MIC5190
Micrel
The V (min) to V
ratio and current will determine the
Package
Power Dissipation
<850mW
<950mW
<1W
IN
OUT
maximum R
required. For example, for a 1.8V (±5%) to
DSON
TSOP-6
1.5V conversion at 5A of load current, dropout voltage can be
calculated as follows (using V (min)):
TSSOP-8
IN
TSSOP-8
V − V
(
)
IN
OUT
PowerPAK™1212-8
SO-8
<1.1W
RDSON
=
=
IOUT
<1.125W
<1.4W
1.71V −1.5V
(
)
PowerPAK™ SO-8 D-Pack
TO-220/TO-263 (D2Pack)
RDSON
5A
>1.4W
RDSON = 42mΩ
Table 1. Power Dissipation and
Running the N-Channel in dropout will seriously affect tran-
sientresponseandPSRR(powersupplyripplerejection).For
this reason, we want to select a MOSFET that has lower than
42mΩ for our example application.
Package Recommendation
In our example, our power dissipation is greater than
2
1.4W, so we’ll choose a TO-263 (D Pack) N-Channel
MOSFET. θ is calculated as follows.
JA
Size is another important consideration. Most importantly,
the design must be able to handle the amount of power being
dissipated.
θ
= θ + θ + θ
JC CS SA
JA
Where θ is the junction-to-case resistance, θ is the
JC
CS
case-to-sinkresistanceandtheθ isthesink-to-ambient
SA
The amount of power dissipated can be calculated as follows
air resistance.
(using V (max)):
IN
2
In the D package we’ve selected, the θ is 2°C/W. The
JC
θ
, assumingweareusingthePCBastheheatsink, can
CS
P = (V – V
) × I
OUT
D
IN
OUT
be approximated to 0.2°C/W. This allows us to calculate
P = (1.89V – 1.5V) × 5A
the minimum θ
:
D
SA
P = 1.95W
θ
θ
θ
= θ – θ – θ
JA CS JC
D
SA
SA
SA
= 31°C/W – 0.2°C/W – 2°C/W
= 28.8°C/W
Nowthatweknowtheamountofpowerwewillbedissipating,
we will need to know the maximum ambient air temperature.
For our case we’re going to assume a maximum of 65°C
ambient temperature. Different MOSFETs have different
maximum operating junction temperatures. Most MOSFETs
are rated to 150°C, while others are rated as high as 175°C.
In this case, we’re going to limit our maximum junction
temperature to 125°C. The MIC5190 has no internal thermal
protection for the MOSFET so it is important that the design
provides margin for the maximum junction temperature. Our
design will maintain better than 125°C junction temperature
with1.95Wofpowerdissipationatanambienttemperatureof
65°C. Our thermal resistance calculates as follows:
Referring to Application Hint 17, Designing PCB Heat
Sinks, the minimum amount of copper area for a D Pack
2
2
2
at 28.8°C/W is 2750mm (or 0.426in ). The solid line
denotes convection heating only (2 oz. copper) and the
dotted line shows thermal resistance with 250LFM air-
flow. The copper area can be significantly reduced by
increasing airflow or by adding external heat sinks.
PC Board Heat Sink
Thermal Resistance vs. Area
T max − T ambient
(
)
(
)
J
J
θJA
θJA
=
=
P
D
125°C − 65°C
1.95W
θJA = 31°C / W
So our package must have a thermal resistance less than
31°C /W. Table 1. shows a good approximation of power
dissipation and package recommendation.
Figure 8. PC Board Heat Sink
Another important characteristic is the amount of gate
capacitance. Large gate capacitance can reduce tran-
sient performance by reducing the ability of the MIC5190
to slew the gate. It is recommended that the MOSFET
used has an input capacitance <10nF (C ).
ISS
December 2005
9
M9999-120105
MICREL [ MICREL SEMICONDUCTOR ]
