AP1187
1A Ultra Low Dropout Positive Adjustable Regulator
Function Description
Introduction
The AP1187 keeps a constant 1.25V between the
VSENSE pin and the ADJ pin. By placing a resistor R1
across these two pins and connecting the VSENSE
and VOUT pin together, a constant current flows
through R1, adding to the IADJ current and into the
R2 resistor producing a voltage equal to the
(1.25/R1)*R2 + IADJ*R2. This voltage is then added
to the 1.25V to set the output voltage. This is
summarized in the above equation.
The AP1187 regulator is a 7-terminal device
designed specifically to provide extremely low
dropout voltages comparable to the PNP type
without the disadvantage of the extra power
dissipation due to the base current associated with
PNP regulators. This is done by bringing out the
control pin of the regulator that provides the base
current to the power NPN and connecting it to a
voltage that is greater than the voltage present at
the VIN pin. This flexibility makes the AP1187 ideal
for applications where dual inputs are available
such as a computer motherboard with an ATX style
power supply that provides 5V and 3.3V to the
board. One such application is the new graphic chip
sets that require anywhere from 2.4V to 2.7V
supply. The AP1187-ADJ can easily be
programmed with the addition of two external
resistors to any voltages within the range of 1.25V
to 15.5V. Another major requirement of these
graphic chips is the need to switch the load current
from zero to several amps in tens of nanoseconds
at the processor pins, which translates to an
approximately 300 to 500ns of current step at the
regulator. In addition, the output voltage tolerances
are also extremely tight and they include the
transient response as part of the specification.
Since the minimum load current requirement of the
AP1187 is 10mA, R1 is typically selected to be a
121Ω resistor so that it automatically satisfies this
condition. Notice that since the IADJ is typically in the
range of 50uA it only adds a small error to the
output voltage and should be considered when very
precise output voltage setting is required.
Load Regulation
Since the AP1187 has separate pins for the output
(VOUT) and the sense (VSENSE), it is ideal for
providing true remote sensing of the output voltage
at the load. This means that the voltage drops due
to parasitic resistance such as PCB traces between
the regulator and the load are compensated for
using remote sensing. Figure following shows a
typical application of the AP1187 with remote
sensing.
The AP1187 is specifically designed to meet the
fast current transient needs as well as providing an
accurate initial voltage, reducing the overall system
cost with the need for fewer number of output
capacitors. Another feature of the device is its true
remote sensing capability that allows accurate
voltage setting at the load rather than at the device.
VOUT
VIN
VIN
AP1187-ADJ
ADJ
VSENSE
VCTRL
VCTRL
R1
R2
RL
Output Voltage Setting
Stability
The AP1187 can be programmed to any voltages in
the range of 1.25V to 15.5V with the addition of R1
and R2 external resistors according to the following
formula:
The AP1187 requires the use of an output capacitor
as part of the frequency compensation in order to
make the regulator stable. Typical designs for the
microprocessor
applications
use
standard
electrolytic capacitors with typical ESR in the range
of 50 to 100mΩ and an output capacitance of
100uF to 1000uF. Fortunately as the capacitance
increases, the ESR decreases resulting in a fixed
RC time constant. The AP1187 takes advantage of
the phenomena in making the overall regulator loop
stable.
VOUT=VREF(1+R2/R1)+IADJ*R2 where: VREF=1.25V & IADJ=50uA Typically
V
VOUT
IN
AP1187-ADJ
VSENSE
VREF
VCTRL
R1+
R2+
ADJ
IADJ=50uA
For most applications
a minimum of 100uF
aluminum electrolytic capacitor insures both
stability and good transient response.
Anachip Corp.
www.anachip.com.tw
Rev.1.1 Aug 23, 2005
4/6
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