Datasheet

LP3906
SNVS456M –AUGUST 2006–REVISED MAY 2013 www.ti.com
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 = 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 -η
1
) / η
1
[V*A]
η
1
= efficiency of buck 1
Power dissipation of Buck2
P
Buck2
= P
IN
– P
OUT
=
Vout
Buck2
* Iout
Buck2
* (1 - η
2
) / η
2
[V*A]
η
2
= 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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