Diodes AP1186 - Manual (Page 5)

AP1186
1.5A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE OR
FIXED-MODE REGULATOR
AP1186 Rev. 1 5 of 9 MARCH 2007
www.diodes.com © Diodes Incorporated
Functional Descriptions
Introduction
The AP1186 regulator is a 5-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 AP1186 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 AP1186-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 AP1186 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 AP1186-ADJ 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:
Vout = Vref (1+R2/R1) + Iadj * R2 where : Vref = 1.25V & Iadj=50uA Typically
AP1186-ADJ
Vin
Vctrl
Adj
Vout
Vsense
R1+
R2+
Vref
Iadj=50uA
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 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.
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.
AP1186-ADJ
Vin
Vctrl
Vin
Vctrl
Vout
Vsense
Adj
R1
R2
R
L