ADP1882ARMZ-1.0-R7 datasheet(24/40 Pages) AD

24 / 40 page

ADP1882/ADP1883

Rev. 0 | 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 0.8 V. Selecting values for RT and RB determines the

minimum output load current of the converter. Therefore, for

a given value of RB, the RT value can be determined using the

following expression:

RT = RB ×

(

−

OUT

INDUCTOR SELECTION

The inductor value is inversely proportional to the inductor

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

LOAD

≈

where KI is typically 0.33.

The equation for the inductor value is given by

VALLEY CURRENT LIMIT (A)

OUT

−

)

(

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 79).

ΔI = 50%

ΔI = 40%

ΔI = 33%

Figure 79. Peak Current vs. Valley Current Threshold for 33%, 40%, and 50%

of Inductor Ripple Current

Table 7. Recommended Inductors

(μH)

DCR

(mΩ)

ISAT

(A)

Dimensions

(mm)

Manufacturer

Model

Number

0.12

0.33

10.2 × 7

Wurth Electronics

744303012

0.22

0.33

10.2 × 7

Wurth Electronics

744303022

0.47

0.8

14.2 × 12.8

Wurth Electronics

744355147

0.72

1.65

10.5 × 10.2

Wurth Electronics

744325072

0.9

1.6

13 × 12.8

Wurth Electronics

744355090

1.2

1.8

10.5 × 10.2

Wurth Electronics

744325120

1.0

3.3

10.5 × 10.2

Wurth Electronics

7443552100

1.4

3.2

14 × 12.8

Wurth Electronics

744318180

2.0

2.6

13.2 × 12.8

Wurth Electronics

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 deter-

mined using the following equation. Note that an accuracy of

1.0% is possible only during steady state conditions, 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:

[]⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

(

ESR

RIPPLE

OUT

where ESR is the equivalent series resistance of the output

capacitors.

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

))

(

ESR

LOAD

DROOP

LOAD

OUT

−

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

a given positive load current step (ΔILOAD).

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