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LT1374

1374fb

APPLICATIONS INFORMATION

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

••

•

CATCH DIODE

The suggested catch diode (D1) is a 1N5821 Schottky, or

its Motorola equivalent, MBR330. It is rated at 3A average

forward current and 30V reverse voltage. Typical forward

voltage is 0.5V at 3A. The diode conducts current only

during switch off time. Peak reverse voltage is equal to

regulator input voltage. Average forward current in normal

operation can be calculated from:

D AVG

OUT

( )=

−

()

This formula will not yield values higher than 3A with

maximum load current of 4.25A unless the ratio of input to

output voltage exceeds 3.4:1. 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 2.6A, 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 5.7A,

determined by peak switch current limit of 6A. With

VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 5.7A:

D AVG

()=

−

()

5 7 15 4

418

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 0.27

µF

capacitor and a FMMD914 diode. The anode is connected

to the regulated output voltage and this generates 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 voltage.

This could be necessary if the regulated output voltage is

very low (< 3V) or if the input voltage is less than 5V.

Efficiency is not affected by the capacitor value, but the

capacitor should have an ESR of less than 1

Ω to ensure

Figure 3. LT1374 Ripple Voltage Waveform

0.5

µs/DIV

1374 F03

20mV/DIV

0.5A/DIV

VOUT AT

IOUT = 1A

VOUT AT

IOUT = 50mA

INDUCTOR

CURRENT

AT IOUT = 1A

INDUCTOR

CURRENT

AT IOUT = 50mA

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