ADP1613ARMZ-R7 datasheet(13/28 Pages) AD

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ADP1612/ADP1613

Rev. A | Page 13 of 28

APPLICATIONS INFORMATION

SETTING THE OUTPUT VOLTAGE

The ADP1612/ADP1613 feature an adjustable output voltage

range of VIN to 20 V. The output voltage is set by the resistor

voltage divider, R1 and R2, (see Figure 34) from the output

voltage (VOUT) to the 1.235 V feedback input at FB. Use the

following equation to determine the output voltage:

VOUT = 1.235 × (1 + R1/R2)

(1)

Choose R1 based on the following equation:

⎟

⎠

⎞

⎜

⎝

⎛

−

235

OUT

(2)

INDUCTOR SELECTION

The inductor is an essential part of the step-up switching

converter. It stores energy during the on time of the power

switch, and transfers that energy to the output through the

output rectifier during the off time. To balance the tradeoffs

between small inductor current ripple and efficiency, induc-

tance values in the range of 4.7 μH to 22 μH are recommended.

In general, lower inductance values have higher saturation

current and lower series resistance for a given physical size.

However, lower inductance results in a higher peak current

that can lead to reduced efficiency and greater input and/or

output ripple and noise. A peak-to-peak inductor ripple current

close to 30% of the maximum dc input current typically yields

an optimal compromise.

For determining the inductor ripple current in continuous

operation, the input (VIN) and output (VOUT) voltages determine

the switch duty cycle (D) by the following equation:

OUT

−

(3)

Using the duty cycle and switching frequency, fSW, determine

the on time by the following equation:

(4)

The inductor ripple current (ΔIL) in steady state is calculated by

(5)

Solve for the inductance value (L) by the following equation:

(6)

Ensure that the peak inductor current (the maximum input

current plus half the inductor ripple current) is below the rated

saturation current of the inductor. Likewise, make sure that the

maximum rated rms current of the inductor is greater than the

maximum dc input current to the regulator.

For CCM duty cycles greater than 50% that occur with input

voltages less than one-half the output voltage, slope compen-

sation is required to maintain stability of the current-mode

regulator. For stable current-mode operation, ensure that the

selected inductance is equal to or greater than the minimum

calculated inductance, LMIN, for the application parameters in

the following equation:

OUT

MIN

−

)

(

(7)

Inductors smaller than the 4.7 μH to 22 μH recommended

range can be used as long as Equation 7 is satisfied for the given

application. For input/output combinations that approach the

90% maximum duty cycle, doubling the inductor is recom-

mended to ensure stable operation. Table 5 suggests a series

of inductors for use with the ADP1612/ADP1613.

Table 5. Suggested Inductors

Manufacturer

Part Series

Dimensions

L × W × H (mm)

Sumida

CMD4D11

5.8 × 4.4 × 1.2

CDRH4D28CNP

5.1 × 5.1 × 3.0

CDRH5D18NP

6.0 × 6.0 × 2.0

CDRH6D26HPNP

7.0 × 7.0 × 2.8

Coilcraft

DO3308P

12.95 × 9.4 × 3.0

DO3316P

12.95 × 9.4 × 5.21

Toko

D52LC

5.2 × 5.2 × 2.0

D62LCB

6.2 × 6.3 × 2.0

D63LCB

6.2 × 6.3 × 3.5

Würth

Elektronik

WE-TPC

Assorted

WE-PD, PD2, PD3, PD4

Assorted

CHOOSING THE INPUT AND OUTPUT CAPACITORS

The ADP1612/ADP1613 require input and output bypass capa-

citors to supply transient currents while maintaining constant

input and output voltages. Use a low equivalent series resistance

(ESR), 10 μF or greater input capacitor to prevent noise at the

ADP1612/ADP1613 input. Place the capacitor between VIN

and GND as close to the ADP1612/ADP1613 as possible.

Ceramic capacitors are preferred because of their low ESR

characteristics. Alternatively, use a high value, medium ESR

capacitor in parallel with a 0.1 μF low ESR capacitor as close

to the ADP1612/ADP1613 as possible.

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