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LT1776

TYPICAL APPLICATIONS

chosen to be significantly above the SHDN pin input

current to minimize its error contribution, but signifi-

cantly below the typical 3.8mA the LT1776 draws in

lockout mode. Resistor R4 is then chosen to yield this

same 200

µA, less 2.5µA, with the desired VIN UVLO

voltage minus 1.25V applied across it. (The 2.5

µA factor

is an allowance to minimize error due to SHDN pin input

current.)

Behavior is as follows: Normal operation is observed at the

nominal input voltage of 40V. As the input voltage is

decreased to roughly 32V, switching action will stop, VOUT

will drop to zero, and the LT1776 will draw its VIN and VCC

quiescent currents from the VIN supply. At a much lower

input voltage, typically 14V 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 pin

lockout threshold is relatively accurate includ-

ing temperature effects, the SHDN pin

shutdown thresh-

old is more coarse, and exhibits considerably more

temperature drift. Nevertheless the shutdown threshold

will always be well below the lockout threshold.)

Minimum Size Inductor Application

Figure 4a employs power path parts that are capable of

delivering the full rated output capability of the LT1776.

Potential users with low output current requirements may

be interested in substituting a physically smaller and less

costly power inductor. The circuit shown in Figure 6a is

topologically identical to the basic application, but speci-

fies a much smaller inductor. This circuit is capable of

delivering up to 400mA at 5V, or, up to 500mA at 3.3V. The

only disadvantage is that due to the increased resistance

in the inductor, the circuit is no longer capable of with-

standing indefinite short circuits to ground. The LT1776

will still current limit at its nominal ILIM value, but this will

overheat the inductor. Momentary short circuits of a few

seconds or less can still be tolerated. Typical efficiency is

shown in Figure 6b.

1776 F05

VIN

VOUT

158k

6.19k

VIN

VCC

VSW

LT1776

SHDN

SYNC

GND

Figure 5. User Programmable Undervoltage Lockout

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