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Power Limit
The protocol decoder and the power limit logic will limit
maximum output current to keep regulation within
recommended V
/I
operating range. The Power Limit
out
out
block adjusts V
voltage reference at the current
REFC
regulation loop in order to limit the maximum output current.
The NCP4371 is designed to give a user a high degree of
freedom to optimize maximum power and current limit
profile of the target application. The user can scale both –
maximum output power and maximum current limit
independently.
The NCP4371 has two constant power curves defined –
"Option A" for Class A only and "Option B" for either Class
A or Class B. Power Option C shall be used for applications
where constant power regulation is not required. If Power
Option C is selected then power limiting curve is ignored.
The applications based on Power Option C operate in
"constant current regulation mode".
In order to scale the power limit curve for the given power
a selection of the current sense resistor has to be done. The
relation between current sense resistor and output power
limit is given by the curves in Figure 18.
20
18
16
POUT Limit Option B
14
12
POUT Limit Option A
10
8
6
15
16
17
18
19
Figure 18. R
SENSE
The characteristics in the Figure 23 cover a range P
15 – 25 W. For powers outside this interval following
formula can be used for R
SENSE
Option A (Class A only) :
Option B (Class A & B) :
20
21
22
23
24
P (W)
vs. P
Limit Curve
OUT
OUT
selection:
+ 168
R
[mW]
(eq. 1)
SENSE
P
max
+ 277
R
[mW]
(eq. 2)
SENSE
P
max
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NCP4371
Once the Power limit is defined by an R_SENSE selection
the user needs to define a maximum output current limit.
This current limit can be given by a connector or cable
maximum current rating.
There are 5 current limit options available for Power
Option A and 8 current limit options for Power Option B and
C. Each power limit option corresponds to a particular
Current Control Reference Voltage (V
the maximum output current for the selected R_SENSE
resistor. The user has to make a selection from current limit
characteristics shown in Figure 19. Each power limit curve
represents a unique device option (see Table Device
Options).
4.0
3.8
3.6
3.4
E
3.2
3.0
2.8
D
2.6
2.4
2.2
C
2.0
1.8
B
1.6
1.4
A
1.2
15
16
17
Power Option A Current Limit Selection
4.4
4.0
3.6
3.2
25
2.8
2.4
2.0
=
1.6
1.2
0.8
15
16
17
Power Option B Current Limit Selection
Figure 19. Current Limit Characteristics
11
), which limits
REFC
18
19
20
21
22
23
P (W)
H
18
19
20
21
22
23
P (W)
24
25
G
F
E
D
C
B
A
24
25
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