Micrel, Inc.
MIC29150/29300/29500/29750
Adjustable Regulator Design
Capacitor Requirements
For stability and minimum output noise, a capacitor on
the regulator output is necessary. The value of this
capacitor is dependent upon the output current; lower
currents
allow
smaller
capacitors.
The
MIC29152BT
MIC29150/29300/29500/29750 regulators are stable
with the following minimum capacitor values at full load:
Device
Full Load Capacitor
MIC29150...............................................10µF
MIC29300...............................................10µF
MIC29500...............................................10µF
MIC29750...............................................22µF
V
V
OUT
IN
R1
R2
This capacitor need not be an expensive low ESR type:
aluminum electrolytics are adequate. In fact, extremely
low ESR capacitors may contribute to instability.
Tantalum capacitors are recommended for systems
where fast load transient response is important.
10µF
22µF
Figure 4. Adjustable Regulator with Resistors
Where the regulator is powered from a source with a
high AC impedance, a 0.1µF capacitor connected
between Input and GND is recommended. This capacitor
should have good characteristics to above 250kHz.
The adjustable regulator versions, MIC29xx2 and
MIC29xx3, allow programming the output voltage
anywhere between 1.25V and the 25V. Two resistors are
used. The resistor values are calculated by:
Minimum Load Current
V
The MIC29150–29750 regulators are specified between
finite loads. If the output current is too small, leakage
currents dominate and the output voltage rises. The
following minimum load current swamps any expected
leakage current across the operating temperature range:
⎛
⎞
OUT
R1 = R2 × ⎜
− 1⎟
⎜
⎟
1.240
⎝
⎠
where VOUT 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
“Minimum Load Current” section).
Device
Minimum Load
MIC29150................................................5mA
MIC29300................................................7mA
MIC29500..............................................10mA
MIC29750..............................................10mA
Error Flag
MIC29xx1 and MIC29xx3 versions feature an Error Flag,
which looks at the output voltage and signals an error
condition when this voltage drops 5% below its expected
value. The error flag is an open-collector output that
pulls low under fault conditions. It may sink 10mA. Low
output voltage signifies a number of possible problems,
including an overcurrent fault (the device is in current
limit) and low input voltage. The flag output is inoperative
during overtemperature shutdown conditions.
Enable Input
MIC29xx1 and MIC29xx2 versions feature an enable
(EN) input that allows ON/OFF control of the device.
Special design allows “zero” current drain when the
device is disabled—only microamperes of leakage
current flows. The EN input has TTL/CMOS compatible
thresholds for simple interfacing with logic, or may be
directly tied to ≤30V. Enabling the regulator requires
approximately 20µA of current.
M9999-013112-B
January 2012
18
MICREL [ MICREL SEMICONDUCTOR ]
