MIC29310/29312

Micrel

MIC29310/29312

8

July 1999

loads. If the output current is too small, leakage currents

dominate and the output voltage rises. A 10mA minimum

load current is necessary for proper regulation.

Adjustable Regulator Design

R1

100k

R2

56.2k

VOUT = 1.240V × [1 + (R1 / R2)]

VIN

4.75V to 5.25V

VOUT

3.45V

MIC29312BT

22µF

10µF

Figure 4. Adjustable Regulator with Resistors

The adjustable regulator version, MIC29312, allows pro-

gramming the output voltage anywhere between 1.25V and

the 15V maximum operating rating of the family. Two resis-

tors are used. Resistors can be quite large, up to 1M

Ω,

because of the very high input impedance and low bias

current of the sense comparator. The resistor values are

calculated by:

V

OUT

R1 = R2

× ( –—— – 1 )

1.240

Where V

component definition. Applications with widely varying load

currents may scale the resistors to draw the minimum load

current required for proper operation (see the table below).

Enable Input

The MIC29312 version features an enable (EN) input that

allows ON/OFF control of the device. Special design allows

“zero” current drain when the device is disabled—only micro-

amperes of leakage current flows. The EN input has TTL/

CMOS compatible thresholds for simple interfacing with

logic, or may be directly tied to V

requires approximately 20

µA of current into the EN pin.

Resistor Value Table for the MIC29312 Adjustable Regulator

Note: This regulator has a minimum load requirement. “Standard” values assume the load meets this requirement. “Minimum

Load” values are calculated to draw 10mA and allow regulation with an open load (the minimum current drawn from the load

may be zero).

Voltage

Standard (

Min. Load (

R1

R2

R1

R2

2.85

100k

76.8k

162

124

2.9

100k

75.0k

165

124

3.0

100k

69.8k

174

124

3.1

100k

66.5k

187

124

3.15

100k

64.9k

191

124

3.3

100k

60.4k

205

124

3.45

100k

56.2k

221

124

3.6

100k

52.3k

237

124

3.8

100k

48.7k

255

124

4.0

100k

45.3k

274

124

4.1

100k

43.2k

287

124