ADP1621ARMZ-R7 datasheet(15/32 Pages) AD

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ADP1621

Rev. A | Page 15 of 32

INPUT CAPACITOR SELECTION

The bulk input capacitor provides a low impedance path for the

inductor ripple current. Capacitor C1 in Figure 1 represents a

bulk input capacitor. Choose a bulk input capacitor whose

impedance at the switching frequency is lower than the

impedance of the voltage source VIN.

The preferred bulk input capacitor is a 10 μF to 100 μF ceramic

capacitor because it has low equivalent series resistance (ESR) and

low impedance. Aluminum electrolytic and aluminum polymer

capacitors can also be used as the bulk input capacitors. The bulk

input capacitor does not need to be placed very close to the IN

and PIN pins. Aluminum electrolytic capacitors are the cheapest

and generally have high ESR values, which increase dramatically at

temperatures less than 0°C. Some aluminum electrolytic capacitors

have ESR less than 20 mΩ, but their capacitances are generally

greater than 800 μF. Aluminum polymer capacitors are more

expensive than the aluminum electrolytic ones, but are generally

cheaper than the ceramic capacitors for the same amount of

capacitance. Polymer capacitors have relatively low ESR, with

some models having less than 10 mΩ.

Regardless of the type of capacitor used, make sure the ripple

current rating of the bulk input capacitor, ICIN,RMS, is greater than

RMS

CIN

(11)

where ΔIL is the peak-to-peak inductor ripple current.

In addition to the bulk input capacitor, a bypass input capacitor is

required. The function of the bypass capacitor is to locally filter the

input voltage to the ADP1621 and maintain the input voltage at a

steady value during switching transitions. The bypass capacitor is

typically a 0.1 μF or greater ceramic capacitor and should be placed

as close as possible to the IN and PIN pins of the ADP1621.

Capacitors C3 and C4 in Figure 1 represent the bypass capacitors.

OUTPUT CAPACITOR SELECTION

The output capacitor maintains the output voltage and supplies

current to the load while the external MOSFET is on.

The value and characteristics of the output capacitor greatly

affect the output voltage ripple and stability of the converter.

The amount of peak-to-peak output voltage ripple, ΔVOUT, can

be approximated by

⎟

⎠

⎞

⎜

⎝

⎛

−

≈

LOAD

OUT

()2

ESL

ESR

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(12)

where ΔIL is the peak-to-peak inductor ripple current, fSW is the

switching frequency, COUT is the output capacitance, ESR is the

effective ESR of COUT, and ESL is the effective equivalent series

inductance of COUT.

Because the output capacitor is typically greater than 40 μF, the

ESR dominates the output capacitance impedance and thus the

output voltage ripple. The use of low ESR, ceramic dielectric

capacitors is preferred, although aluminum electrolytic,

tantalum, OS-CON™ (from Sanyo), and aluminum polymer

capacitors can be used. At higher switching frequencies, the ESL

of the output capacitor may also be a factor in determining the

output voltage ripple. Multiple capacitors can be connected in

parallel to reduce the effective ESR and ESL. Keep in mind that

the capacitance of a given capacitor typically degrades with

increased temperature and bias voltage. Consult the capacitor

manufacturer’s data sheet when determining the actual

capacitance of a capacitor under certain conditions.

Ensure that the output capacitor ripple current rating, ICOUT,RMS,

is greater than

LOAD

RMS

COUT

−

(13)

DIODE SELECTION

The diode conducts the inductor current to the output capacitor

and load while the MOSFET is off. The average diode current is

the load current:

LOAD

AVE

DIODE

(14)

The rms diode current in continuous conduction mode is given by

LOAD

RMS

DIODE

−

(15)

where D is the duty cycle.

The power dissipated in the diode is

LOAD

DIODE

(16)

where VD is the forward-voltage drop of the diode.

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