MIC5159 Datasheet, PDF (7/23 Pages) Micrel Semiconductor – PROGRAMMABLE CURRENT LIMIT UCAP LDO REGULATOR CONTROLLER

MIC5159
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:
R_SENSE
=


50mV ^
I_{OUT }
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:
R_SENSE
=

50mV
5A

R_SENSE = 10mΩ
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 maxi-
mum 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:
( ) _RDS(ON)

= _
V_IN(min) − V_OUT
I_OUT (max)


− R_SENSE
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:
R_DS(ON)
=


(2.5V − 1.8V)
5A

−
10mΩ
R_DS(ON) = 130mΩ
Micrel
According to the above calculation, the minimum R_DS(ON) is
130mΩ 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 130mΩ 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 param-
eters:
• 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 θ_JA for the MOSFET can be calculated
using the following formula:
θ _JA
=


(T_J
(max)
P_D
−
T_A
)


Where T_J (max) is equal to the maximum die temperature of
the P-Channel.
θ_JA = θ_JC + θ_CS + θ_SA
Example
For the same regulator, 2.5V_IN to 1.8V_OUT at 5A with an
ambient temperature of 60°C:
P_D = (2.5V – 1.8V) × 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 θ_JA to maintain a maximum T_J of 150°C (max.)
T_J according to a typical MOSFET data sheet is as follows:
θ_JA
=
(150°C − 60°C)
3.5W
θ_JA = 25.71°C/W
The heatsink and MOSFET must have a combined thermal
resistance to meet the above criteria.
April 2004
7
M9999-041204