Datasheet

R
T
JA

=
T
J
- T
A
P
INTERNAL
R
T
=
'T
Power
LM26420
SNVS579F –FEBRUARY 2009–REVISED MARCH 2013 www.ti.com
Due to Dead-Time-Control Logic in the converter, there is a small delay (~4nS) between the turn ON and OFF of
the TOP and BOTTOM FET. During this time, the body diode of the BOTTOM FET is conducting with a voltage
drop of V
BDIODE
(~.65V). This allows the inductor current to circulate to the output, until the BOTTOM FET is
turned ON and the inductor current passes through the FET. There is a small amount of power loss due to this
body diode conducting and it can be calculated as follows:
P
BDIODE
= 2x(V
BDIODE
x I
OUT
x F
SW
x T
BDIODE
) (33)
Typical Application power losses are:
P
LOSS
= ΣP
COND
+ P
SW
+ P
BDIODE
+ P
IND
+ P
Q
(34)
P
INTERNAL
= ΣP
COND
+ P
SW
+ P
BDIODE
+ P
Q
(35)
Table 1. Power Loss Tabulation
V
IN
5.0V V
OUT
1.2V
I
OUT
2.0A P
OUT
2.4W
F
SW
550kHz
V
BDIODE
0.65V P
BDIODE
5.7mW
I
Q
8.4mA P
Q
42mW
T
RISE
1.5nS P
SWR
4.1mW
T
FALL
1.5nS P
SWF
4.1mW
R
DSON_TOP
75mΩ P
COND_TOP
81mW
R
DSON_BOT
55mΩ P
COND_BOT
167mW
IND
DCR
20mΩ P
IND
80mW
D 0.262 P
LOSS
384mW
η 86.2% P
INTERNAL
304mW
These calculations assume a junction temperature of 25°C. The R
DSON
values will be larger due to internal
heating and therefore the internal power loss (P
INTERNAL
) must be first calculated to estimate the rise in junction
temperature.
Thermal Definitions
T
J
= Chip junction temperature
T
A
= Ambient temperature
R
θJC
= Thermal resistance from chip junction to device case
R
θJA
= Thermal resistance from chip junction to ambient air
Heat in the LM26420 due to internal power dissipation is removed through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the
transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).
Heat Transfer goes as:
Silicon → package → lead frame → PCB
Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural
convection occurs when air currents rise from the hot device to cooler air.
Thermal impedance is defined as:
(36)
Thermal impedance from the silicon junction to the ambient air is defined as:
(37)
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