MIC5190 Micrel Semiconductor, MIC5190 Datasheet - Page 9

• Manufacturer Part Number: MIC5190
• Description: Ultra High-Speed High-Current Active Filter/LDO Controller
• Manufacturer: Micrel Semiconductor

December 2005
The V
maximum R
1.5V conversion at 5A of load current, dropout voltage can be
calculated as follows (using V
Running the N-Channel in dropout will seriously affect tran-
sient response and PSRR (power supply ripple rejection). For
this reason, we want to select a MOSFET that has lower than
42m
Size is another important consideration. Most importantly,
the design must be able to handle the amount of power being
dissipated.
The amount of power dissipated can be calculated as follows
(using V
Now that we know the amount of power we will be dissipating,
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
with 1.95W of power dissipation at an ambient temperature of
65 C. Our thermal resistance calculates as follows:
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.
MIC5190
P
P
P
R
R
R
IN
for our example application.
D
D
D
DSON
DSON
DSON
(min) to V
IN
= (V
= (1.89V – 1.5V)
= 1.95W
(max)):
DSON
JA
JA
JA
IN
42
1 71V 1 5V
V
IN
m
– V
125 C 65 C
31
T max
I
required. For example, for a 1.8V ( 5%) to
OUT
J
OUT
V
5A
OUT
1.95W
C W
OUT
ratio and current will determine the
)
P
T ambient
I
OUT
D
J
5A
IN
(min)):
9
In our example, our power dissipation is greater than
1.4W, so we’ll choose a TO-263 (D
MOSFET.
Where
case-to-sink resistance and the
air resistance.
In the D
be approximated to 0.2 C/W. This allows us to calculate
the minimum
Referring to
Sinks
at 28.8 C/W is 2750mm
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.
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
CS
Package
TSOP-6
TSSOP-8
TSSOP-8
PowerPAK™1212-8
SO-8
PowerPAK™ SO-8 D-Pack
TO-220/TO-263 (D
, assuming we are using the PCB as the heat sink, can
, the minimum amount of copper area for a D
JA
SA
SA
SA
2
JC
package we’ve selected, the
=
= 31 C/W – 0.2 C/W – 2 C/W
= 28.8 C/W
=
Table 1. Power Dissipation and
is the junction-to-case resistance,
Thermal Resistance vs. Area
JA
Figure 8. PC Board Heat Sink
JA
JC
Application Hint 17, Designing PCB Heat
Package Recommendation
–
is calculated as follows.
SA
+
PC Board Heat Sink
:
CS
2
CS
Pack)
–
+
JC
2
SA
(or 0.426in
www.DataSheet4U.com
SA
is the sink-to-ambient
Power Dissipation
2
2
Pack) N-Channel
JC
). The solid line
ISS
<850mW
<950mW
<1.125W
is 2 C/W. The
<1.1W
<1.4W
>1.4W
M9999-120105
<1W
).
CS
2
Micrel
is the
Pack