AP1187
1A Ultra Low Dropout Positive Adjustable Regulator
Features
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General Description
AP1187 is a 1A regulator with extremely low
dropout voltage. This product is specifically
designed to provide well regulated supply for
applications requiring 2.8V or lower voltages from
3.3V ATX power supplies where high efficiency of a
switcher can be achieved without the cost and
complexity associated with switching regulator. One
such application is the new graphic chip sets that
require anywhere from 2.4V to 2.7V supply.
Adjustable output voltage
0.6V Maximum Dropout voltage at 1.0A Load
current
Built-in Thermal Shutdown
Output Current Limiting
Fast transient response
Good noise rejection
Package: SOP-7L
Applications
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PC Motherboard
LCD Monitor
Graphic Card
DVD-Video Player
NIC/Switch
Telecom Equipment
ADSL Modem
Printer and other Peripheral Equipment
Pin Assignments
Pin Descriptions
Name
ADJ
Description
A resistor divider from this pin to the V
OUT
pin and ground sets the output voltage.
This pin is the positive side of the
reference that allows remote load
sensing to achieve excellent load
regulation. A minimum of 10uF capacitor
must be connected from this pin to
ground to insure stability.
The input of the regulator.
This pin is the supply pin for the internal
control circuit as well as the base drive
for the pass transistor.
Not used.
The output of the regulator.
( Top View )
ADJ 1
V
SENSE
2
AP1187
V
IN
3
V
CTRL
4
SOP-7L
7 NC
6 V
OUT
V
SENSE
5 V
IN
V
IN
V
CTRL
NC
V
OUT
This datasheet contains new product information. Anachip Corp. reserves the rights to modify the product specification without notice. No liability is assumed as a result of the use of
this product. No rights under any patent accompany the sale of the product.
Rev.1.1
1/6
Aug 23, 2005
AP1187
1A Ultra Low Dropout Positive Adjustable Regulator
Function Description
Introduction
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.
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.
Output Voltage Setting
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:
V
OUT
=V
REF
(1+R2/R1)+I
ADJ
*R2 where: V
REF
=1.25V & I
ADJ
=50uA Typically
V
IN
AP1187-ADJ
V
CTRL
ADJ
I
ADJ
=50uA
V
SENSE
V
REF
R1+
V
OUT
The AP1187 keeps a constant 1.25V between the
V
SENSE
pin and the ADJ pin. By placing a resistor R1
across these two pins and connecting the V
SENSE
and V
OUT
pin together, a constant current flows
through R1, adding to the I
ADJ
current and into the
R2 resistor producing a voltage equal to the
(1.25/R1)*R2 + I
ADJ
*R2. This voltage is then added
to the 1.25V to set the output voltage. This is
summarized in the above equation.
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 I
ADJ
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
(V
OUT
) and the sense (V
SENSE
), 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.
V
IN
V
CTRL
V
IN
AP1187-ADJ
V
CTRL
V
SENSE
ADJ
R1
R2
R
L
V
OUT
R2+
Stability
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.
For most applications a minimum of 100uF
aluminum electrolytic capacitor insures both
stability and good transient response.
Anachip Corp.
www.anachip.com.tw
4/6
Rev.1.1 Aug 23, 2005
AP1187
1A Ultra Low Dropout Positive Adjustable Regulator
Function Description (Continued)
Thermal Design
The AP1187 incorporates an internal thermal
shutdown that protects the device when the junction
temperature exceeds the allowable maximum
junction temperature. Although this device can
operate with junction temperatures in the range of
150℃, it is recommended that the selected heat
sink be chosen such that during maximum
continuous load operation the junction temperature
is kept below this number. The example below
shows the steps in selecting the proper surface
mount package.
Package
SOP7
Copper Area
1.0” X 1.0”
Assuming, the following conditions:
V
OUT
=2.5V
V
IN
=3.3V
V
CTRL
=5V
I
OUT
=1A DC Avg.
Calculate the maximum power dissipation using the
following equation:
P
D
= I
OUT
*(V
IN
-V
OUT
) + (I
OUT
/60) * (V
CTRL
- V
OUT
)
P
D
=1*(3.3-2.5) + (1/60)*(5-2.5) =0.84W
Using table below select the proper package and
the amount of copper board needed.
Max Pd (Ta=25
o
C)
1.7W
Max Pd (Ta=45
o
C)
1.4W
Θ
JA
(
o
C
/ W
)
65
o
Note: Above table is based on the maximum junction temperature of 135 C.
As shown in the above table, any of the two packages will do the job. For low cost applications the SOP7 package is
recommended.
Marking Information
(Top View)
7
5
Logo
Part number
AP1187
YY WW X X
Blank: normal
L: Lead Free Package
ID code: internal
Xth week: 01~52
Year: "01" =2001
"02" =2002
~
1
4
SOP-7L
Anachip Corp.
www.anachip.com.tw
5/6
Rev.1.1 Aug 23, 2005
Analogue and Mixed Signal
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