LT1500CS datasheet(13/16 Pages) LINER

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LT1500/LT1501

APPLICATIONS INFORMATION

and it should be no more than 300k to keep bias current

errors under 1%, giving:

DIV

BAT

124

31 24

124

()

–.

VBAT = low battery voltage

RDIV = Thevenin divider resistance = R3 in parallel with R4

There is about 20mV of hysteresis at the LBI pin. Hyster-

esis can be increased by adding a resistor (R5) from the

output (LBO) back to LBI. This resistor can be calculated

from the following equation, but note that the equation for

R4 will have to be changed when R5 is added.

VmV V

HYST

BAT

()

(

)

–

VCC = supply voltage for LBO pull-up resistor

VHYST = desired hysteresis at the battery

R4 (When R5 is Used) =

R3 R5

()( )

−

()+−

()

124

1 24

R V

BAT

The LBO pin is open collector. The external pull-up resistor

value is determined by user needs. Generally the resistor

is 100k to 1M to keep current drain low, but the LBO pin

can sink several milliamperes if needed.

Example: low battery voltage = 2.5V, desired hysteresis =

200mV, VCC = 5V.

Use RDIV = 150k

150 2 5

124

302

301 5

0017 25

()

.– .

k (use 301k, 1%)

.56M (Use 10M)

301 10

1 24

2 5 1 24

301 5 1 24

()( )( )

−

()+−

()

272k (Use 274k 1%)

The total divider resistance will be 274k + 301k = 575k, and

this will draw about 7

µA from a fully charged battery.

Synchronizing

The SYNC pin on the LT1500 can be used to synchronize

switching frequency to an external clock. The pin should

be driven with a 50ns to 300ns pulse which will trigger the

switch to an on state. There is a fairly restricted range over

which synchronizing will work, because the period between

sync pulses must be greater than the natural on-time of

the regulator when it is running unsynchronized, and the

sync frequency must be greater than the unsynchronized

switching frequency. This puts the following restrictions

on synchronized operation:

SYNC

NATURAL

SYNC

NATURAL

OUT

()

–

(Use Minimum V )

fNATURAL is the natural unsynchronized switching fre-

quency of the regulator. It is a function of load current, so

a careful check must be done to ensure that the above

conditions are met under all load and input voltage condi-

tions.

Soft Start (SS)

The LT1500 can be soft started by connecting a capacitor

to the SS pin. This pin is the base of a PNP transistor

whose emitter is tied to the VC pin. Soft start action will

occur over the range of 0V to 0.8V on the SS pin and the

pin is clamped at 1.2V with an internal clamp. An internal

µA pull-up current and the external capacitor value

determine soft start time. In a typical application a 0.22

µF

capacitor is sufficient to limit input surges and prevent

output overshoot, even with overcompensation on the VC

pin. Output voltages greater than 6V with very large output

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