LT1376CS datasheet(14/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

regulator input voltage. Average forward current in normal

operation can be calculated from:

D AVG

OUT

( )=

−

()

This formula will not yield values higher than 1A with

maximum load current of 1.25A unless the ratio of input

to output voltage exceeds 5:1. The only reason to consider

a larger diode is the worst-case condition of a high input

voltage and

overloaded (not shorted) output. Under short-

circuit conditions, foldback current limit will reduce diode

current to less than 1A, but if the output is overloaded and

does not fall to less than 1/3 of nominal output voltage,

foldback will not take effect. With the overloaded condi-

tion, output current will increase to a typical value of 1.8A,

determined by peak switch current limit of 2A. With

VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 1.8A:

D AVG

( )=

−

()

1 8 15 4

132

This is safe for short periods of time, but it would be

prudent to check with the diode manufacturer if continu-

ous operation under these conditions must be tolerated.

BOOST PIN CONSIDERATIONS

For most applications, the boost components are a 0.1

µF

capacitor and a 1N914 or 1N4148 diode. The anode is

connected to the regulated output voltage and this gener-

ates a voltage across the boost capacitor nearly identical

to the regulated output. In certain applications, the anode

may instead be connected to the unregulated input volt-

age. This could be necessary if the regulated output

voltage is very low (< 3V) or if the input voltage is less than

6V. Efficiency is not affected by the capacitor value, but the

capacitor should have an ESR of less than 2

Ω to ensure

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 0.1uF 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 10mA +

IOUT/ 75. At peak load current of 1.25A, this gives a total

drain of 27mA. Capacitor ripple voltage is equal to the

product of on time and drain current divided by capacitor

value;

∆V = tON • 27mA/C. To keep capacitor ripple voltage

to less than 0.5V (a slightly arbitrary number) at the worst-

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

0.1

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:

mA I

MIN

OUT

()(

)

() −

()

f = Switching frequency

VOUT = Regulated output voltage

VIN = Minimum input voltage

This formula can yield capacitor values substantially less

than 0.1

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 temperature and

output overload.

SHUTDOWN FUNCTION AND UNDERVOLTAGE

LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1376. 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

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