LP3906 datasheet(36/38 Pages) NSC

36 / 38 page

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 125oCof

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 +PLD02 +PBUCK1 +PBUCK2 + (0.0001A *

Vin) [Watts].

Power dissipation of LDO1

LDO1 =(VinLDO1- VoutLDO1) * IoutLDO1 [V*A]

Power dissipation of LDO2

LDO2 =(VinLDO2 - VoutLDO2) * IoutLDO2 [V*A]

Power dissipation of Buck1

Buck1 =PIN –POUT =

Vout

Buck1* IoutBuck1 *(1-

1)/

1 [V*A]

1 = efficiency of buck 1

Power dissipation of Buck2

Buck2 =PIN –POUT =

Vout

Buck2 * IoutBuck2 *(1-

2)/

2 [V*A]

2 = 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

IN vs ILOAD Operating Ranges

20197864

LP3906 Datasheet(PDF) 36 Page - National Semiconductor (TI)

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