AS7620 datasheet(9/16 Pages) AMSCO

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AS7620

Datasheet - D e t a i l e d D e s c r i p t i o n

8.4 VOUT Selection

The AS7620-B features a 3.3V fixed output voltage. The AS7620-A provides an adjustable output from 1.2V (VREF) up to virtually VIN (see

100% Duty Cycle Operation on page 11). To select the desired VOUT, the related resistor divider has to be tuned according to the following

formula:

(EQ 1)

Where:

Rh is the high side resistor of the output divider

Rl is the resistor of the output divider

Note:

It is suggested to select resistors in the range of hundreds of k

Ω in order to limit the current consumption.

8.5 Current Limit

The current is sensed during the on time of the internal PMOS. Three different current limit thresholds can be selected by the ILIM pin:

1. 240mA (typ) ILIM shorted to GND

2. 720mA (typ) ILIM shorted to VOUT (from 1.5V to 3.6V)

3. 1000mA (typ) ILIM floating

This threshold is intended as peak current limit. If the current reaches the threshold during the on time, the PMOS is turned off and it will be

turned on again only when the current approaches 0A and the feedback voltage is equal or lower than VREF. The maximum output current is

ILIM/2.

8.6 Switching Frequency

The switching frequency (fsw) changes according to the application conditions and, in particular, to the output current in order to optimize the

efficiency in any load condition. Anyway, it is always possible to estimate the fsw during the design process. As described in the Regulation (refer

to page 8) – the converter always works in DCM and, normally, the peak current into the inductor is the CL threshold (ILIM). The average of the

inductor current must be equal to the output current. The following formula provides the relationship between inductor current and output current:

IOUT =

(EQ 2)

Consequently, the fsw can be expressed as following:

(EQ 3)

Figure 18. Chart Illustrating the Fsw vs. IOUT in a Standard Application (VIN=24V, VOUT=3.3V, L=10µH, ILIM=1A)

OUT

REF

-------









•

Tsw

----------

---

ILIM

•

OUT

–

(

) V

OUT

•

--------------------------------------------------

•

Fsw

OUT

•

OUT

–

(

) V

OUT

•

ILIM

•

----------------------------------------------------------------------------

Fsw vs. Iout

100

150

200

250

300

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Iout (A)

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