LT1576CS8 datasheet(15/28 Pages) LINER

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LT1576/LT1576-5

APPLICATIONS INFORMATION

Threshold voltage for lockout is about 2.44V. A 3.5

µA bias

current flows

out of the pin at threshold. This internally

generated current is used to force a default high state on

the shutdown pin if the pin is left open. When low shut-

down current is not an issue, the error due to this current

can be minimized by making RLO 10k or less. If shutdown

current is an issue, RLO can be raised to 100k, but the error

due to initial bias current and changes with temperature

should be considered.

()

−

()

−

()

244

3 5

to 100k 25k suggested

.. µ

VIN = Minimum input voltage

Keep the connections from the resistors to the shutdown

pin short and make sure that interplane or surface capaci-

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

tor values are used, the shutdown pin should be bypassed

with a 1000pF capacitor to prevent coupling problems

from the switch node. If hysteresis is desired in the

undervoltage lockout point, a resistor RFB can be added to

the output node. Resistor values can be calculated from:

–

2.44V

0.4V

GND

VSW

LT1576

INPUT

RFB

RHI

1576 F04

OUTPUT

SHDN

STANDBY

TOTAL

SHUTDOWN

3.5

µA

RLO

Figure 4. Undervoltage Lockout

V V

LO IN

OUT

−+

()+

[]

−

()

()(

)

2 44

244

3 5

∆∆

∆

25k suggested for RLO

VIN = Input voltage at which switching stops as input

voltage descends to trip level

∆V = Hysteresis in input voltage level

Example: output voltage is 5V, switching is to stop if input

voltage drops below 12V and should not restart unless

input rises back to 13.5V.

∆V is therefore 1.5V and

VIN = 12V. Let RLO = 25k.

−+

()+

[]

−

()

()=

25 12 2 44 15 5 1

244

25 3 5

25 10 33

235

110

110 5 1 5

366

.. /

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