LM26480 datasheet(28/30 Pages) NSC

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High VIN-High Load Operation

Additional inforamtion is provided when the IC is operated at

extremes of VIN and regulator loads. These are described in

terms of the junction temperature and buck output ripple man-

agement.

Junction Temperature

The maximum junction temperature T

J-MAX-OP of 125°C of the

IC package.

The following equations demonstrate junction temperature

determination, ambient temperature T

A-MAX and total chip

power ust be controlled to keep T

J below this maximum:

J-MAX-OP = TA-MAX + (θJA) [°C/Watt] * (PD-MAX) [Watts]

Total IC power dissipation P

D-MAX is the sum of the individual

power dissipation of the four regulators plus a minor amount

for chip overhead. Chip overhead is bias, TSD and LDO ana-

log.

D-MAX = PLOD1 + PLDO2 +PBUCK1 + PBUCK2 + (0.0001A

* VIN) [Watts].

Power dissipation of LDO1 (PLDO1) = (VINLDO1 − VOUTL-

DO1) * IOUTLDO1 [V*A]

Power dissipation of LDO2 (PLDO2) = (VINLDO2 − VOUTL-

DO2) * IOUTLDO2 [V*A]

Power dissipation of Buck1 (PBuck1) = POUT − PIN = VOUT-

BUCK1 − IOUTBUCK1 * (1 −

η2)/ η2 [V*A]

η1 = efficiency of Buck1

Power dissipation of Buck2 (PBuck2) = POUT − PIN = VOUT-

BUCK2 − IOUTBUCK2 * (1 −

η2)/ η2 [V*A]

η2 = efficiency of Buck2

Where

η is the efficiency for the specific condition is taken

from efficiency graphs.

If VIN and ILOADincrease, the output ripple associated with

the Buck Regulators also increases. This mainly occurs with

IN > 5.2V and a load current greater than 1.20A. To ensure

operation in this area of operation, it is recommended that the

system designer circumvents the output ripple issues by in-

stalling Schottky diodes on the bucks(s) that are expected to

perform under these extreme conditions.

LM26480 Datasheet(PDF) 28 Page - National Semiconductor (TI)

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