Micrel, Inc. MIC37150/51/52/53
October 2007 10
M9999-101907
load current variations and input voltage variations.
Large output capacitors are not required to obtain this
performance. A standard 10µF output capacitor,
preferably tantalum, is all that is required. Larger values
help to improve performance even further.
By virtue of its low dropout voltage, this device does not
saturate into dropout as readily as similar NPN-based
designs. When converting from 3.3V to 2.5V, 2.5V to
1.8V or 1.65V, or 2.5V to 1.5V, the NPN-based
regulators are already operating in dropout, with typical
dropout requirements of 1.2V or greater. To convert
down to 2.5V without operating in dropout, NPN-based
regulators require an input voltage of 3.7V at the very
least. The MIC37150/51/52/53 regulator will provide
excellent performance with an input as low as 3.0V or
2.25V, respectively. This gives the PNP-based
regulators a distinct advantage over older, NPN-based
linear regulators.
Minimum Load Current
The MIC37150/51/52/53 regulator is specified between
finite 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 operation.
Error Flag
The MIC37151 and MIC37153 feature an error flag
circuit that monitors the output voltage and signals an
error condition when the voltage 5% below the nominal
output voltage. The error flag is an open-collector output
that can sink 10mA during a fault condition.
Low output voltage can be caused by a number of
problems, including an overcurrent fault (device in
current limit) or low input voltage. The flag is inoperative
during overtemperature shutdown.
Enable Input
The MIC37151/37152/37153 also features an enable
input for on/off control of the device. Its shutdown state
draws “zero” current (only microamperes of leakage).
The enable input is TTL/CMOS compatible for simple
logic interface, but can be connected to up to V
IN
. When
enabled, it draws approximately 15µA.
Adjustable Regulator Design
IN
R1
V
OUT
V
IN
C
OUT
R2
EN
OUT
ADJ
GND
MIC37152
ENABLE
SHUTDOWN
V
OUT
= 1.240V (1 + )
R1
R2
Figure 1. Adjustable Regulator with Resistors
The MIC37152 and MIC37153 allow programming the
output voltage anywhere between 1.24V and the 5.5V
maximum operating rating of the family. Two resistors
are used. Resistors can be quite large, up to 1MΩ,
because of the very high input impedance and low bias
current of the sense comparator: The resistor values are
calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−= 1
1.240
V
R2R1
OUT
Where V
OUT
is the desired output voltage. Figure 1
shows component definition. Applications with widely
varying load currents may scale the resistors to draw the
minimum load current required for proper operation (see
above).
