Datasheet
LM26480
SNVS543I –JANUARY 2008–REVISED MAY 2013
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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 management.
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:
T
J-MAX-OP
= T
A-MAX
+ (θ
JA
) [°C/Watt] * (P
D-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 analog.
P
D-MAX
= PLOD1 + PLDO2 +PBUCK1 + PBUCK2 + (0.0001A * VIN) [Watts].
Power dissipation of LDO1 (PLDO1) = (VINLDO1 − VOUTLDO1) * IOUTLDO1 [V*A]
Power dissipation of LDO2 (PLDO2) = (VINLDO2 − VOUTLDO2) * IOUTLDO2 [V*A]
Power dissipation of Buck1 (PBuck1) = POUT − PIN = VOUTBUCK1 − IOUTBUCK1 * (1 − η2)/ η2 [V*A]
η1 = efficiency of Buck1
Power dissipation of Buck2 (PBuck2) = POUT − PIN = VOUTBUCK2 − 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 V
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 installing Schottky diodes on the
bucks(s) that are expected to perform under these extreme conditions.
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