ADP1871 datasheet(24/44 Pages) AD

Part #	ADP1871

Description	Synchronous Buck Controller with Constant On-Time and Valley Current Mode

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ADP1870/ADP1871

Rev. 0 | Page 24 of 44

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 determines 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:

V

6

.

0

V)

6

.

0

(

−

×

=

OUT

B

T

V

R

INDUCTOR SELECTION

The inductor value is inversely proportional to the inductor

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

3

LOAD

I

L

I

K

I

≈

×

=

Δ

where KI is typically 0.33.

The equation for the inductor value is given by

IN

OUT

SW

L

OUT

IN

V

f

I

V

L

×

Δ

−

=

)

(

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).

52

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

48

50

6

8

10

12

14

16

18

20

22

24

26

28

30

VALLEY CURRENT LIMIT (A)

ΔI = 50%

ΔI = 40%

ΔI = 33%

Figure 79. Peak Inductor Current vs. Valley Current Limit for 33%, 40%, and

50% of Inductor Ripple Current

Table 8. Recommended Inductors

L

(μH)

DCR

(mΩ)

ISAT

(A)

Dimensions

(mm)

Manufacturer

Model

Number

0.12

0.33

55

10.2 × 7

Würth Elek.

744303012

0.22

0.33

30

10.2 × 7

Würth Elek.

744303022

0.47

0.67

50

13.2 × 12.8

Würth Elek.

744355147

0.72

1.3

35

10.5 × 10.2

Würth Elek.

744325072

0.9

1.6

28

13 × 12.8

Würth Elek.

744355090

1.2

1.8

25

10.5 × 10.2

Würth Elek.

744325120

1.0

3.3

20

10.5 × 10.2

Würth Elek.

7443552100

1.4

3.2

24

14 × 12.8

Würth Elek.

744318180

2.0

2.6

22

13.2 × 12.8

Würth Elek.

7443551200

0.8

2.5

16.5

12.5 × 12.5

AIC Technology

CEP125U-R80

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 only during steady state conditions,

not during load transients.)

OUT

RR

V

×

=

Δ

)

01

.

0

(

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:

[]⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛

×

Δ

−

Δ

×

Δ

=

)

(

8

1

ESR

I

V

f

I

C

L

RIPPLE

SW

L

OUT

where ESR is the equivalent series resistance of the output

capacitors.

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

))

(

2

ESR

I

V

f

I

C

LOAD

DROOP

SW

LOAD

OUT

×

Δ

−

Δ

×

Δ

×

=

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

a given positive load current step (ΔILOAD).

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