MIC5159 MICREL [Micrel Semiconductor], MIC5159 Datasheet - Page 7

• Manufacturer Part Number: MIC5159
• Description: PROGRAMMABLE CURRENT LIMIT UCAP LDO REGULATOR CONTROLLER
• Manufacturer: MICREL [Micrel Semiconductor]

Applications Information
The MIC5159 is a high performance voltage regulator con-
troller. 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:
Where I
For example, the current sense resistor for a 2.5V
1.8V
P-Channel MOSFET Selection
The P-Channel MOSFET selected for use with the MIC5159
must satisfy the following requirements:
To prevent damage to the P-Channel MOSFET, the maxi-
mum input voltage (V
source breakdown voltage (BV
input voltage (V
gate threshold voltage (V
a given output current and dropout requirement, the ON-
resistance (R
be determined. The minimum R
MOSFET is calculated as follows:
Where I
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
April 2004
MIC5159
• Input voltage
• Gate threshold
• Load current
• Dropout voltage (input-to-output differential)
• Thermal performance
OUT
R
R
R
R
R
R
SENSE
DS(ON)
OUT
DS(ON)
SENSE
SENSE
DS(ON)
OUT
, 5A, linear regulator calculates as follows:
(max) is the maximum output current and R
is the maximum output current.
DS(ON)
= 10m
= 130m
IN
DS(ON)
(min)) must be greater than or equal to the
50mV
50mV
I
V (
2.5V 1.8V
OUT
5A
) of the P-Channel MOSFET must also
IN
I
OUT
IN
min)
5A
is calculated as:
(max)) must be less than its drain-
GS
(
max)
) of the P-Channel MOSFET. For
V
DS
OUT
). In addition, the minimum
DS(ON)
10m
R
of the P-Channel
SENSE
SENSE
IN
to
7
According to the above calculation, the minimum R
130m for a 2.5V to 1.8V LDO with 5A of output current. For
this design, the R
than 130m
voltage conditions.
Placing two or more P-Channel FETs in parallel can reduce
the total R
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 param-
eters:
First, calculate the maximum power dissipation of the
regulator:
Ground current can generally be ignored. The amount of
power dissipated by ground current and input voltage is
minimal. Minimum
using the following formula:
Where T
the P-Channel.
Example
For the same regulator, 2.5V
ambient temperature of 60 C:
Where V
I
The P-Channel MOSFET must be able to dissipate 3.5W.
The minimum
T
The heatsink and MOSFET must have a combined thermal
resistance to meet the above criteria.
OUT
J
according to a typical MOSFET data sheet is as follows:
• Maximum ambient temperature (T
• Output current (I
• Output voltage (V
• Input voltage (V
.
P
P
P
JA
JA
D
D
D
JA
JA
J
= (V
= (2.5V – 1.8V)
= 3.5W
IN
=
= 25.71 C/W
(max) is equal to the maximum die temperature of
DS(ON)
is the maximum V
JC
150 C 60 C
IN
over the required temperature, current, and
T (
JA
+
– V
J
3.5W
DS(ON)
max)
to maintain a maximum T
CS
of the regulator. This also aids thermal
OUT
P
JA
+
D
IN
)
OUT
for the MOSFET can be calculated
for the FETs should maintain better
OUT
)
SA
T
I
5A
)
OUT
A
)
IN
IN
to 1.8V
and I
OUT
OUT
A
)
J
is the maximum
of 150 C (max.)
at 5A with an
M9999-041204
DS(ON)
Micrel
is