ADP1874ARQZ-0.3-R7 datasheet(21/44 Pages) AD

21 / 44 page

ADP1874/ADP1875

Rev. 0 | Page 21 of 44

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RON (mΩ)

RES = 47kΩ

ACS = 3V/V

RES = 22kΩ

ACS = 6V/V

RES = NO RES

ACS = 12V/V

RES = 100kΩ

ACS = 24V/V

Because the ADP1874/ADP1875 are based on valley current

control, the relationship between ICLIM and ILOAD is

⎟

⎠

⎞

⎜

⎝

⎛ −

LOAD

CLIM

where:

is the ratio between the inductor ripple current and the

desired average load current (see Figure 70).

ICLIM

is the desired valley current limit.

ILOAD

is the current load.

Establishing KI helps to determine the inductor value (see the

Inductor Selection section), but in most cases KI = 0.33.

LOAD CURRENT

VALLEY CURRENT LIMIT

RIPPLE CURRENT =

ILOAD

Figure 71. Valley Current-Limit Value vs. RON of the Lower-Side MOSFET

for Each Programming Resistor (RES)

The valley current limit is programmed as outlined in Table 7

and Figure 71. The inductor chosen must be rated to handle the

peak current, which is equal to the valley current from Table 7

plus the peak-to-peak inductor ripple current (see the Inductor

Selection section). In addition, the peak current value must be

used to compute the worst-case power dissipation in the MOSFETs

(see Figure 72).

Figure 70. Valley Current Limit to Average Current Relation

When the desired valley current limit (ICLIM) has been determined,

the current-sense gain can be calculated as follows:

INDUCTOR

CURRENT

VALLEY CURRENT-LIMIT

THRESHOLD (SET FOR 25A)

∆I = 33%

OF 30A

AMPLIFIER

OUTPUT

SWING

AMPLIFIER

OUTPUT

2.4V

35A

30A

32.25A

37A

49A

39.5A

∆I = 45%

OF 32.25A

∆I = 65%

OF 37A

MAXIMUM DC LOAD

CURRENT

CLIM

where:

RON is the channel impedance of the lower-side MOSFET.

ACS is the current-sense gain multiplier (see Table 6 and Table 7).

Although the ADP1874/ADP1875 have only four discrete current-

sense gain settings for a given RON variable, Table 7 and Figure 71

outline several available options for the valley current setpoint

based on various RON values.

Table 7. Valley Current Limit Program (See Figure 71)

Valley Current Level

47 kΩ

22 kΩ

Open

100 kΩ

RON

(mΩ)

ACS = 3 V/V

ACS = 6 V/V

ACS = 12 V/V

ACS = 24 V/V

1.5

38.9

29.2

2.5

23.3

39.0

19.5

3.5

33.4

16.7

4.5

26.0

23.4

11.7

5.5

21.25

10.6

23.3

11.7

5.83

31.0

15.5

7.75

7.5

26.0

13.0

6.5

3.25

Figure 72. Valley Current-Limit Threshold in Relation to Inductor Ripple Current

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