Datasheet

MIC5219
DS20006021A-page 14  2018 Microchip Technology Inc.
The actual power dissipation of the regulator circuit can
be determined using one simple equation.
EQUATION 4-2:
Substituting P
D(MAX)
for P
D
and solving for the
operating conditions that are critical to the application
will give the maximum operating conditions for the
regulator circuit. For example, if we are operating the
MIC5219-3.3YM5 at room temperature, with a
minimum footprint layout, we can determine the
maximum input voltage for a set output current.
EQUATION 4-3:
The thermal resistance, junction-to-ambient, for the
minimum footprint is 220°C/W, taken from Ta b le 4- 1 .
The maximum power dissipation number cannot be
exceeded for proper operation of the device. Using the
output voltage of 3.3V, and an output current of
150 mA, we can determine the maximum input voltage.
Ground current, maximum of 3 mA for 150 mA of
output current, can be taken from the Electrical
Characteristics section of the data sheet.
EQUATION 4-4:
Therefore, a 3.3V application at 150 mA of output
current can accept a maximum input voltage of 6.2V in
a SOT23-5 package. For a full discussion of heat
sinking and thermal effects on voltage regulators, refer
to the “Regulator Thermals” section of Microchip’s
Designing with Low-Dropout Voltage Regulators
handbook.
4.7 Peak Current Applications
The MIC5219 is designed for applications where high
start-up currents are demanded from space
constrained regulators. This device will deliver 500 mA
start-up current from a SOT23-5 or MSOP-8 package,
allowing high power from a very low profile device. The
MIC5219 can subsequently provide output current that
is only limited by the thermal characteristics of the
device. You can obtain higher continuous currents from
the device with the proper design. This is easily proved
with some thermal calculations.
If we look at a specific example, it may be easier to
follow. The MIC5219 can be used to provide up to
500 mA continuous output current. First, calculate the
maximum power dissipation of the device, as was done
in the Thermal Considerations section. Worst case
thermal resistance (θ
JA
= 220°C/W for the
MIC5219-x.xYM5), will be used for this example.
EQUATION 4-5:
Assuming a 25°C room temperature, we have a
maximum power dissipation value calculated in
Equation 4-6.
EQUATION 4-6:
Then we can determine the maximum input voltage for
a 5 volt regulator operating at 500 mA, using worst
case ground current.
P
D
V
IN
V
OUT
–I
OUT
 V
IN
+ I
GND
=
P
DMAX
125C 25C–
220C/W
-----------------------------------
455mW==
455mW V
IN
3.3V–150mA V
IN
+3mA=
455mW 150mA V
IN
 3mA+ V
IN
 495mW–=
950mW 153mA V
IN
=
V
IN MAX
6.2V
MAX
=
P
DMAX
T
JMAX
T
A
–

JA
--------------------------------
=
P
DMAX
125C 25C–
220C/W
-----------------------------------
455mW==