AP1186
1.5A Ultra Low Dropout Positive Adjustable or Fixed-mode Regulator
Functional Descriptions(Continued)
Vout = Vref (1+R2/R1) + Iadj * R2 where : Vref = 1.25V & Iadj=50uA Typically
Vin
AP1186-ADJ
Vctrl
Adj
Iadj=50uA
Vsense
Vref
R1+
Vout
R2+
The AP1186-ADJ 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 ans 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.
Since the minimum load current requirement of the
AP1186-ADJ 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 AP1186 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 AP1186 with remote
sensing.
Vin
Vin
AP1186-ADJ
Vout
Vsense
R1
R2
R
L
Vctrl
Vctrl
Adj
Stability
The AP1186-XXX 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 AP1186-XXX 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
5/8
Rev.1.1 Sep.7, 2004
ANACHIP [ ANACHIP CORP ]
