Micrel, Inc.
MIC5219
Peak Current Applications
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
MIC5219-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” .)
TheMIC5219isdesignedforapplicationswherehighstart-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
profiledevice.TheMIC5219cansubsequentlyprovideoutput
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.
Theinformationusedtodeterminethesafeoperatingregions
can be obtained in a similar manner such as 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 multi-
plied 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.
If we look at a specific example, it may be easier to follow.
TheMIC5219canbeusedtoprovideupto500mAcontinuous
output current. First, calculate the maximum power dissipa-
tion of the device, as was done in the thermal considerations
section. Worst case thermal resistance (θ = 220°C/W for
the MIC5219-x.xBM5), will be used for this example.
JA
TJ (max ) − TA
(
)
PD (max ) =
θJA
Assuming a 25°C room temperature, we have a maximum
power dissipation number of
125 °C − 25°C
(
)
% DC
ꢀ 100 ꢀ
ꢀ
ꢀ
PD (max ) =
Avg.P = ꢀ
ꢀ V – V
I
+ V I
IN GND
(
)
D
IN
OUT
OUT
220 °C /W
P (max) = 455mW
D
% DC
ꢀ
ꢀ
ꢀ
455mW = ꢀ
ꢀ 100 ꢀ
(
8V – 5V 500mA + 8V × 20mA
)
Then we can determine the maximum input voltage for a
5-voltregulatoroperatingat500mA, usingworstcaseground
current.
% Duty Cycle
ꢀ
455mW = ꢀ
ꢀ 100
ꢀ
ꢀ1.66W
ꢀ
P (max) = 455mW = (V – V
) I
+ V I
IN GND
D
IN
OUT OUT
% Duty Cycle
100
I
= 500mA
= 5V
OUT
0.274 =
V
I
OUT
% Duty Cycle Max = 27.4%
= 20mA
GND
455mW = (V – 5V) 500mA + V × 20mA
With an output current of 500mAand a three-volt drop across
the MIC5219-xxBMM, the maximum duty cycle is 27.4%.
IN
IN
2.995W = 520mA × V
IN
Applications also call for a set nominal current output with a
greateramountofcurrentneededforshortdurations.Thisisa
trickysituation,butitiseasilyremedied.Calculatetheaverage
power dissipation for each current section, then add the two
numbers giving the total power dissipation for the regulator.
For example, if the regulator is operating normally at 50mA,
but for 12.5% of the time it operates at 500mA output, the
total power dissipation of the part can be easily determined.
First, calculate the power dissipation of the device at 50mA.
We will use the MIC5219-3.3BM5 with 5V input voltage as
our example.
2.955W
520mA
VIN(max ) =
= 5.683V
Therefore, to be able to obtain a constant 500mAoutput cur-
rent from the 5219-5.0BM5 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 en-
able pin can increase the power dissipation of the device by
maintaining a lower average power figure. This is ideal for
applicationswherehighcurrentisonlyneededinshortbursts.
Figure1showsthesafeoperatingregionsfortheMIC5219-x.
xBM5 at three different ambient temperatures and at differ-
ent 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.A1" square cop-
per trace on the PC board reduces the thermal resistance of
thedevice.Thisimprovedthermalresistanceimprovespower
dissipation and allows for a larger safe operating region.
P × 50mA = (5V – 3.3V) × 50mA + 5V × 650µA
D
P × 50mA = 173mW
D
However, this is continuous power dissipation, the actual
on-time for the device at 50mA is (100%-12.5%) or 87.5%
of the time, or 87.5% duty cycle. Therefore, P must be mul-
D
tiplied by the duty cycle to obtain the actual average power
dissipation at 50mA.
Figures3and4showsafeoperatingregionsfortheMIC5219-x.
June 2009
10
M0371-061809
MICREL [ MICREL SEMICONDUCTOR ]
