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LT1777

TYPICAL APPLICATIONS

Figure 14. Output Node Ripple

µs/DIV

(b) After Output Filter

µs/DIV

1777 F13a

(a) Before Output Filter

VOUT NODE

AC COUPLED

2mV/DIV

1777 F14b

User Programmable Undervoltage Lockout

Figure 15 uses a resistor divider between VIN and ground

to drive the SHDN node. This is a simple, cost-effective

way to add a user-programmable undervoltage lockout

(UVLO) function. Resistor R5 is chosen to have approxi-

mately 200

µA through it at the nominal SHDN pin lockout

threshold of roughly 1.25V. The somewhat arbitrary value

of 200

µA was chosen to be significantly above the SHDN

pin input current to minimize its error contribution, but

significantly below the typical 2.5mA the LT1777 draws in

lockout mode. Resistor R4 is then chosen to yield this

same 200

µA, less 2.5µA, with the desired VIN UVLO volt-

age minus 1.25V across it. (The 2.5mA factor is an allow-

ance to minimize error due to SHDN pin input current.)

Behavior is as follows: Normal operation is observed at the

nominal input voltage of 24V. As the input voltage is

decreased to roughly 18V, switching action will stop, VOUT

will drop to zero, and the LT1777 will draw its VIN and VCC

quiescent currents from the VIN supply. At a lower input

voltage, typically 10V or so at 25

°C, the voltage on the

SHDN pin will drop to the shutdown threshold, and the part

will draw its shutdown current only from the VIN rail. The

resistive divider of R4 and R5 will continue to draw power

from VIN. (The user should be aware that while the SHDN

lockout threshold is relatively accurate including

temperature effects, the SHDN pin shutdown threshold is

more coarse, and exhibits considerably more temperature

drift. Nevertheless the

shutdown threshold will always be

well below the lockout threshold.)

Figure 15. User Programmable UVLO

84.5k

6.19k

100pF

SHDN

LT1777

1777 F15

VIN

VOUT NODE

AC COUPLED

2mV/DIV

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