Micrel, Inc.

MIC5231

November 2008

6

M9999-110408-B

Application Information

Input Capacitor

A 0.1µF (or larger) capacitor should be placed from the

IN (supply input) to GND (ground) if there is more than

20cm of wire between IN and the ac filter capacitor or if

supplied from a battery.

Output Capacitors

The MIC5231 does not require an output capacitor for

stability. A 1µF or larger capacitor is recommended

between OUT (output) and GND to improve the

regulator’s transient response. A 0.1µF capacitor can be

used to reduce overshoot recovery time at the expense

of overshoot amplitude. The ESR (effective series

resistance) of this capacitor has no effect on regulator

stability, but low-ESR capacitors improve high frequency

transient response. The value of this capacitor may be

increased without limit, but values larger than 10µF tend

to increase the settling time after a step change in input

voltage or output current.

The MIC5231 has no minimum load current; it will

remain stable and in regulation with no load (other than

the internal voltage divider). This is especially important

in

real-time

clock

and

CMOS

RAM

keep-alive

applications.

Minimum Load Current

The MIC5231 does not require a minimum load for

proper operation. This allows the device to operate in

applications where very light output currents are required

for keep-alive purposes. This is important for powering

SRAM or Flash memory in low-power modes for

handheld devices.

Safe Operating Conditions

The MIC5231 does not incorporate current limit or

thermal shutdown in the design. The output pass

element is approximately 15Ω, therefore, when a short

occurs from the output to ground, the current is self-

limited. The pass element has a positive temperature

coefficient, such that when the device gets hot, the

output impedance goes up, limiting the current even

further. The maximum junction temperature for the

device is +125°C, and it is important that this is not

exceeded for any length of time.

Thermal Considerations

The MIC5231 is not intended for sourcing currents that

would cause a large power loss in the device, but since it

is not current limited, it is possible to source more than

the rated 10mA. At this point, it is important to ensure

that the die temperature does not exceed +125°C.

Power dissipation in the regulator is calculated as

follows:

(

)

GND

IN

OUT

OUT

IN

D

I

V

I

V

V

P

⋅

+

−

=

The MIC5231 consumes only 0.65µA over load and

does not need to consider that contribution in the power

dissipation equation, therefore the equation is simplified.

(

)

OUT

OUT

IN

D

I

V

V

P

−

=

The MIC5231, in the IttyBitty SOT-23-5 package, has a

thermal resistance, junction-to-ambient, of 235°C/W.

Using this number, the power dissipation capability of

that package, without exceeding a +125°C junction

temperature rating, can easily be calculated.

JA

A

J(MAX)

D(MAX)

θ

T

T

P

−

=

C/W

235

T

C

125

P

A

D(MAX)

°

−

°

=

If the device is being operated at +85°C, the maximum

power dissipation allowed can easily be determined.

C/W

235

C

85

C

125

°

°

−

°

=

170mW

Therefore, the device can only dissipate 170mW

maximum. If the MIC5231 is powered off of a 12V

source and the output voltage is 3.3V, the maximum

output current can be calculated.

(

)

OUT

OUT

IN

D(MAX)

I

V

V

P

−

=

(

)

OUT

I

3.3V

12V

170mW

−

=

8.7V

170mW

19.5mA

Therefore, the device can source almost 20mA at an

ambient of +85°C before the die temperature exceeds

+125°C.