LP3906 Datasheet, PDF (36/38 Pages) National Semiconductor (TI) – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C Compatible Interface

Application Notes (Continued)
High Vin-High Load Operation
Additional information 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 TJ-MAX-OP of 125^oC of
the IC package.
The following equations demonstrate junction temperature
determination, ambient temperature TA-MAX and Total chip
power must be controlled to keep TJ below this maximum:
TJ-MAX-OP = TA-MAX + (θJA) [ ˚C/ Watt] * (PD-MAX)
[Watts]
Total IC power dissipation PD-MAX is the sum of the indi-
vidual power dissipation of the four regulators plus a minor
amount for chip overhead. Chip overhead is Bias, TSD &
LDO analog.
PD-MAX = P_LDO1 + P_LD02 + P_BUCK1 + P_BUCK2 + (0.0001A *
Vin) [Watts].
Power dissipation of LDO1
P_LDO1 = (Vin_LDO1- Vout_LDO1) * Iout_LDO1 [V*A]
Power dissipation of LDO2
P_LDO2 = (Vin_LDO2 - Vout_LDO2) * Iout_LDO2 [V*A]
Power dissipation of Buck1
P_Buck1 = P_IN – P_OUT =
Vout_Buck1* Iout_Buck1 * (1 -η₁) / η₁ [V*A]
η₁ = efficiency of buck 1
Power dissipation of Buck2
P_Buck2 = P_IN – P_OUT =
Vout_Buck2 * Iout_Buck2 * (1 - η₂) / η₂ [V*A]
η₂ = efficiency of buck 2
η is the efficiency for the specific condition taken from effi-
ciency graphs.
Buck Output Ripple Management
If Vin and ILoad increase, the output ripple associated with
the Buck Regulators also increases. The figure below shows
the safe operating area. To ensure operation in the area of
concern it is recommended that the system designer circum-
vents the output ripple issues to install schottky diodes on
the Bucks(s) that are expected to perform under these ex-
treme corner conditions.
(Schottky diodes are recommended to reduce the output
ripple if the system requirements include this shaded area of
operation. Vin > 5.2 V and Iload > 1.24 A)
LP3906 Buck Converter
V_IN vs I_LOAD Operating Ranges
20197864
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