MIC5159-1.8YM6 MICREL [Micrel Semiconductor], MIC5159-1.8YM6 Datasheet - Page 7

• Manufacturer Part Number: MIC5159-1.8YM6
• Description: Programmable Current Limit ?Cap LDO Regulator Controller
• Manufacturer: MICREL [Micrel Semiconductor]

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:
Where I
the current sense resistor for a 2.5V
linear regulator calculates as follows:
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
maximum input voltage (V
drain-source breakdown voltage (BV
minimum input voltage (V
equal to the gate threshold voltage (V
Channel MOSFET. For a given output current and
dropout requirement, the ON-resistance (R
P-Channel MOSFET must also be determined. The
minimum R
calculated as follows:
Where I
R
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
Micrel, Inc.
June 2006
SENSE
•
•
•
•
•
is the current sense resistor.
OUT
R
Input voltage
Gate threshold
Load current
Dropout voltage (input-to-output differential)
Thermal performance
R
R
R
OUT(max)
SENSE
DS(ON)
SENSE
SENSE
is the maximum output current. For example,
DS(ON)
= 10mΩ
=
=
=
is the maximum output current and
⎛
⎜
⎜
⎝
⎛
⎜
⎝
⎛
⎜
⎜
⎝
50mV
50mV
(
I
of the P-Channel MOSFET is
V
OUT
5A
IN(MIN)
I
OUT(MAX)
IN(min)
⎞
⎟
⎟
⎠
⎞
⎟
⎠
IN(max)
−
DS(ON)
V
) must be greater than or
OUT
) must be less than its
is calculated as:
)
⎞
⎟
⎟
⎠
DS
−
IN
). In addition, the
R
to 1.8V
SENSE
GS
DS(ON)
) of the P-
OUT
) of the
, 5A,
7
According to the above calculation, the minimum R
is 130mΩ for a 2.5V to 1.8V LDO with 5A of output
current. For this design, the R
maintain
temperature, current, and voltage conditions.
Placing two or more P-Channel FETs in parallel can
reduce the total R
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,
application-specific parameters:
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 θ
calculated using the following formula:
Where T
of 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 θ
150°C (max.) T
sheet is as follows:
OUT
.
•
•
•
•
R
Maximum ambient temperature (T
Output current (I
Output voltage (V
Input voltage (V
P
θ
P
P
R
θ
θ
J(max)
IN
JA
D
D
D
DS(ON)
JA
JA
DS(ON)
better
is the maximum V
= (V
= (2.5V–1.8V) × 5A
= 3.5W
= θ
=
=
thermal
is equal to the maximum die temperature
⎛
⎜
⎜
⎝
JC
(
= 130mΩ
IN
150
(
J
T
=
+ θ
– V
J(MAX)
according to a typical MOSFET data
⎛
⎜
⎝
DS(ON)
than
°
3.5W
(
C
CS
2.5V
OUT
P
−
D
design
JA
+ θ
JA
IN
) × I
−
60
OUT
5A
of the regulator. This also aids
)
OUT
T
−
to maintain a maximum T
130mΩ
SA
°
for the MOSFET can be
A
1
C
)
OUT
)
8 .
)
)
IN
IN
⎞
⎟
⎟
⎠
V
DS(ON)
and I
to 1.8V
requires
)
⎞
⎟
⎠
−
over
10
for the FETs should
OUT
mΩ
OUT
is the maximum
M9999-062706
A
the
the
)
at 5A with an
MIC5159
following
required
DS(ON)
J
of