MIC29310/29312
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
Micrel
current of the sense comparator. The resistor values are
calculated by:
V
OUT
R1 = R2
×
( –—— – 1 )
1.240
Where V
O
is the desired output voltage. Figure 4 shows
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
IN
. Enabling the regulator
requires approximately 20µA of current into the EN pin.
MIC29312BT
VIN
4.75V to 5.25V
10µF
R1
100k
22µF
R2
56.2k
VOUT
3.45V
VOUT = 1.240V
×
[1 + (R1 / R2)]
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 1MΩ,
because of the very high input impedance and low bias
Resistor Value Table for the MIC29312 Adjustable Regulator
Voltage
2.85
2.9
3.0
3.1
3.15
3.3
3.45
3.6
3.8
4.0
4.1
Standard (
Ω
)
R1
100k
100k
100k
100k
100k
100k
100k
100k
100k
100k
100k
R2
76.8k
75.0k
69.8k
66.5k
64.9k
60.4k
56.2k
52.3k
48.7k
45.3k
43.2k
Min. Load (
Ω
)
R1
162
165
174
187
191
205
221
237
255
274
287
R2
124
124
124
124
124
124
124
124
124
124
124
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).
MIC29310/29312
8
July 1999
MICREL [ MICREL SEMICONDUCTOR ]
