Micrel, Inc.
Where the regulator is powered from a source with high
AC impedance, a 0.1µF capacitor connected between
Input and GND is recommended. This capacitor should
have good characteristics to above 250kHz.
MIC29510/29512
Where V
O
is the desired output voltage. Figure 4 shows
component definition.
Adjustable Regulator Design
Transient Response and 5V to 3.3V Conversion
The MIC29510/2 have excellent response to variations
in input voltage and load current. By virtue of their low
dropout voltage, these devices do not saturate into
dropout as readily as similar NPN-based designs. A 3.3V
output Micrel LDO will maintain full speed and
performance with an input supply as low as 4.2V, and
will still provide some regulation with supplies down to
3.8V, unlike NPN devices that require 5.1V or more for
good performance and become nothing more than a
resistor under 4.6V of input. Micrel’s PNP regulators
provide superior performance in “5V to 3.3V” conversion
applications, especially when all tolerances are
considered.
Adjustable Regulator Design
The adjustable regulator version, MIC29512, allows
programming the output voltage anywhere between
1.25V and the 16V maximum operating rating of the
family. Two resistors are used. Resistors can be quite
large, up to 100kΩ, because of the very high input
impedance and low bias current of the sense
comparator. The resistor values are calculated by:
⎞
⎛
V
R1
=
R2
× ⎜
OUT
−
1
⎟
⎟
⎜
1.240
⎠
⎝
Figure 4. Adjustable Regulator with Resistors
Enable Input
The MIC29512 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 microamperes 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.
Voltage
Standard (
Ω
)
R1
100k
100k
100k
100k
100k
100k
100k
93.1k
100k
100k
100k
100k
R2
76.8k
75.0k
69.8k
66.5k
64.9k
60.4k
56.2k
51.1k
52.3k
48.7k
45.3k
43.2k
2.85
2.9
3.0
3.1
3.15
3.3
3.45
3.525
3.6
3.8
4.0
4.1
Resistor Value Table for the MIC29512 Adjustable Regulator
May 2006
8
M9999-051706
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
