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LT1578C

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1578/LT1578-2.5

APPLICATIONS INFORMATION

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 0.33µF 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 1.25A, this gives a total drain of 25mA.

Capacitor ripple voltage is equal to the product of on time

and drain current divided by capacitor value;

∆V = (tON)(25mA/C). To keep capacitor ripple voltage to

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

case condition of tON = 4.7µs, the capacitor needs to be

0.24µF. 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:

( ( )( )( ) ) CMIN =

IOUT / 50 VOUT / VIN

f VOUT − 3V

f = Switching frequency

VOUT = Regulated output voltage

VIN = Minimum input voltage

This formula can yield capacitor values substantially less

than 0.24µF, 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 LT1578. Typically, UVLO is used in situations where

the input supply is current limited, or has a relatively high

source resistance. It is particularly useful for input sup-

plies with foldback current limiting. 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 and latch under low source voltage

RFB

INPUT

RHI

C1

RLO

LT1578

IN

SHDN

2.42V

–

3.5µA

+

+

0.4V

–

GND

VSW

STANDBY

TOTAL

SHUTDOWN

OUTPUT

+

1578 F04

Figure 4. Undervoltage Lockout

14

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