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LT3436

3436fa

APPLICATIONS INFORMATION

shutdown pin can be used. The threshold voltage of the

shutdown pin comparator is 1.35V. A 3

µA internal current

source defaults the open pin condition to be operating (see

Typical Performance Graphs). Current hysteresis is added

above the SHDN threshold. This can be used to set voltage

hysteresis of the UVLO using the following:

135

−

()

+µ

VH – Turn-on threshold

VL – Turn-off threshold

Example: switching should not start until the input is

above 4.75V and is to stop if the input falls below 3.75V.

VH = 4.75V

VL = 3.75V

475

3 75

143

135

475

1 35

143

50 4

−

()

+µ

Keep the connections from the resistors to the SHDN pin

short and make sure that the interplane or surface capaci-

tance to the switching nodes are minimized. If high resis-

tor values are used, the SHDN pin should be bypassed with

a 1nF capacitor to prevent coupling problems from the

switch node.

CATCH DIODE

The suggested catch diode (D1) is a B220A Schottky. It is

rated at 2A average forward current and 20V reverse

voltage. Typical forward voltage is 0.5V at 2A. The diode

conducts current only during switch off time. Peak reverse

voltage is equal to regulator output voltage. Average

forward current in normal operation is equal to output

current.

SHUTDOWN AND UNDERVOLTAGE LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT3436. Typically, UVLO is used in situations where

the input supply is

current limited, or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so source current increases as

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. UVLO

prevents the regulator from operating at source voltages

where these problems might occur.

Figure 4. Undervoltage Lockout

1.35V

GND

INPUT

3436 F04

SHDN

VCC

LT3436

µA

An internal comparator will force the part into shutdown

below the minimum VIN of 2.6V. This feature can be used

to prevent excessive discharge of battery-operated sys-

tems. If an adjustable UVLO threshold is required, the

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