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AS7620

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

9 Application Information

9.1 Input Capacitors

An input capacitor is required to sustain the peak current requested by the turning on of the internal PMOS. When used, the capacitor helps to

reduce the noise and saves the input battery life. The input capacitor has to withstand the input RMS current, which can be calculated by the

following formula:

(EQ 4)

OSCON, polymer, aluminum or MLCCs. Tantalum types are not recommended for their weakness in withstanding big inrush currents.

9.2 Output Inductor

The inductor together with the output capacitor represents the output filter. Using the AS7620, the inductor is charged and completely discharged

at every switching cycle being that the converter is forced to work in DCM. Values from 10µH to 100µH are suitable to work with AS7620 and its

selection should consider the following statements:

Bigger inductor implies lower bandwidth

The inductor must be rated to withstand the peak current (ILIM) and the RMS current

9.3 Output Capacitor

The output capacitor together with the inductor represents the output filter. The bigger the capacitance is, the lower will be the output ripple.

Usually, low ESR MLCCs are preferred as they are inexpensive and small in size. Any value from 10µF is suitable, considering the load transient

specifications of the application as well.

9.4 Free Wheeling Diode

The inductor current is forced through the diode during the off-times. The average current flowing through it is

The reverse voltage must be higher than the input voltage and safely it is common to consider 30% more. Normally, a schottky diode is preferred

because of its low forward voltage.

9.5 Stability

Even though the hysteretic voltage mode is intrinsically stable, an excessive noise at the FB could cause instability. For this reason care must be

taken drawing the layout, reducing the noise and shielding the FB path from it. The main noise generator is the switching node, which is

(inductor and diode). It is preferable to draw the FB path as far as possible from the LX node and, perhaps, shielding it with a GND track. Another

recommendation is to use low ESL output capacitors, thus avoiding electrolytic parts. A big ESL adds a square wave contribution on the FB that

can make the device work improperly.

9.6 100% Duty Cycle Operation

Thanks to the PMOS structure of the internal switch, the device can actually work at 100% duty cycle. This feature is very helpful during the load

transient, because the maximum power can be transferred to the output in order to recover as fastest. The device will try to work at 100% duty

whenever the FB voltage is lower than the upper window’s threshold. Moreover, in this condition, the safety is always guaranteed by the current

limit.

Irms

I

OUT

V

OUT

V

IN

V

OUT

–

(

)

•

V

IN

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

•

=

Irms

I

OUT

2

IL

2

∆

3

-----------

+

=

1

2

---

fsw

ILIM

2

L

•

V

OUT

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

•

•