SLUS593B 鈭?DECEMBER 2003 鈭?REVISED APRIL 2004
TPS40054
TPS40055
TPS40057
DESIGN EXAMPLE
11. Calculate the current limit resistor (R
ILIM
)
The current limit set point depends on t
START
, V
O
,C
O
and I
LOAD
at start-up as shown in equation (15). For this
design,
I
LIM
u
360
mF
3.3 V
)
8.0 A
+
9.2 A
1 ms
(68)
For this design, set I
LIM
for 11.0 A
DC
minimum. From equation (16), with I
OC
equal to the DC output surge current
plus one-half the ripple current of 3.2 A and R
DS(on)
is increased 30% (1.3 * 0.008) to allow for MOSFET heating.
R
ILIM
+
12.6 A
1.12
0.0104W
)
(* 0.075)
+
11.7 kW
*
7.5 kW
+
4.2 kW
^
4.22 kW
10
mA
10
mA
(69)
12. Calculate loop compensation values
Calculate the DC modulator gain (A
MOD
) from equation (19)
A
MOD
+
10
+
5.0
2
A
MOD(dB)
+
20
log (5)
+
14 dB
(70)
Calculate the output filter L-C
O
poles and C
O
ESR zeros from (23) and (25)
f
LC
+
and
f
Z
+
2p
1
ESR
C
O
+
2p
1
0.006
360
mF
+
73.7 kHz
(72)
1
2p L
C
O
+
1
2p 2.9
mH
360
mF
+
4.93 kHz
(71)
Select the close-loop 0 dB crossover frequency, f
C
. For this example f
C
= 20 kHz.
Select the double zero location for the Type III compensation network at the output filter double pole at 4.93kHz.
Select the double pole location for the Type III compensation network at the output capacitor ESR zero at
73.7 kHz.
The amplifier gain at the crossover frequency of 20 kHz is determined by the reciprocal of the modulator gain
AMOD at the crossover frequency from equation (27).
A
MOD(f)
+
A
MOD
f
LC
f
C
2
+
5
4.93 kHz
20 kHz
2
+
0.304
(73)
And also from equation (27).
G
+
1
A
MOD(f)
+
1
+
3.29
0.304
(74)
Choose R1 = 100 k鈩?/div>
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