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Micrel, Inc.
Application Information
Grounding
Both the AGND and PGND must be connected to the
exposed backside pad. The exposed backside pad also
improves thermal performance. A large ground plane
decreases thermal resistance to ambient air.
Thermal Considerations and the SOA
The SOA (safe operating area) of the MIC2299 is shown
in the typical characteristics section. This graph
represents the maximum continuous output power
capability of the part when used on a minimal evaluation
board layout. This is a 2 layer board of 1 ounce copper,
utilizing the bottom layer as a ground plane heat sink.
The total area of the GND copper is approximately 1.3
square inches. This approaches a thermal resistance of
45 o C/W. An alternative layout with more copper area for
heat sinking will increase the area under the SOA curve.
Note that the SOA is for continuous power and not peak
power and is effectively a thermal limitation. The SOA is
true for a time constant of approximately >1 seconds.
Therefore, any load transient with a period of < 3s can
exceed the SOA curve power up to a maximum limited
by the current limit of the MIC2299. Figure 8 shows the
theoretical output current limit of the MIC2299 using the
Evaluation Board inductor value of 2.2μH with a DCR
50m ? .
MIC2299
This simplifies to I RMS = I PK ? DC when there is no DC
level.
The graph in Figure 9 shows the peak LED current
which can be pulsed at a given duty cycle (DC) to stay
within SOA limits of 400mA to 700mA.
Figure 9. Duty Cycle vs. Peak Current
for Fixed RMS Current
Example
Two series connected high brightness white LEDs with a
Vf max of 4.2V and peak current of 800mA require
pulses of 300ms at 3 second intervals. Power source is
a Li-ion cell of 3V min.
?
?
?
?
Looking at the SOA curves, this cannot be
driven continuously.
The time constant of the driver is <3 seconds, so
we can look at the peak current capability of the
driver in Figure 8.
Looking at Figure 8, the MIC2299 can achieve
more than the required 800mA peak current at
8.4V
Reading from the SOA curve in the typical
characteristics section, the MIC2299 at 3V, 50 o C
and 8.4V output voltage, can provide 580mA
RMS.
(
)
Figure 8. Peak Output Current vs. V OUT
If our load is within these limits, it is possible to drive the
load at some repetition rate or duty cycle (DC). This is
allowed as long as we limit the RMS current to below the
SOA limit.
The RMS current for a pulsed current is known to be
I RMS = I PK ? PK ? DC + I DC where the current pulse I PK-PK
sits on a DC level of I DC .
October 2007
12
?
Now looking at the curve in Figure 9, using the
next lower value of 500mA RMS current, we can
see that the 850mA peak can be driven at a duty
cycle of ~33% (or 1 second out of every 3
seconds). That is well within our target of
300ms.
M9999-101907-B
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