June 2000

5

MIC5231

MIC5231

Micrel

Applications Information

Input Capacitor

A 0.1

µF (or larger) capacitor should be placed from the IN

(supply input) to GND (ground) if there is more than 20 cm 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 stabil-

ity. A 1

µF or larger capacitor is recommended between OUT

(output) and GND to improve the regulator’s transient re-

sponse. A 0.1

µF 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 capaci-

tor may be increased without limit, but values larger than

10

µF 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 approxi-

mately 15 ohms, 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:

P

The MIC5231 consumes only 0.65uA over load and does not

need to consider that contribution in the power dissipation

equation, therefore the equation is simplified.

P

The MIC5231, in the IttyBitty SOT23-5 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.

P

P

If the device is being operated at 85

°C, the maximum power

Block Diagram

IN

OUT

GND

EN

MIC5231

Bandgap

Reference