MIC5191 Datasheet, PDF (9/13 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter/LDO Controller

MIC5191
The V_IN(min) to V_OUT ratio and current will determine the
maximum R_DSON required. For example, for a 1.8V (±5%) to
1.5V conversion at 5A of load current, dropout voltage can be
calculated as follows (using V_IN(min):
( ) _{RDSON =}
V_IN − V_OUT
I_OUT
( ) ^{1.71V − 1.5V}
R_DSON =
5A
R_DSON = 42mΩ
For performance reasons, we do not want to run the N-
Channel in dropout. This will seriously affect transient re-
sponse and PSRR (power supply ripple rejection). For this
reason, we want to select a MOSFET that has lower than
42mΩ for our example application.
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_IN(max)):
P_D = (V_IN – V_OUT) × I_OUT
P_D = (1.89V – 1.5V) × 5A
P_D = 1.95W
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, though 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 MIC5191 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:
( ) ( ) _θJA
=
T_J
max
− T_J
P_D
ambient
θ _JA
=
125°C − 65°C
1.95W
θ_JA = 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.
Micrel
Package
Power Dissipation
TSOP-6
<850mW
TSSOP-8
<950mW
TSSOP-8
<1W
PowerPAK™ 1212-8
<1.1W
SO-8
<1.125W
PowerPAK™SO-8 D-Pack
TO-220/TO-263 (D²pack)
<1.4W
>1.4W
Table 1. Power Dissipation and
Package Recommendation
In our example, our power dissipation is greater than
1.4W, so we’ll choose a TO-263 (D²Pack) N-Channel
MOSFET. θ_JA is calculated as follows.
θ_JA = θ_JC + θ_CS + θ_SA
Where θ_JC is the junction to case resistance, θ_CS is the
case-to-sink resistance and the θ_SA is the sink-to-ambi-
ent air resistance.
In the D² package we’ve selected, the θ_JC is 2°C/W. The
θ_CS, assuming we are using the PCB as the heat sink,
can be approximated to 0.2°C/W. This allows us to
calculate the minimum θ_SA:
θ_SA= θ_JA– θ_CS – θ_JC
θ_SA= 31°C/W – 0.2°C/W – 2°C/W
θ_SA= 28.8°C/W
Referring to Application Hint 17, Designing PCB Heat
Sinks, the minimum amount of copper area for a D²pack
at 28.8°C/W is 2750mm² (or 0.426in² ). 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
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).
April 2004
9
M9999-042804