Micrel, Inc. MIC5216
March 2007
9
M9999-032307
V
IN
= 6.2V
MAX
Therefore, a 3.3V application at 150mA of output current
can accept a maximum input voltage of 6.2V in a SOT-
23-5 package. For a full discussion of heat sinking and
thermal effects on voltage regulators, refer to the
Regulator Thermals section of Micrel’s
Designing with
Low-Dropout Voltage Regulators
handbook.
Peak Current Applications
The MIC5216 is designed for applications where high
start-up currents are demanded from space constrained
regulators. This device will deliver 500mA start-up
current from a SOT-23-5 or MM8 package, allowing high
power from a very low profile device. The MIC5216 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 MIC5216 can be used to provide up to
500mA 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 MIC5216-
x.xBM5), will be used for this example.
JA
AJ(MAX)
D(MAX)
θ
TT
P
−
=
Assuming room temperature, we have a maximum
power dissipation number of
()
C/W220
C25C125
P
D(MAX)
°
°−°
=
P
D(MAX)
= 455mW
Then we can determine the maximum input voltage for a
five-volt regulator operating at 500mA, using worst case
ground current.
P
D(MAX)
= 455mW = (V
IN
– V
OUT
) I
OUT
+ V
IN
I
GND
I
OUT
= 500mA
V
OUT
= 5V
I
GND
=20mA
455mW = (V
IN
– 5V) 500mA + V
IN
× 20mA
2.995mW = 520mA × V
IN
5.683V
520mA
2.955W
V
IN(MAX)
==
Therefore, to be able to obtain a constant 500mA output
current from the 5216-5.0BM5 at room temperature, you
need extremely tight input-output voltage differential,
barely above the maximum dropout voltage for that
current rating.
You can run the part from larger supply voltages if the
proper precautions are taken. Varying the duty cycle
using the enable pin can increase the power dissipation
of the device by maintaining a lower average power
figure. This is ideal for applications where high current is
only needed in short bursts. Figure 1 shows the safe
operating regions for the MIC5216-x.xBM5 at three
different ambient temperatures and at different output
currents. The data used to determine this figure
assumed a minimum footprint PCB design for minimum
heat sinking. Figure 2 incorporates the same factors as
the first figure, but assumes a much better heat sink. A
1” square copper trace on the PC board reduces the
thermal resistance of the device. This improved thermal
resistance improves power dissipation and allows for a
larger safe operating region.
Figures 3 and 4 show, safe operating regions for the
MIC5216-x.xBMM, the power MSOP package part.
These graphs show three typical operating regions at
different temperatures. The lower the temperature, the
larger the operating region. The graphs were obtained in
a similar way to the graphs for the MIC5216-x.xBM5,
taking all factors into consideration and using two
different board layouts, minimum footprint and 1” square
copper PC board heat sink. (For further discussion of PC
board heat sink characteristics, refer to Application Hint
17, “Designing PC Board Heat Sinks”.
The information used to determine the safe operating
regions can be obtained in a similar manner to that used
in determining typical power dissipation, already
discussed. Determining the maximum power dissipation
based on the layout is the first step, this is done in the
same manner as in the previous two sections. Then, a
larger power dissipation number multiplied by a set
maximum duty cycle would give that maximum power
dissipation number for the layout. This is best shown
through an example. If the application calls for 5V at
500mA for short pulses, but the only supply voltage
available is 8V, then the duty cycle has to be adjusted to
determine an average power that does not exceed the
maximum power dissipation for the layout.
()
GNDINOUTOUTIND
I VI VV
100
%DC
Avg.P +−
⎟
⎠
⎞
⎜
⎝
⎛
=
()
20mA V 8500mA V58V
100
%DC
455mW ×+−
⎟
⎠
⎞
⎜
⎝
⎛
=
