AS1322 datasheet(11/18 Pages) AMSCO

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AS1322

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

9.1 Output Voltage Ripple

The AS1322 is designed to work at high efficiency. In order to reduce the output ripple the following improvements are recommended:

Use a higher output capacitor, up to 44µF and a higher input capacitor (22µF).

Use smaller values for the resistor divider. R1 should be about 300k

Ω. To avoid a high leakage current from pin VOUT through the resistor

divider to GND, R1 should not be less than 100k

Ω.

To reduce the output ripple it’s also possible to speed up the feedback loop. To achieve this, place a 22pF (C4 in Figure 20) capacitor in

parallel to R1. Via C4 the fast transients are shorted to the FB pin and the feedback loop is even faster. A 1M

Ω resistor for R1 slows down

the FB loop.

Due to noise and to their non linear behavior, the use of potentiometers is not recommended.

Figure 20. AS1322 - Typical Application for lower Output Voltage Ripple

Note:

For correct measurements of the output ripple connect the oscilloscope probe as close as possible to the positive plate of the COUT

and connect the GND of the oscilloscope probe to the negative plate of the COUT. This will reduce the inductive coupling and will

deliver a more accurate measurement result.

The output ripple is getting higher as VIN is getting closer to VOUT. Figure 21 shows that the above mentioned improvements reduce the output

voltage ripple. If VIN is higher than VOUT the AS1322 stops switching and VIN is connected to VOUT via the inductor and the internal P-FET.

Figure 21. Output Voltage Ripple vs. Input Voltage; VOUT = 2.8V, IOUT = 0.8mA

AS1322

22µF

196k

Ω

250k

Ω

Off

4.7µH

2xAA Battery

22µF

VOUT = 2.8V

GND

SHDNN

VOUT

22µF

22pF

VIN

100

125

0.5

1.5

2.5

3.5

Input Voltage (V)

Cout = 44µF

Cout = 66µF

Cout = 44µF + C4 = 22pF

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