Micrel, Inc. MIC37252
February 2006
7
M9999-021606
(408) 955-1690
Transient Response and 5V to 3.3V or 3.3V to
1.8VConversions
The MIC37252 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 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 1.8V, the NPN-
based regulators are already operating in dropout, with
typical dropout requirements of 2V or greater. To convert
down to1.8V without operating in dropout, NPN-based
regulators require an input voltage of 3.8V at the very
least. The MIC37252 regulator will provide excellent
performance with an input as low as 3.0V. This gives the
PNP-based regulators a distinct advantage over older
NPN-based linear regulators.
Minimum Load Current
The MIC37252 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.
Enable Input
The MIC37252 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 VIN. When enabled, it
draws approximately 15µA.
Adjustable Regulator Design
Figure 2. Adjustable Regulator with Resistors
The MIC37252 allows programming of the output voltage
anywhere between 1.24V and the 5.5V maximum
operating rating of the family. Two resistors are used.
Resistors values are recommended to be less than 10kΩ
in value to improve stability. The resistor values can be
sized to draw the mini-mum load current for this device.
The resistor values are calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−= 1
1.240
V
R2R1
OUT
Where V
OUT
is the desired output voltage. Figure 2
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).
