Micrel, Inc.
Transient Response and 3.3V to 2.5V, 2.5V to 1.8V
or 1.65V, or 2.5V to 1.5V Conversions
The MIC37300/01/02/03 has excellent transient
response to variations in input voltage and load
current. The device has been designed to respond
quickly to load current variations and input voltage
variations. Large output capacitors are not required to
obtain this performance. A standard 47µF 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 MIC37300/01/02/03 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 MIC37300/01/02/03 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 MIC37301 and MIC37303 feature an error
flag
circuit that monitors the output voltage and signals an
error condition when the voltage is 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.
MIC37300/01/02/03
Enable Input
The MIC37301/02/03 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 up to VIN. When
enabled, it draws approximately 15µA.
Adjustable Regulator Design
MIC37302
V
IN
ENABLE
SHUTDOWN
IN
EN
OUT
R1
ADJ
GND
R2
V
OUT
C
OUT
R1
⎞
⎛
V
OUT
= 1.240V
⎜
1+
⎟
⎝
R2
⎠
Figure 1. Adjustable Regulator with Resistors
The MIC37302 and MIC37303 allow programming the
output voltage any-where 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:
⎞
⎛
V
R1
=
R2
⎜
OUT
−
1
⎟
⎟
⎜
1.240
⎠
⎝
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).
October 2009
10
M9999-102909
MICREL [ MICREL SEMICONDUCTOR ]
