Micrel, Inc.

MIC2298

October 2007

11

M9999-101707-B

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 MIC2298 is shown

in the typical characteristics sub-section. This graph

represents the maximum continuous output power

capability of the part when used in the evaluation board

layout. 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 second.

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 MIC2298. Figure 8 shows the

theoretical output current limit of the MIC2298 using the

Evaluation Board inductor value of 4.7µH.

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

(

)

DC

PK

PK

RMS

I

DC

I

I

+

⋅

=

−

This simplifies to

DC

I

I

PK

RMS

⋅

=

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 (Page 4, Max DC

LED

Current),

these

cannot

be

driven

continuously.

•

The time constant of the driver is < 3 seconds,

so one can look at the peak current capability of

the driver in Figure 8.

•

Looking at Figure 8, the MIC2298 can achieve

more than the required 800mA peak current at

8.4V.

•

Reading from the SOA curve in the typical

characteristics sub-section (Page 4, Max DC

LED Current), the MIC2298 at 3V, 50

8.4V output voltage, can provide 580mA RMS.

•

Now looking at the curve in Figure 9, using the

next lower value of 500mA RMS current, one

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.