Diodes AP2127 Manual do Utilizador descarregar pdf (Página 12)

AP2127

Document number: DS36478 Rev. 3 - 2

12 of 27

www.diodes.com

January 2014

P2127

Product Line o

Diodes Incorporated

Application Notes

Input Capacitor

A 1µF ceramic capacitor is recommended to connect between V

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

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:

⎟

⎠

⎞

⎜

⎝

⎛

REFOUT

1VV

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

REF

OUT

For AP2127, the resistor at the low side (R

) 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

R2π

0.7kHz ≤

××

≤

The recommended value of the feed-forward capacitor for different resistor divider ratios is shown in the table below.

Output Voltage R1 R2

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

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 26 27

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