LT1339C datasheet(11/20 Pages) LINER

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LT1339

APPLICATIONS INFORMATION

impedance of the IAVG pin. The integrator corner fre-

quency is typically set 1 to 2 orders of magnitude below the

oscillator frequency and follows the relation:

f–3dB = (3.2)(10

–6)/CAVG

The average current limit function can be disabled by

shorting the IAVG pin directly to SGND.

Soft Start Programming

The current control pin (VC) limits sensed inductor current

to zero at voltages less than a transistor VBE, to full average

current limit at VC = VBE + 1.8V. This generates a 1.8V full

regulation range for average load current. An internal

voltage clamp forces the VC pin to a VBE – 100mV above

the SS pin voltage. This 100mV “dead zone” assures 0%

duty cycle operation at the start of the soft start cycle, or

when the soft start pin is pulled to ground. Given the

typical soft start current of 8

µA and a soft start timing

capacitor CSS, the start-up delay time to full available

average current will be:

tSS = (1.5)(10

5)(CSS)

Boost Supply

The VBOOST supply is bootstrapped via an external capaci-

tor. This supply provides gate drive to the topside switch

FET. The bootstrap capacitor is charged from 12VIN through

a diode when the switch node is pulled low.

The diode reverse breakdown voltage must be greater than

VIN + 12VIN. The bootstrap capacitor should be at least 100

times greater than the total input capacitance of the

topside FET. A capacitor in the range of 0.1

µF to 1µF is

generally adequate for most applications.

Shutdown Function — Input Undervoltage Detect and

Threshold Hysteresis

The LT1339 RUN/SHDN pin uses a bandgap generated

reference threshold of about 1.25V. This precision thresh-

old allows use of the RUN/SHDN pin for both logic-level

shutdown applications and analog monitoring applica-

tions such as power supply sequencing.

Because an LT1339 controlled converter is a power trans-

fer device, a voltage that is lower than expected on the

input supply could require currents that exceed the sourc-

ing capabilities of that supply, causing the system to lock

up in an undervoltage state. Input supply start-up protec-

tion can be achieved by enabling the RUN/SHDN pin using

a resistor divider from the input supply to ground. Setting

the divider output to 1.25V when that supply is almost fully

enabled prevents the LT1339 regulator from drawing large

currents until the input supply is able to provide the

required power.

If additional hysteresis is desired for the enable function,

an external feedback resistor can be used from the LT1339

regulator output. If connection to the regulator output is

not desired, the 5VREF internal supply pin can be used.

Figure 3 shows a resistor connection on a 48V to 5V

converter that yields a 40V VIN start-up threshold for

regulator enable and also provides about 10% input

referred hysteresis.

The shutdown function can be disabled by connecting the

RUN/SHDN pin to the 12VIN rail. This pin is internally

clamped to 2.5V through a 20k series input resistance and

will therefore draw about 0.5mA when tied directly to 12V.

This additional current can be minimized by making the

connection through an external resistor (100k is typically

used).

Inductor Selection

The inductor for an LT1339 converter is selected based on

output power, operating frequency and efficiency require-

ments. Generally, the selection of inductor value can be

reduced to desired maximum ripple current in the inductor

(

∆I). Forabuckconverter,theminimuminductorvaluefor

a desired maximum operating ripple current can be deter-

mined using the following relation:

10k

300k

VOUT

1339 • F03

5VREF

LT1339

RUN/SHDN

390k

VIN

48V

OPTION 2

OPTION 1

Figure 3. Input Supply Sequencing Programming

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