Datasheet

ADP1878/ADP1879 Data Sheet
Rev. B | Page 20 of 40
When the desired valley current limit (I
CLIM
) has been determined,
the current sense gain can be calculated as follows:



1.4V




where:
R
ON
is the channel impedance of the low-side MOSFET.
A
CS
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 R
ON
variable, Table 7 and Figure 71
outline several available options for the valley current setpoint
based on various R
ON
values.
Table 7. Valley Current Limit Program (See Figure 71)
R
ON
(mΩ)
Valley Current Level (A)
1
47 kΩ, 22 kΩ, Open, 100 kΩ,
A
CS
= 3 V/V A
CS
= 6 V/V A
CS
= 12 V/V A
CS
= 24 V/V
1.5 38.9
2 29.2
2.5 23.3
3 39.0 19.5
3.5 33.4 16.7
4.5 26.0 13
5 23.4 11.7
5.5 21.25 10.6
10 23.3 11.7 5.83
15 31.0 15.5 7.75 3.87
18 26.0 13.0 6.5 3.25
1
Blank cells are not applicable.
Figure 71. Valley Current-Limit Value vs. R
ON
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
VALLEY CURRENT LIMIT (A)
R
ON
(mΩ)
39
37
35
33
31
29
27
25
23
21
19
17
15
13
11
9
7
5
3
RES = 47kΩ
A
CS
= 3V/V
RES = 22kΩ
A
CS
= 6V/V
RES = NO RES
A
CS
= 12V/V
RES = 100kΩ
A
CS
= 24V/V
09441-071
INDUCTOR
CURRENT
VALLEY CURRENT-LIMIT
THRESHOLD (SET FOR 25A)
∆I = 33%
OF 30A
CS AMP
OUTPUT
SWING
CURRENT
SENSE
AMPLIFIER
OUTPUT
2.4V
1V0A
35A
30A
32.25A
37A
49
A
39.5A
∆I = 45%
OF 32.25A
∆I = 65%
OF 37A
MAXIMUM DC LOAD
CURRENT
09441-072