MIC2937A/29371/29372

Micrel

October 1999

25

MIC2937A/29371/29372

Applications Information

External Capacitors

A 10

µF (or greater) capacitor is required between the

MIC2937A output and ground to prevent oscillations due to

instability. Most types of tantalum or aluminum electrolytics

will be adequate; film types will work, but are costly and

therefore not recommended. Many aluminum electrolytics

have electrolytes that freeze at about –30

°C,sosolidtantalums

are recommended for operation below –25

°C. The important

parameters of the capacitor are an effective series resistance

of about 5

Ω 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

0.5

µF 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 22

µF (or greater)

capacitor should be used.

The MIC2937A/29371 will remain in regulation with a minimum

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

version with external resistors, the current through these

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

A 0.1

µF 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 whenever

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 voltage.

(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 limiting, 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

OUT

= 4.75). Since the MIC29371’s dropout

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

(approximately 4.75V) does not vary with load.

* SEE APPLICATIONS INFORMATION

*

*

OUTPUT

VOLTAGE

INPUT

VOLTAGE

ERROR

NOT

VALID

NOT

VALID

5V

1.3V

4.75V

Figure 1. ERROR Output Timing

The error comparator has an NPN open-collector output

which 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

250

µA, this sink current adds to battery drain in a low battery

condition. Suggested values range from 100k to 1M

Ω. The

resistor is not required if this output is unused.

Programming the Output Voltage (MIC29372)

The MIC29372 may programmed for any output voltage

between 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

REF

x { 1 + R

–

R

1

where V

REF

is the nominal 1.235 reference voltage and I

FB

is

the Adjust pin bias current, nominally 20nA. The minimum

recommended load current of 1

µA forces an upper limit of

1.2M

Ω on the value of R

2

, if the regulator must work with no

load (a condition often found in CMOS in standby), I

FB

will

produce a –2% typical error in V

OUT

which may be eliminated

at room temperature by trimming R

choosing R

2

= 100k reduces this error to 0.17% while increasing

the resistor program current to 12

µA. Since the MIC29372

typically draws 100

µA 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. This is relatively inefficient, as increasing the

capacitor from 1

µF to 220µF only decreases the noise from

430

µV to 160µV

Noise can be reduced by a factor of four with the adjustable