LT1376CS8 datasheet(13/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

discharge surges, such as when the regulator output is

dead shorted, do not harm the capacitors.

Unlike the input capacitor, RMS ripple current in the

output capacitor is normally low enough that ripple cur-

rent rating is not an issue. The current waveform is

triangular with a typical value of 200mARMS. The formula

to calculate this is:

Output Capacitor Ripple Current (RMS):

Lf V

RIPPLE RMS

OUT

( ) =

() −

()

()( )( )

029

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor’s ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic ca-

pacitors remain capacitive to beyond 300kHz and usually

resonate with their ESL before ESR becomes effective.

They are appropriate for input bypassing because of their

high ripple current ratings and tolerance of turn-on surges.

For further information on ceramic and other capacitor

types please refer to Design Note 95.

OUTPUT RIPPLE VOLTAGE

Figure 3 shows a typical output ripple voltage waveform

for the LT1376. Ripple voltage is determined by the high

frequency impedance of the output capacitor, and ripple

current through the inductor. Peak-to-peak ripple current

through the inductor into the output capacitor is:

VL f

OUT

-P =

() −

()

()( )( )

For high frequency switchers, the sum of ripple current

slew rates may also be relevant and can be calculated

from:

Peak-to-peak output ripple voltage is the sum of a

triwave

created by peak-to-peak ripple current times ESR, and a

square wave created by parasitic inductance (ESL) and

ripple current slew rate. Capacitive reactance is assumed

to be small compared to ESR or ESL.

ESR

ESL

RIPPLE =

()( )+()

P-P

Example: with VIN =10V, VOUT = 5V, L = 10µH, ESR = 0.1Ω,

ESL = 10nH:

RIPPLE

P-P

() −

()

()













()( )+













−

510 5

10 10 10

500 10

10 10

0 05 0 01 60

••

•

VOUT AT IOUT = 1A

VOUT AT IOUT = 50mA

INDUCTOR CURRENT

AT IOUT = 1A

0.5

µs/DIV

1375/76 F03

Figure 3. LT1376 Ripple Voltage Waveform

INDUCTOR CURRENT

AT IOUT = 50mA

20mV/DIV

0.5A/DIV

CATCH DIODE

The suggested catch diode (D1) is a 1N5818 Schottky, or

its Motorola equivalent, MBR130. It is rated at 1A average

forward current and 30V reverse voltage. Typical forward

voltage is 0.42V at 1A. The diode conducts current only

during switch off time. Peak reverse voltage is equal to

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