LT1302CS8 datasheet(8/16 Pages) LINER

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LT1302/LT1302-5

APPLICATIONS INFORMATION

and there is no low frequency component. The total ESR

is under 0.03

Ω. If a single 100µF aluminum electrolytic

capacitor is used instead, the converter mode-hops be-

tween current mode and Burst Mode due to high ESR,

causing the voltage comparator to trip as shown in Figure

5. The ripple voltage is now over 500mVP-P and contains

a low frequency component. Maximum allowable output

capacitor ESR can be calculated by the following formula:

ESR

MAX

OUT

REF

× 1

where,

VOS = 15mV

VREF = 1.24V

500

µs/DIV

1302 F04

VOUT

50mV/DIV

AC COUPLED

ILOAD

Figure 4. Low ESR Output Capacitor Results in Stable

Operation. Ripple Voltage is Under 30mVP-P

500

µs/DIV

1302 F05

VOUT

200mV/DIV

AC COUPLED

ILOAD

Figure 5. Inexpensive Electrolytic Capacitor Has High

ESR, Resulting in Mode-Hop, Ripple Voltage Amplitude Is

Over 500mVP-P and Includes Low Frequency Component

510mA

10mA

510mA

10mA

Input Capacitor

The input supply should be decoupled with a good quality

electrolytic capacitor close to the LT1302 to provide a

stable input supply. Long leads or traces from power

source to the switcher can have considerable impedance

at the LT1302’s switching frequency. The input capacitor

provides a low impedance at high frequency. A 0.1

µF

ceramic capacitor is required right at the VIN pin. When the

input voltage can be above 5V, a 10

Ω/1µF decoupling

network for VIN is recommended as detailed in Figure 6.

This network is also recommended when driving a trans-

former.

Figure 6. A 10

Ω/1µF Decoupling Network at VIN Is

Recommended When Input Voltage Is Above 5V

VIN

PGND

µF

LT1302

• • •

GND

µF

100

µF

Ω

1302 F06

VIN > 5V

Table 2 lists capacitor vendors along with device types.

Table 2. Recommended Capacitors

VENDOR

SERIES

TYPE

PHONE NO.

AVX

TPS

Surface Mount

(803) 448-9411

Sanyo

OS-CON

Through Hole

(619) 661-6835

Sprague

595D

Surface Mount

(603) 224-1961

Diode Selection

A 2A Schottky diode such as Motorola MBRS130LT3 has

been found to be the best available. Other choices include

1N5821 or MBRS130T3. Do not use “general purpose”

diodes such as 1N4001. They are much too slow for use

in switching regulator applications.

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