lm26480sqx-bf National Semiconductor Corporation, lm26480sqx-bf Datasheet - Page 28

• Manufacturer Part Number: lm26480sqx-bf
• Description: Lm26480 Externally Programmable Dual High-current Step-down Dc/dc And Dual Linear Regulators
• Manufacturer: National Semiconductor Corporation

www.national.com
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
IC package.
The following equations demonstrate junction temperature
determination, ambient temperature T
power ust be controlled to keep T
T
Total IC power dissipation P
power dissipation of the four regulators plus a minor amount
for chip overhead. Chip overhead is bias, TSD and LDO ana-
log.
P
* VIN) [Watts].
J-MAX-OP
D-MAX
= PLOD1 + PLDO2 +PBUCK1 + PBUCK2 + (0.0001A
= T
A-MAX
+ (θ
JA
) [°C/Watt] * (P
D-MAX
J
is the sum of the individual
below this maximum:
J-MAX-OP
A-MAX
D-MAX
) [Watts]
of 125°C of the
and total chip
28
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
V
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.
IN
> 5.2V and a load current greater than 1.20A. To ensure