LT1578CS8 datasheet(13/28 Pages) LINER

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LT1578/LT1578-2.5

APPLICATIONS INFORMATION

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

regulator input voltage. Average forward current in normal

operation can be calculated from:

µs/DIV

1578 F03

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 = 30µH, ESR = 0.1Ω,

ESL = 10nH:

RIPPLE

P-P

() −

()

()















()( )+















=+=

−

510 5

10 30 10

200 10

042

30 10

033 10

0 42

0 1

10 10

0 33 10

0 042 0 003

••

•

.•

•

. •

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 0.33

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 1

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

VOUT AT

IOUT = 1A

INDUCTOR

CURRENT

AT IOUT = 1A

VOUT AT

IOUT = 50mA

INDUCTOR

CURRENT

AT IOUT = 50mA

20mV/DIV

200mA/DIV

20mV/DIV

200mA/DIV

Figure 3. LT1578 Ripple Voltage Waveform

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