LT1950IGN datasheet(11/20 Pages) LINER

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LT1950

sn1950 1950fs

APPLICATIO S I FOR ATIO

Shutdown and Undervoltage Lockout

Figure 4 shows how to program undervoltage lockout

(UVLO) for the VIN supply. 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, hence source

current increases as source voltage drops. This looks like

a negative load resistance to the source and can cause the

source to current limit or latch low under low source

voltage conditions. An internally set undervoltage lockout

(UVLO) threshold prevents the regulator from operating at

source voltages where these problems might occur. 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. Alternatively, UVLO threshold is adjustable. The

shutdown threshold voltage of the SHDN pin is 1.32V.

Forcing the SHDN pin below this 1.32V threshold causes

VREF to be disabled and stops switching at the GATE pin.

If the SHDN pin is left open circuit, a permanent 3

µA flows

out of the pin to ensure that the pin defaults high to allow

normal operation. Voltages above the 1.32V threshold

cause an extra 7

µA to be sourced out of the pin, providing

current hysteresis. This can be used to set voltage hyster-

esis of the UVLO threshold using the following equations:

132

+µ

–

(– .

)

VH = Turn on threshold

VL = Turn off threshold

Example: switching should not start until the input is

above 11V and is to stop if the input falls below 9V.

VH = 11V

VL = 9V

286

132

1 32

286

+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-

tance values are used, the SHDN pin should be bypassed

with a 1nF capacitor to prevent coupling problems from

the switch node.

Figure 4. Undervoltage Lockout

µA

1.32V

GND

VREF

VIN

1950 F04

LT1950

Generating VIN2 Supply Using BOOST Pin

The LT1950’s BOOST pin is used to provide a “boosted”

11V supply at the VIN2 pin for VIN voltages as low as 3V.

Since VIN2 supplies the output driver for the GATE pin of

the IC, it is advantageous to generate a boosted VIN2. This

architecture achieves high GATE drive for an external

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