LP3906 datasheet(16/47 Pages) TI

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LP3906

SNVS456M – AUGUST 2006 – REVISED MAY 2013

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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 125ºC 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 individual power dissipation of the four regulators plus a

minor amount for chip overhead. Chip overhead is Bias, TSD & LDO analog.

PD-MAX = PLDO1 + PLD02 + 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 = PIN – POUT =

VoutBuck1* IoutBuck1 * (1 -η1) / η1 [V*A]

η1 = efficiency of buck 1

Power dissipation of Buck2

PBuck2 = PIN – POUT =

VoutBuck2 * IoutBuck2 * (1 - η2) / η2 [V*A]

η2 = efficiency of buck 2

η is the efficiency for the specific condition taken from efficiency graphs.

Buck Output Ripple Management

If Vin and ILoad increase, the output ripple associated with the Buck Regulators also increases. Figure 32 shows

the safe operating area. To ensure operation in the area of concern it is recommended that the system designer

circumvents the output ripple issues to install schottky diodes on the Bucks(s) that are expected to perform under

these extreme 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)

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