LT1576 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1576

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1576/LT1576-5

APPLICATIONS INFORMATION

Ripple Current in the Input and Output Capacitors

Positive-to-negative converters have high ripple current in

both the input and output capacitors. For long capacitor

lifetime, the RMS value of this current must be less than

the high frequency ripple current rating of the capacitor.

The following formula will give an approximate value for

RMS ripple current. This formula assumes continuous

mode and large inductor value. Small inductors will give

somewhat higher ripple current, especially in discontinu-

ous mode. The exact formulas are very complex and

appear in Application Note 44, pages 30 and 31. For our

purposes here I have simply added a fudge factor (ff). The

value for ff is about 1.2 for higher load currents and

L ≥10µH. It increases to about 2.0 for smaller inductors at

lower load currents.

( )( ) Capacitor IRMS = ff IOUT

VOUT

VIN

ff = Fudge factor (1.2 to 2.0)

Diode Current

Average diode current is equal to load current. Peak diode

current will be considerably higher.

Peak diode current:

Continuous Mode =

( ) ( )(( )()( ) ) IOUT

VIN + VOUT

VIN

+

2L

VIN VOUT

f VIN + VOUT

( )( ) Discontinuous Mode = 2 IOUT VOUT

( )( )L f

Keep in mind that during start-up and output overloads,

average diode current may be much higher than with

normal loads. Care should be used if diodes rated less than

1A are used, especially if continuous overload conditions

must be tolerated.

26

Share Link: