Micrel, Inc.
MIC5159
June 2006
7
M9999-062706
Application Information
The MIC5159 is a high performance voltage regulator
controller. When used with an external P-Channel
MOSFET and a tiny ceramic output capacitor, it forms a
wide variety of simple, inexpensive ultra-low-dropout
voltage regulators.
Current Sense Resistor Selection
A current sense resistor placed between the input and
the current sense pin (IS) allows for programmability of
the current limit. This resistor can simply be calculated
by:
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
OUT
SENSE
I
50mV
R
Where I
OUT
is the maximum output current. For example,
the current sense resistor for a 2.5V
IN
to 1.8V
OUT
, 5A,
linear regulator calculates as follows:
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
5A
50mV
R
SENSE
R
SENSE
= 10m&
P-Channel MOSFET Selection
The P-Channel MOSFET selected for use with the
MIC5159 must satisfy the following requirements:
" Input voltage
" Gate threshold
" Load current
" Dropout voltage (input-to-output differential)
" Thermal performance
To prevent damage to the P-Channel MOSFET, the
maximum input voltage (V
IN(max)
) must be less than its
drain-source breakdown voltage (BV
DS
). In addition, the
minimum input voltage (V
IN(min)
) must be greater than or
equal to the gate threshold voltage (V
GS
) of the P-
Channel MOSFET. For a given output current and
dropout requirement, the ON-resistance (R
DS(ON)
) of the
P-Channel MOSFET must also be determined. The
minimum R
DS(ON)
of the P-Channel MOSFET is
calculated as follows:
(
)
SENSE
OUT(MAX)
OUT
IN(MIN)
DS(ON)
R
I
V
V
R
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
Where I
OUT(max)
is the maximum output current and
R
SENSE
is the current sense resistor.
For example, the MIC5159-1.8BM6 is used with an
external MOSFET to form a 5A LDO with an input of
2.5V. Either a 2.5V or 1.8V gate threshold MOSFET can
be selected. The minimum R
DS(ON)
is calculated as:
)
m&
10
5A
V
8
.
1
2.5V
R
DS(ON)
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
R
DS(ON)
= 130m&
According to the above calculation, the minimum R
DS(ON)
is 130m& for a 2.5V to 1.8V LDO with 5A of output
current. For this design, the R
DS(ON)
for the FETs should
maintain better than 130m& over the required
temperature, current, and voltage conditions.
Placing two or more P-Channel FETs in parallel can
reduce the total R
DS(ON)
of the regulator. This also aids
thermal dissipation by sharing the current and heat
between the multiple FETs.
Thermal Considerations
Linear regulators are simple to use. The most
complicated design parameters to consider are thermal
characteristics. Since the MIC5159 offers no thermal
protection, thermal design requires the following
application-specific parameters:
" Maximum ambient temperature (T
A
)
" Output current (I
OUT
)
" Output voltage (V
OUT
)
" Input voltage (V
IN
)
First, calculate the maximum power dissipation of the
regulator:
P
D
= (V
IN
V
OUT
) ?I
OUT
Ground current can generally be ignored. The amount of
power dissipated by ground current and input voltage is
minimal. Minimum ?/DIV>
JA
for the MOSFET can be
calculated using the following formula:
)
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
D
A
J(MAX)
JA
P
T
T
?/DIV>
Where T
J(max)
is equal to the maximum die temperature
of the P-Channel.
?/DIV>
JA
= ?/DIV>
JC
+ ?/DIV>
CS
+ ?/DIV>
SA
Example
For the same regulator, 2.5V
IN
to 1.8V
OUT
at 5A with an
ambient temperature of 60癈:
P
D
= (2.5V1.8V) ?5A
P
D
= 3.5W
Where V
IN
is the maximum V
IN
and I
OUT
is the maximum
I
OUT
.
The P-Channel MOSFET must be able to dissipate
3.5W. The minimum ?/DIV>
JA
to maintain a maximum T
J
of
150癈 (max.) T
J
according to a typical MOSFET data
sheet is as follows:
)
3.5W
C
60
C
150
?/DIV>
JA
?/DIV>
?/DIV>
=
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