LT1374CS8 datasheet(14/32 Pages) LINER

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LT1374

1374fb

APPLICATIONS INFORMATION

that it can be recharged fully under the worst-case condi-

tion of minimum input voltage. Almost any type of film or

ceramic capacitor will work fine.

WARNING! Peak voltage on the BOOST pin is the sum of

unregulated input voltage plus the voltage across the

boost capacitor. This normally means that peak BOOST

pin voltage is equal to input voltage plus output voltage,

but

when the boost diode is connected to the regulator

input, peak BOOST pin voltage is equal to twice the input

voltage. Be sure that BOOST pin voltage does not exceed

its maximum rating.

For nearly all applications, a 0.27

µF boost capacitor works

just fine, but for the curious, more details are provided

here. The size of the boost capacitor is determined by

switch drive current requirements. During switch on time,

drain current on the capacitor is approximately IOUT/ 50. At

peak load current of 4.25A, this gives a total drain of 85mA.

Capacitor ripple voltage is equal to the product of on time

and drain current divided by capacitor value;

∆V = (tON)(85mA/C). To keep capacitor ripple voltage to

less than 0.6V (a slightly arbitrary number) at the worst-

case condition of tON = 1.8µs, the capacitor needs to be

0.27

µF. Boost capacitor ripple voltage is not a critical

parameter, but if the minimum voltage across the capaci-

tor drops to less than 3V, the power switch may not

saturate fully and efficiency will drop. An

approximate

formula for absolute minimum capacitor value is:

MIN

OUT

= ()(

)

() −

()

f = Switching frequency

VOUT = Regulated output voltage

VIN = Minimum input voltage

This formula can yield capacitor values substantially less

than 0.27

µF, but it should be used with caution since it

does not take into account secondary factors such as

capacitor series resistance, capacitance shift with tem-

perature and output overload.

SHUTDOWN FUNCTION AND

UNDERVOLTAGE LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1374. 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.

–

2.38V

0.4V

GND

VSW

LT1374

INPUT

RFB

RHI

1374 F04

OUTPUT

SHDN

STANDBY

TOTAL

SHUTDOWN

3.5

µA

RLO

Figure 4. Undervoltage Lockout

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