Ap2127 – Diodes AP2127 User Manual
Page 12
AP2127
Document number: DS36478 Rev. 3 - 2
12 of 27
January 2014
© Diodes Incorporated
AP2127
A Product Line of
Diodes Incorporated
Application Notes
Input Capacitor
A 1µF ceramic capacitor is recommended to connect between V
IN
and GND pins to decouple input power supply glitch and noise. The amount of
the capacitance may be increased without limit. This input capacitor must be located as close as possible to the device to assure input stability
and less noise. For PCB layout, a wide copper trace is required for both V
IN
and GND.
Output Capacitor
The output capacitor is required to stabilize and help transient response for LDO. The AP2127 is stable with very small ceramic output capacitor
with a low ESR 1µF or higher of X7R or X5R MLCC capacitor, which will be sufficient at full temperature ranges. Additional capacitance helps to
reduce undershoot and overshoot during transient. Place output capacitor as close as possible to V
OUT
and GND pins, and keep the leads as short
as possible.
Adjustable Operation
For adjustable version, the output voltage is calculated by:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
2
1
REF
OUT
R
R
1
V
V
Where V
REF
= 0.8V (the internal reference voltage)
Rearranging the equation will give the following that is used for adjusting the output to a particular voltage:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
1
V
V
R
R
REF
OUT
2
1
For AP2127, the resistor at the low side (R
2
) can be selected from 5kΩ to 200kΩ.
In order to improve the stability and to decrease the noise level of the adjustable version, a feed-forward capacitor is suggested to be placed
between V
OUT
and ADJ pins (Figure 1). It’s recommended that this feed-forward capacitor value can be calculated as:
15kHz
ff
C
1
R
2π
1
0.7kHz
≤
Ч
Ч
≤
The recommended value of the feed-forward capacitor for different resistor divider ratios is shown in the table below.
Output Voltage
R1
R2
C
ff
1.2V 7.5kΩ 15kΩ 2.7nF
1.6V 7.5kΩ 7.5kΩ 2.7nF
1.8V 22.5kΩ 18kΩ 1nF
1.9V 7.5kΩ 5.49kΩ 2.7nF
2.5V 38.3kΩ 18kΩ 560pF
3.3V 56.2kΩ 18kΩ 390pF
4.0V 120kΩ 30.1kΩ 180pF
Table 1 Output Voltage Setting Guide