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LTC3440

3440fb

APPLICATIO S I FOR ATIO

Short-Circuit Improvements

The LTC3440 is current limited to 2.7A peak to protect the

IC from damage. At input voltages above 4.5V a current

limit condition may produce undesirable voltages to the IC

due to the series inductance of the package, as well as the

Figure 8. Error Amplifier with Type III Compensation

1.22V

3440 F08

CP1

CZ1

VOUT

CP2

–

ERROR

AMP

Figure 7. Error Amplifier with Type I Compensation

1.22V

3440 F07

CP1

VOUT

–

ERROR

AMP

Which is extremely close to DC

POLE

ZERO

POLE

232

≈

••

•

••

•

••

•

••

•

traces and external components. Following the recom-

mendations for output voltage >4.3V and input voltage

>4.5V will improve this condition. Additional short-circuit

protection can be accomplished with some external cir-

cuitry.

In an overload or short-circuit condition the LTC3440

voltage loop opens and the error amp control voltage on

the VC pin slams to the upper clamp level. This condition

forces boost mode operation in order to attempt to provide

more output voltage and the IC hits a peak switch current

limit of 2.7A. When switch current limit is reached switches

B and D turn on for the remainder of the cycle to reverse

the volts • seconds on the inductor. Although this prevents

current run away, this condition produces four switch

operation producing a current foldback characteristic and

the average input current drops. The IC is trimmed to

guarantee greater than 1A average input current to meet

the maximum load demand, but in a short-circuit or

overload condition the foldback characteristic will occur

producing higher peak switch currents. To minimize this

affect during this condition the following circuits can be

utilized.

Restart Circuit

For a sustained short-circuit the circuit in Figure 9 will

force a soft-start condition. The only design constraint is

that R2/C2 time constant must be longer than the soft-

start components R1/C1 to ensure start-up.

Figure 9. Soft-Start Reset Circuitry for a Sustained Short-Circuit

10nF

4.7nF

VOUT

VIN

SOFT-START

SO/SS

NMOS

VN2222

NMOS

VN2222

1N4148

3440 F09

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