MIC2937A/29371/29372

Micrel, Inc.

MIC2937A/29371/29372

8

May 2006

Applications Information

External Capacitors

A10µF(orgreater)capacitorisrequiredbetweentheMIC2937A

outputandgroundtopreventoscillationsduetoinstability.Most

types of tantalum or aluminum electrolytics will be adequate;

film types will work, but are costly and therefore not recom-

mended. Many aluminum electrolytics have electrolytes that

freeze at about –30°C, so solid tantalums are recommended

for operation below –25°C. The important parameters of the

capacitor are an effective series resistance of about 5Ω or

less and a resonant frequency above 500kHz. The value of

this capacitor may be increased without limit.

At lower values of output current, less output capacitance is

required for output stability. The capacitor can be reduced to

0.5µF for current below 10mA or 0.15µF for currents below

1 mA. Adjusting the MIC29372 to voltages below 5V runs the

error amplifier at lower gains so that more output capacitance

is needed. For the worst-case situation of a 750mA load at

1.23V output (Output shorted to Adjust) a 22µF (or greater)

capacitor should be used.

TheMIC2937A/29371willremaininregulationwithaminimum

load of 5mA. When setting the output voltage of the MIC29372

versionwithexternalresistors,thecurrentthroughtheseresis-

tors may be included as a portion of the minimum load.

A 0.1µF capacitor should be placed from the input to ground

if there is more than 10 inches of wire between the input and

the AC filter capacitor or if a battery is used as the input.

Error Detection Comparator Output (MIC29371)

A logic low output will be produced by the comparator when-

ever the MIC29371 output falls out of regulation by more than

approximately 5%. This figure is the comparator’s built-in

offset of about 75mV divided by the 1.235V reference volt-

age. (Refer to the block diagram on Page 1). This trip level

remains “5% below normal” regardless of the programmed

output voltage of the MIC29371. For example, the error flag

trip level is typically 4.75V for a 5V output or 11.4V for a 12V

output. The out of regulation condition may be due either to

low input voltage,extremely high input voltage, current limit-

ing, or thermal limiting.

Figure 1 is a timing diagram depicting the ERROR signal

and the regulated output voltage as the MIC29371 input is

ramped up and down.The ERROR signal becomes valid (low)

at about 1.3V input. It goes high at about 5V input (the input

voltage at which V

voltage is load-dependent (see curve in Typical Performance

Characteristics), the input voltage trip point (about 5V) will

vary with the load current. The output voltage trip point (ap-

proximately 4.75V) does not vary with load.

* SEE APPLICATIONS INFORMATION

Figure 1. ERROR Output Timing

TheerrorcomparatorhasanNPNopen-collectoroutputwhich

requires an external pull-up resistor. Depending on system

requirements, this resistor may be returned to the 5V output

or some other supply voltage. In determining a value for this

resistor, note that while the output is rated to sink 250µA, this

sink current adds to battery drain in a low battery condition.

Suggested values range from 100k to 1MΩ. The resistor is

not required if this output is unused.

Programming the Output Voltage (MIC29372)

The MIC29372 may programmed for any output voltage be-

tween its 1.235V reference and its 26V maximum rating. An

external pair of resistors is required, as shown in Figure 3.

The complete equation for the output voltage is

V

where V

theAdjust pin bias current, nominally 20nA.The minimum rec-

ommended load current of 1µAforces an upper limit of 1.2MΩ

on the value of R

condition often found in CMOS in standby), I

a –2% typical error in V

temperature by trimming R

R

resistorprogramcurrentto12µA.SincetheMIC29372typically

draws 100µA at no load with SHUTDOWN open-circuited,

this is a negligible addition.

Reducing Output Noise

In reference applications it may be advantageous to reduce

the AC noise present at the output. One method is to reduce

the regulator bandwidth by increasing the size of the output

capacitor.Thisisrelativelyinefficient,asincreasingthecapaci-

tor from 1µF to 220µF only decreases the noise from 430µV

to 160µV

be reduced by a factor of four with the adjustable regulators

*

*

OUTPUT

VOLTAGE

INPUT

VOLTAGE

ERROR NOT

VALID

NOT

VALID

5V

1.3V

4.75V