ADP1850ACPZ-R7 datasheet(18/32 Pages) AD

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ADP1850

Data Sheet

Rev. C | Page 18 of 32

INPUT CAPACITOR SELECTION

The input current to a buck converter is a pulse waveform. It is

zero when the high-side switch is off and approximately equal

to the load current when it is on. The input capacitor carries the

input ripple current, allowing the input power source to supply

only the direct current. The input capacitor needs sufficient

ripple current rating to handle the input ripple, as well as an

ESR that is low enough to mitigate input voltage ripple. For the

usual current ranges for these converters, it is good practice to

use two parallel capacitors placed close to the drains of the

high-side switch MOSFETs (one bulk capacitor of sufficiently

high current rating and a 10 μF ceramic decoupling capacitor,

typically).

Select an input bulk capacitor based on its ripple current rating.

First, determine the duty cycle of the output.

OUT

D =

The input capacitor RMS ripple current is given by

)

(

RMS

−

where:

IO is the output current.

D is the duty cycle

The minimum input capacitance required for a particular load is

ESR

MIN

)

(

)

(

−

where:

VPP is the desired input ripple voltage.

RESR is the equivalent series resistance of the capacitor.

If an MLCC capacitor is used, the ESR is near 0, then the

equation is simplified to

MIN

−

)

(

The capacitance of MLCC is voltage dependent. The actual

capacitance of the selected capacitor must be derated according to

the manufacturer’s specification. In addition, add more bulk

capacitance, such as by using electrolytic or polymer capacitors,

as necessary for large step load transients. Make sure the

current ripple rating of the bulk capacitor exceeds the

maximum input current ripple of a particular design.

INPUT FILTER

Normally a 0.1 µF or greater value bypass capacitor from the

input pin (VIN) to AGND is sufficient for filtering out any

unwanted switching noise. However, depending on the PCB

layout, some switching noise can enter the ADP1850 internal

circuitry; therefore, it is recommended to have a low pass filter

at the VIN pin. Connecting a resistor, between 2 Ω and 5 Ω, in

series with VIN and a 1 µF ceramic capacitor between VIN and

AGND creates a low pass filter that effectively filters out any

unwanted glitches caused by the switching regulator. Keep in

mind that the input current could be larger than 100 mA when

driving large MOSFETs. A 100 mA across a 5 Ω resistor creates

a 0.5 V drop, which is the same voltage drop in VCCO. In this

case, a lower resistor value is desirable.

ADP1850

VIN

AGND

2Ω TO 5Ω

1µF

Figure 32. Input Filter Configuration

BOOST CAPACITOR SELECTION

To lower system component count and cost, the ADP1850 has

an integrated rectifier (equivalent to the boost diode) between

VCCO and BSTx. Choose a boost ceramic capacitor with a

value between 0.1 µF and 0.22 µF; this capacitor provides the

current for the high-side driver during switching.

INDUCTOR SELECTION

The output LC filter smoothes the switched voltage at SWx. For

most applications, choose an inductor value such that the

inductor ripple current is between 20% and 40% of the

maximum dc output load current. Generally, a larger inductor

current ripple generates more power loss in the inductor and

larger voltage ripples at the output. Check the inductor data

sheet to make sure that the saturation current of the inductor is

well above the peak inductor current of a particular design.

Choose the inductor value by the following equation:

OUT

∆

−

where:

L is the inductor value.

fSW is the switching frequency.

VOUT is the output voltage.

VIN is the input voltage.

∆I

L is the peak-to-peak inductor ripple current.

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