ADP1878ACPZ-1.0-R7 datasheet(20/40 Pages) AD

20 / 40 page

ADP1878/ADP1879

Data Sheet

Rev. A | Page 20 of 40

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

the current sense gain can be calculated as follows:

1.4V

where:

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

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

Although the ADP1878/ADP1879 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)

RON

(mΩ)

Valley Current Level (A)1

47 kΩ,

22 kΩ,

Open,

100 kΩ,

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

1 Blank cells are not applicable.

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

for Each Programming Resistor (RES)

The valley current limit is programmed as listed in Table 7 and

shown in Figure 71. The inductor that is 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 72. Valley Current-Limit Threshold in Relation to Inductor Ripple Current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

RON (mΩ)

RES = 47kΩ

ACS = 3V/V

RES = 22kΩ

ACS = 6V/V

RES = NO RES

ACS = 12V/V

RES = 100kΩ

ACS = 24V/V

INDUCTOR

CURRENT

VALLEY CURRENT-LIMIT

THRESHOLD (SET FOR 25A)

∆I = 33%

OF 30A

CS AMP

OUTPUT

SWING

CURRENT

SENSE

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

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