Datasheet

Data Sheet ADP1874/ADP1875
Rev. A | Page 21 of 44
Because the ADP1874/ADP1875 are based on valley current
control, the relationship between I
CLIM
and I
LOAD
is
×=
2
1
I
LOADCLIM
K
II
where:
K
I
is the ratio between the inductor ripple current and the
desired average load current (see Figure 70).
I
CLIM
is the desired valley current limit.
I
LOAD
is the current load.
Establishing K
I
helps to determine the inductor value (see the
Inductor Selection section), but in most cases K
I
= 0.33.
LOAD CURRENT
VALLEY CURRENT LIMIT
RIPPLE CURRENT =
I
LOAD
3
09347-068
Figure 70. Valley Current Limit to Average Current Relation
When the desired valley current limit (I
CLIM
) has been determined,
the current-sense gain can be calculated as follows:
ONCS
CLIM
RA
I
×
=
V4.1
where:
R
ON
is the channel impedance of the lower side MOSFET.
A
CS
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 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
47 22 Open 100
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 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
09347-069
Figure 71. Valley Current-Limit Value vs. R
ON
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).
INDUCTOR
CURRENT
VALLEY CURRENT-LIMIT
THRESHOLD (SET FOR 25A)
ΔI = 33%
OF 30A
CS
AMPLIFIER
OUTPUT
SWING
CS
AMPLIFIER
OUTPUT
2.4V
1V0A
35A
37A
49A
39.5A
ΔI = 45%
OF 32.25A
ΔI = 65%
OF 37A
MAXIMUM DC LOAD
CURRENT
09347-070
32.25A
30A
Figure 72. Valley Current-Limit Threshold in Relation to Inductor Ripple Current