Datasheet

ADP1872/ADP1873 Data Sheet
Rev. B | Page 20 of 40
Table 6. Valley Current Limit Program
1
R
ON
(mΩ)
Valley Current Level
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
Refer to Figure 70 for more information and a graphical representation.
1234567891011121314151617181920
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
08297-069
Figure 70. 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 6
and Figure 70. The inductor chosen must be rated to handle the
peak current, which is equal to the valley current from Table 6
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 71).
INDUCTOR
CURRENT
VALLEY CURRENT LIMIT
THRESHOLD (SET FOR 25A)
∆I = 33%
OF 30A
COMP
OUTPUT
SWING
COMP
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
08297-070
Figure 71. Valley Current-Limit Threshold in Relation to Inductor Ripple Current
HICCUP MODE DURING SHORT CIRCUIT
A current-limit violation occurs when the current across the
source and drain of the lower side MOSFET exceeds the current-
limit setpoint. When 32 current-limit violations are detected,
the controller enters idle mode and turns off the MOSFETs for
6 ms, allowing the converter to cool down. Then, the controller
re-establishes soft start and begins to cause the output to ramp
up again (see Figure 72). While the output ramps up, COMP is
monitored to determine if the violation is still present. If it is still
present, the idle event occurs again, followed by the full-chip
power-down sequence. This cycle continues until the violation
no longer exists. If the violation disappears, the converter is
allowed to switch normally, maintaining regulation.
HS
CLIM
ZERO
CURRENT
REPEATED CURRENT LIMIT
VIOLATION DETECTED
A PREDETERMINED NUMBER
OF PULSES IS COUNTED TO
ALLOW THE CONVERTER
TO COOL DOWN
SOFT START IS
REINITIALIZED T
O
MONITOR IF THE
VIOLATION
STILL EXISTS
08297-071
Figure 72. Idle Mode Entry Sequence Due to Current-Limit Violations