ADP1879 Analog Devices, ADP1879 Datasheet - Page 20

• Manufacturer Part Number: ADP1879
• Description: Synchronous Buck Controller with Constant On-Time and Valley Current Mode with Power Saving Mode
• Manufacturer: Analog Devices

ADP1878/ADP1879
When the desired valley current limit (I
the current sense gain can be calculated as follows:
where:
R
A
Although the
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
1
Blank cells are not applicable.
ON
ON
CS
is the current sense gain multiplier (see Table 6 and Table 7).
Figure 71. Valley Current-Limit Value vs. R
is the channel impedance of the low-side MOSFET.
39
37
35
33
31
29
27
25
23
21
19
17
15
13
11
47 kΩ,
A
31.0
26.0
9
7
5
3
CS
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
= 3 V/V
RES = 100kΩ
A
ADP1878/ADP1879
CS
1.4 V
= 24V/V
for Each Programming Resistor (RES)
ON
values.
22 kΩ,
A
23.3
15.5
13.0
Valley Current Level (A)
CS
= 6 V/V
RES = NO RES
A
ON
CS
R
ON
= 12V/V
variable, Table 7 and Figure 71
have only four discrete current
(mΩ)
Open,
A
39.0
33.4
26.0
23.4
21.25
11.7
7.75
6.5
CLIM
ON
CS
of the Low-Side MOSFET
= 12 V/V
) has been determined,
RES = 22kΩ
A
RES = 47kΩ
A
CS
1
CS
= 6V/V
= 3V/V
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 20 of 40
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
INDUCTOR
CURRENT
0A
ΔI = 33%
OF 30A
VALLEY CURRENT-LIMIT
THRESHOLD (SET FOR 25A)
35A
30A
MAXIMUM DC LOAD
OF 32.25A
ΔI = 45%
CURRENT
39.5A
32.25A
ΔI = 65%
OF 37A
49A
Data Sheet
37A
OUTPUT
CS AMP
SWING
CURRENT
SENSE
AMPLIFIER
OUTPUT
2.4V
1V