LTC7103 datasheet(22/48 Pages) AD

22 / 48 page

LTC7871

Rev. 0

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Upon removal of the short, VLOW soft starts using the

internal soft-start, thus reducing output overshoot. In

the absence of this feature, the output capacitors would

have been charged at current limit, and in applications

with minimal output capacitance this may have resulted

in output overshoot. Current limit foldback is not disabled

during an overcurrent recovery. The load must drop below

the folded back current limit threshold in order to restart

from a hard short.

In both buck and boost modes of operation, forcing a

voltage on the SETCUR pin regulates the average current.

Zero average inductor current can be obtained by forcing

0V on SETCUR.

The LTC7871 has additional overcurrent fault compara-

tors to monitor the current of each channel. If there is

any catastrophic failure in the system which causes one

or more channel’s inductor current to be higher than the

overcurrent fault threshold, all the channels will be shut

down and both the PWMEN and the FAULT pins will be

pulled down to SGND.

Another way to protect against overcurrent is to moni-

tor the IMON pin voltage. If the IMON voltage indicates

excessive current, an external circuit can be used to shut

down the system.

Inductor Value Calculation

Given the desired input and output voltages, the inductor

value and operating frequency, fOSC, directly determine

the inductor’s peak-to-peak ripple current:

IRIPPLE =

VLOW

VHIGH

VHIGH – VLOW

fOSC •L

⎛

⎝⎜

⎞

⎠⎟

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors, and output voltage

ripple. Thus, highest efficiency operation is obtained at

low frequency with a small ripple current. Achieving this,

however, requires a large inductor.

A reasonable starting point is to choose a ripple current

that is about 40% of the maximum inductor current. Note

that the largest ripple current occurs at the highest VHIGH

voltage. To guarantee that ripple current does not exceed

The R1 • C1 time constant should be selected such that:

= 4 •R1•C1 for R1= 10 •R2

The R3 • C2 time constant should be selected such that:

R3 •C2

R1•R2

+R2

•C1

If a 6.8μH inductor and a 1mΩ sense resistor are selected

and C1 and C2 are chosen to be 0.1µF, then the values for

R1, R2 and R3 will be 16.9k, 1.69k and 1.5k, respectively

when the nearest standard value is chosen.

Pre-Biased Output Start-Up

There may be situations that require the power supply to

start up with a prebias on the VLOW output capacitors. In

this case, it is desirable to start up without discharging

that output prebias. The LTC7871 can safely power up

into a prebiased output without discharging it.

The LTC7871 accomplishes this by disabling both the

top and bottom MOSFETs until the SS pin voltage and the

internal soft-start voltage are above the VFBLOW pin volt-

age. When VFBLOW is higher than SS or the internal soft-

start voltage, the error amp output is parked at its zero

current level. Disabling both top and bottom MOSFETs

prevents the prebiased output voltage from being dis-

charged. When SS and the internal soft-start both cross

1.32V or VFBLOW, whichever is lower, both top and bottom

MOSFETs are enabled.

Overcurrent Fault Protection

In the buck mode, when the output of the power supply

is loaded beyond its preset current limit, the regulated

output voltage will collapse depending on the load. The

VLOW rail may be shorted to ground through a very low

impedance path or it may be a resistive short, in which

case the output will collapse partially, until the load cur-

rent equals the preset current limit. The controller will

continue to source current into the short. The amount of

current sourced depends on the ILIM pin setting and the

VFBLOW voltage as shown in the Current Foldback graph

in the Typical Performance Characteristics section.

APPLICATIONS INFORMATION

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