ADP1875 Analog Devices, ADP1875 Datasheet - Page 21

• Manufacturer Part Number: ADP1875
• Description: Synchronous Buck Controller with Constant On-Time, Valley Current Mode, and Power Saving Mode
• Manufacturer: Analog Devices

Because the ADP1874/ADP1875 are based on valley current
control, the relationship between I
where:
K
desired average load current (see Figure 70).
I
I
Establishing K
Inductor Selection section), but in most cases K
When the desired valley current limit (I
the current-sense gain can be calculated as follows:
where:
R
A
Although the ADP1874/ADP1875 have only four discrete current-
sense gain settings for a given R
outline several available options for the valley current setpoint
based on various R
Table 7. Valley Current Limit Program (See Figure 71)
R
(mΩ)
1.5
2
2.5
3
3.5
4.5
5
5.5
10
15
18
CLIM
LOAD
ON
ON
I
CS
is the ratio between the inductor ripple current and the
is the current-sense gain multiplier (see Table 6 and Table 7).
is the channel impedance of the lower-side MOSFET.
is the desired valley current limit.
I
is the current load.
I
CLIM
CLIM
Figure 70. Valley Current Limit to Average Current Relation
47 kΩ
A
31.0
26.0
CS
=
=
= 3 V/V
I
A
LOAD
I
CS
helps to determine the inductor value (see the
1
LOAD CURRENT
4 .
×
VALLEY CURRENT LIMIT
×
ON
V
R
⎛ −
⎜
⎝
ON
1
values.
22 kΩ
A
23.3
15.5
13.0
CS
K
2
Valley Current Level
= 6 V/V
I
⎞
⎟
⎠
ON
RIPPLE CURRENT =
variable, Table 7 and Figure 71
CLIM
CLIM
Open
A
39.0
33.4
26.0
23.4
21.25
11.7
7.75
6.5
and I
CS
= 12 V/V
) has been determined,
LOAD
I
I
LOAD
is
= 0.33.
3
100 kΩ
A
38.9
29.2
23.3
19.5
16.7
13
11.7
10.6
5.83
7.5
3.25
CS
= 24 V/V
Rev. 0 | Page 21 of 44
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
Figure 72. Valley Current-Limit Threshold in Relation to Inductor Ripple Current
0A
Figure 71. Valley Current-Limit Value vs. R
∆I = 33%
OF 30A
39
37
35
33
31
29
27
25
23
21
19
17
15
13
11
VALLEY CURRENT-LIMIT
THRESHOLD (SET FOR 25A)
9
7
5
3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
RES = 100kΩ
35A
A
CS
30A
= 24V/V
for Each Programming Resistor (RES)
MAXIMUM DC LOAD
OF 32.25A
∆I = 45%
CURRENT
39.5A
RES = NO RES
A
32.25A
CS
R
ON
= 12V/V
(mΩ)
ADP1874/ADP1875
∆I = 65%
ON
OF 37A
of the Lower-Side MOSFET
49A
AMPLIFIER
37A
RES = 22kΩ
A
OUTPUT
RES = 47kΩ
A
SWING
CS
CS
CS
= 6V/V
= 3V/V
AMPLIFIER
OUTPUT
CS
2.4V
1V