MIC5191_06 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
MIC5191
compensation resistor, creating a higher mid-band gain.
100
225
80
180
60
135
Increasing Cout reduces
the load resistance and
output capacitor pole
40
allowing for an increase
90
in mid-band gain
20
45
0
0
-20
0.01 0.1
-45
1
10 100 1000 10000 100000
Frequency (KHz)
Figure 7. Increasing Output Capacitance
This will have the effect of both decreasing the voltage
drop as well as returning closer and faster to the
regulated voltage during the recovery time.
MOSFET Selection
The typical pass element for the MIC5191 is an N-
Channel MOSFET. There are multiple considerations
when choosing a MOSFET. These include:
• V_IN to V_OUT differential
• Output Current
• Case Size/Thermal Characteristics
• Gate Capacitance (C_ISS<10nF)
• Gate to Source threshold
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
R _DSON
=
(1.71V − 1.5V)
5A
R_DSON = 42mΩ
For performance reasons, we do not want to run the N-
Channel in dropout. This will seriously affect transient
response 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 import-
antly, 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
December 2006
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:
θ _JA
=
T_J (max) − T_J (ambient)
P_D
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.
Package
TSOP-6
TSSOP-8
TSSOP-8
PowerPAK™ 1212-8
SO-8
PowerPAK™ SO-8 D-Pack
TO-220/TO-263 (D²pack)
Power Dissipation
<850mW
<950mW
<1W
<1.1W
<1.125W
<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
10
M9999-122206