MIC5159_06 Datasheet, PDF (7/23 Pages) Micrel Semiconductor – Programmable Current Limit μCap LDO Regulator Controller

Micrel, Inc.
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:
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
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:
( ) _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:
MIC5159
R _DS(ON)
=
⎜⎛ ^{(2.5V − 1.8V)}⎟⎞ − 10mΩ
⎝ ^5A ⎠
R_DS(ON) = 130mΩ
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 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 θ_JA for the MOSFET can be
calculated using the following formula:
( ) _θJA
= _{⎜⎜⎝⎛}
T_J(MAX) − T_A
P_D
⎟⎟⎠⎞
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
June 2006
7
M9999-062706