2 = 2.32k. R1 VIN SHUT LT1074 V 15 2 3 . V 5 ( ) R1= 2 3 . k 4 = 12. k 5 ( ) − R2 GND 2 3 . V 5 5k 2.35 ( −0.8 12.5 )( ) R3 = = 3. k 9 LT1074•PD08 ⎛ 12.5⎞
Figure 6. Undervoltage Lockout
20 − 2.35 1+ ⎝⎜ 2.32⎠⎟ sn1074 1074fds 10