Excelsys ultimod, Руководство дизайнера, Страница: 6 / 15

Using equation (5)
I
trim
=FI
lim
+ CF- 0.53
I
trim
=1.45V
Using equation (7)
R
Itrim
1320I
trim
2.47-
I
trim
R
Itrim
= 1,873ohms
Foldback Current Limit Programming
Foldback Current Limit can be achieved using the circuit
below.
The onset of Foldback current limit (I
fb1
) can be programmed
using the formula below as can the actual end point (I
fb2
).
To set the final Foldback current limit point (I
fb2
), the value R
1
in
parallel with R
2
is equivalent to R
Itrim
in the previous Straight Line
current limit example.
To set I
fb1
, point, we must calculate the ratio of R
1
to R
2
.
To get the value of R
1
:
R1 = (R
Itrim
)Vout (9)
[FIf
b1
(1 + R
Itrim
) - 2R
Itrim
+1 -VD]
1320 1320
R2 = (R1)(R
Itrim
) (10)
R1 - R
Itrim
VD is the voltage drop across BAW56W. This can be assumed
to be 0.53V for most calculations, however it will vary slightly
due to temperature. Refer to BAW56W datasheet for further
details.
If Current Limit is Less than 85% of Maximum Current:
VD =0.53 (11)
If Current Limit is Greater than 85% of Maximum Current:
VD =1.325 -0.442 I
trim
(12)
Example:
To set the foldback current limit of an XgH set at 5V to the
following levels, I
fb1
=30A and I
fb2
=30A, determine the values
of R
1
to R
2
required.
F = 0.0468 for XgH
CF = 1.042 for XgH
I
max
= 36A for XgH
I
lim
= 20 = 55.5% (less than 85%)
I
max
36
Example:
To set the current limit of XgH to 20A, determine the external
voltage to be applied to the I
trim
pin.
I
lim
= 20A
F = 0.0468 for XgH
CF = 1.043 for XgH
I
max
= 36A for XgH (Imax = max current rating of XgH)
I
lim
= 20 = 55.5% (less than 85%)
I
max
36
Using equation (5)
I
trim
=FI
lim
+ CF- 0.53
I
trim
=1.45V
Note that application of any voltage >2.5V to I
trim
will not increase
current limit beyond the powerMods normal current limit.
Straight Line Current Limit Using an External Resistor
The formula below will calculate the required external resistor
value required to set the current limit of a powerMod :
If Current Limit is Less than 85% of Maximum Current:
R
Itrim
= 1320
Itrim
(7)
2.47-
I trim
If Current Limit is Greater than 85% of Maximum Current:
R
Itrim
= 1320 I
trim
(8)
1.675 - 0.558 I
trim
Example:
To set the current limit of XgH to 30A, what resistance must be
placed between the I
trim
pin and -V.
I
lim
= 30A
F = 0.0468 for XgH
CF = 1.043 for XgH
I
max
= 36A for XgH
I
lim
= 20 = 55.5% (less than 85%)
I
max
36
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6
UItiMod Designer Manual
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Current Limit Adjustment Table
Current Limit Programming
powerMod F CF
XgG 0.0395 1.12
XgH 0.0468 1.043
XgJ 0.0933 1.015
XgK 0.1742 1.009
XgL 0.3182 1.065
Foldback Current Limit