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LT1959

APPLICATIONS INFORMATION

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

available in smaller case sizes. These are ideal for input

bypassing because of their high ripple rating and tolerance

to turn-on surges. As output capacitors, caution must be

used. Solid tantalum capacitor’s ESR generates a loop

“zero” at 5kHz to 50kHz that is beneficial in giving accept-

able loop phase margin. Ceramic capacitors remain ca-

pacitive to beyond 300kHz and usually resonate with their

ESL before ESR becomes effective. When using ceramic

output capacitors, the loop compensation pole frequency

must be reduced by a typical factor of 10.

OUTPUT RIPPLE VOLTAGE

Figure 3 shows a typical output ripple voltage waveform

for the LT1959. 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

1374 F03

INDUCTOR CURRENT

AT IOUT = 50mA

20mV/DIV

0.5A/DIV

Figure 3. LT1959 Ripple Voltage Waveform

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

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