ADP1879ACPZ-0.3-R7 datasheet(24/40 Pages) AD

24 / 40 page

ADP1878/ADP1879

Data Sheet

Rev. A | Page 24 of 40

APPLICATIONS INFORMATION

FEEDBACK RESISTOR DIVIDER

The required resistor divider network can be determined for a

given VOUT value because the internal band gap reference (VREF)

is fixed at 0.6 V. Selecting values for RT and RB determine the

minimum output load current of the converter. Therefore, for a

given value of RB, the RT value can be determined through the

following expression:

0.6V

0.6 V

INDUCTOR SELECTION

The inductor value is inversely proportional to the inductor

ripple current. The peak-to-peak ripple current is given by

∆

where KI is typically 0.33.

The equation for the inductor value is given by

∆

where:

VIN is the high voltage input.

VOUT is the desired output voltage.

fSW is the controller switching frequency (300 kHz, 600 kHz, and

1.0 MHz).

When selecting the inductor, choose an inductor saturation

rating that is above the peak current level, and then calculate

the inductor current ripple (see the Valley Current-Limit

Setting section and Figure 81).

Figure 81. Peak Inductor Current vs. Valley Current Limit for 33%, 40%, and

50% of Inductor Ripple Current

Table 8. Recommended Inductors

(μH)

DCR

(mΩ)

ISAT

(A)

Dimensions

(mm)

Manufacturer

Model

Number

0.12

0.33

10.2 × 7

Würth Elek.

744303012

0.22

0.33

10.2 × 7

Würth Elek.

744303022

0.47

0.8

14.2 × 12.8

Würth Elek.

744355147

0.72

1.65

10.5 × 10.2

Würth Elek.

744325072

0.9

1.6

14 × 12.8

Würth Elek.

744318120

1.2

1.8

10.5 × 10.2

Würth Elek.

744325120

1.0

3.8

10.2 × 10.2

Würth Elek.

7443552100

1.4

3.2

14 × 12.8

Würth Elek.

744318180

2.0

2.6

10.2 × 10.2

Würth Elek.

7443551200

0.8

27.5

Sumida

CEP125U-0R8

OUTPUT RIPPLE VOLTAGE (ΔVRR)

The output ripple voltage is the ac component of the dc output

voltage during steady state. For a ripple error of 1.0%, the output

capacitor value needed to achieve this tolerance can be determined

using the following equation. (Note that an accuracy of 1.0% is

possible during steady state conditions only, not during load

transients.)

ΔVRR = (0.01) × VOUT

OUTPUT CAPACITOR SELECTION

The primary objective of the output capacitor is to facilitate the

reduction of the output voltage ripple; however, the output capacitor

also assists in the output voltage recovery during load transient

events. For a given load current step, the output voltage ripple

generated during this step event is inversely proportional to the

value chosen for the output capacitor. The speed at which the

output voltage settles during this recovery period depends on

where the crossover frequency (loop bandwidth) is set. This

crossover frequency is determined by the output capacitor, the

equivalent series resistance (ESR) of the capacitor, and the

compensation network.

To calculate the small signal voltage ripple (output ripple voltage) at

the steady state operating point, use the following equation:

∆

where ESR is the equivalent series resistance of the output

capacitors.

To calculate the output load step, use the following equation:

∆

where ΔVDROOP is the amount that VOUT is allowed to deviate for

a given positive load current step (ΔILOAD).

VALLEY CURRENT LIMIT (A)

∆I = 50%

∆I = 40%

∆I = 33%

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