MIC2920A/29201/29202/29204

Micrel, Inc.

M9999-021505

8

February 2005

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

(MIC29202/29204)

The MIC29202/29204 may be programmed for any output

voltage between its 1.235V reference and its 26V maxi-

mum rating, using an external pair of resistors, 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

2

=100kreducesthiserrorto0.17%whileincreasingtheresistor

program current to 12µA. Since the MIC29202/29204 typically

draws 110µAat no load with SHUTDOWN open-circuited, this

is a negligible addition. The MIC29204 may be pin-strapped

for 5V using the internal voltage divider by tying Pin 1 (output)

to Pin 2 (sense) and Pin 7 (Adjust) to Pin 6 (V Tap).

Configuring the MIC29201-3.3BM

For the MIC29201-3.3BM, the output (Pin 1) and sense pin

(pin 2), must be connected to ensure proper operation. They

are not connected internally.

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-

Figure 1. ERROR Output Timing

* SEE APPLICATIONS INFORMATION

Applications Information

External Capacitors

A10µF(orgreater)capacitorisrequiredbetweentheMIC2920A

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 2.2µF for current below 10mA or 1µF for currents below

1mA. Adjusting the MIC29202/29204 to voltages below 5V

runs the error amplifier at lower gains so that more output ca-

pacitance is needed. For the worst-case situation of a 500mA

load at 1.23V output (Output shorted to Adjust) a 47µF (or

greater) capacitor should be used.

The MIC2920A/29201 will remain in regulation with a mini-

mum load of 1mA. When setting the output voltage of the

MIC29202/29204 versions with external resistors, the current

through these resistors may be included as a portion of the

minimum load.

A 0.1µF capacitor should be placed from the MIC2920A 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

(MIC29201/MIC29204)

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

ever the MIC29201/29204 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

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

remains “5% below normal” regardless of the programmed

output voltage of the MIC29201/29204. 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, cur-

rent limiting, or thermal limiting.

Figure 1 is a timing diagram depicting the ERROR signal and

the regulated output voltage as the MIC29201/29204 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

dropout voltage is load-dependent (see curve in Typical Per-

formance Characteristics), the input voltage trip point (about

5V) will vary with the load current. The output voltage trip point

(approximately 4.75V) does not vary with load.

The error comparator has an NPN open-collector outputwhich

requires an external pull-up resistor. Depending on system

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

4.75V

E R R O R

OUTPUT

VOLTAGE

INPUT

5V

1.3V

VOLTAGE

NOT *

VALID

NOT *

VALID