Datasheet


SLVS612 − APRIL 2006
24
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DESIGN EXAMPLE
10. 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 (12). For this
design,
I
LIM
u
940 mF 1.25 V
1ms
) 8.0 A + 9.2 A
For this design, set I
LIM
for 11.0 A
DC
minimum. From equation (13), 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 0.0104W
8.6 mA
)
(0.03)
8.6 mA
+ 15.24 kW * 3.5 kW + 11.74 kW ^ 11.8 W
11. Calculate loop compensation values
Calculate the DC modulator gain (A
MOD
) from equation (14)
A
MOD
+
12
2
+ 6.0 A
MOD(dB)
+ 20 log
(
6
)
+ 15.6 dB
Calculate the output filter L-C
O
poles and C
O
ESR zeros from (16) and (17)
f
LC
+
1
2p L C
O
Ǹ
+
1
2p 2.9 mH 940 mF
Ǹ
+ 3.05 kHz
and
f
Z
+
1
2p ESR C
O
+
1
2p 0.006 940 mF
+ 28.2 kHz
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 3.05 kHz.
Select the double pole location for the Type III compensation network at the output capacitor ESR zero at
28.2 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 (22).
A
MOD(f)
+ A
MOD
ǒ
f
LC
f
C
Ǔ
2
+ 6
ǒ
3.05 kHz
20 kHz
Ǔ
2
+ 0.14
And also from equation (22).
G +
1
A
MOD(f)
+
1
0.14
+ 7.14
Choose R1 = 100 kΩ
(60)
(61)
(62)
(63)
(64)
(65)
(66)