LT1302CS8 datasheet(7/16 Pages) LINER

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LT1302/LT1302-5

OPERATIO

CMP1 stays on and the peak switch current is regulated

by the voltage on the VC pin (A1’s output). VC drives the

base of Q1. As the VC voltage rises, Q2 conducts less

current, resulting in less drop across R5. Q4’s peak

current must then increase in order for A2 to trip. This

current mode control results in good stability and immu-

nity to input voltage variations. Because this is a linear,

closed-loop system, frequency compensation is required.

A series RC from VC to ground provides the necessary

pole-zero combination.

The LT1302-5 incorporates feedback resistors R1 and

R2 into the device. Output voltage is set at 5.05V in Burst

Mode, dropping to 4.97V in current mode.

APPLICATIONS INFORMATION

Inductor Selection

Inductors used with the LT1302 must fulfill two require-

ments. First, the inductor must be able to handle current

of 2.5A to 3A without runaway saturation. Rod or drum

core units usually saturate gradually and it is acceptable to

exceed manufacturers’ published saturation currents by

20% or so. Second, it should have low DCR, under 0.05

Ω

so that copper loss is kept low. Inductance value is not

critical. Generally, for low voltage inputs down to 2V, a

µHinductorisrecommended(suchasCoilcraftDO3316-

103). For inputs above 4V to 5V use a 22

µH unit (such as

Coilcraft DO3316-223). Switching frequency can reach up

to 400kHz so the core material should be able to handle

high frequency without loss. Ferrite or molypermalloy

cores are a better choice than powdered iron. If EMI is a

concern a toroidal inductor is suggested, such as Coiltronics

CTX20-4.

For a boost converter, duty cycle can be calculated by the

following formula:

DC = 1–

OUT









A special situation exists where the VOUT/VIN differential is

high, such as a 2V-to-12V converter. The required duty

cycle is higher than the LT1302 can provide, so the

converter must be designed for

discontinuous operation.

This means that inductor current goes to zero during the

switch off-time. In the 2V-to-12V case, inductance must

be low enough so that current in the inductor can reach

2A in a single cycle. Inductor value can be defined by:

≤

−

()×

With the 2V input a value of 3.3

µH is acceptable. Since the

inductance is so low, usually a smaller core size can be

used. Efficiency will not be as high as for the continuous

case since peak currents will necessarily be higher.

Table 1 lists inductor suppliers along with appropriate part

numbers.

Table 1. Recommended Inductors

VENDOR

PART NO.

VALUE(

µH)

PHONE NO.

Coilcraft

DO3316-103

(708) 639-6400

DO3316-153

DO3316-223

Coiltronics

CTX10-2

(407) 241-7876

CTX20-4

Dale

LPT4545-100LA

(605) 665-9301

LPT4545-200LA

Sumida

CD105-100

(708) 956-0666

CD105-150

CDR125-220

Capacitor Selection

The output capacitor should have low ESR for proper

performance. A high ESR capacitor can result in “mode-

hopping” between current mode and Burst Mode at high

load currents because the output voltage will increase by

ISW × ESR when the inductor current is flowing into the

diode. Figure 4 shows output voltage of an LT1302-5

boost converter with two 220

µF AVX TPS capacitors at the

output. Ripple voltage at a 510mA load is about 30mVP-P

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