Datasheet

G +
1
A
MOD(f)
+
1
0.304
+ 3.29
A
MOD(f)
+ A
MOD
ǒ
f
LC
f
C
Ǔ
2
+ 5
ǒ
4.93 kHz
20 kHz
Ǔ
2
+ 0.304
f
Z
+
1
2p ESR C
O
+
1
2p 0.006 360 mF
+ 73.7 kHz
f
LC
+
1
2p L C
O
Ǹ
+
1
2p 2.9 mH 360 mF
Ǹ
+ 4.93 kHz
A
MOD
+
10
2
+ 5.0 A
MOD(dB)
+ 20 log
(
5
)
+ 14 dB
3
ILIM
6 6
14 0.0104 0.020 42.86 10
R 18.24 k 18.7k
1.12 8.5 10 8.5 10
-
- -
´ - ´
= + = W @ W
´ ´ ´
ILIM
360 F 3.3 V
I 8.0 A 9.2 A
1 ms
m ´
> + =
SS / SD
2.35 A
C 1ms 3.36nF 3300pF
0.7 V
m
= ´ = @
ESR 10.3m - 4.3 m 6.0 m= W W = W
1
33mV 3.2 A ESR
8 97 F 300kHz
æ ö
æ ö
= +
ç ÷
ç ÷
ç ÷
´ m ´
è ø
è ø
TPS40054
TPS40055
TPS40057
SLUS593H DECEMBER 2003REVISED JULY 2012
www.ti.com
Using Equation 6 calculate the ESR required to meet the output ripple requirements.
(65)
(66)
For this design example two (2) Panasonic SP EEFUEOJ1B1R capacitors, (6.3 V, 180 µF, 12 m) are used.
10. Calculate the soft-start capacitor (C
SS/SD
)
This design requires a soft-start time (t
START
) of 1 ms. C
SS/SD
can be calculated using Equation 13:
(67)
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 14. For this
design,
(68)
For this design, add I
ILIM
(9.2 A) to one-half the ripple current (1.6 A) and increase this value by 30% to allow for
tolerances. This yields a overcurrent setpoint (I
OC
) of 14 A. R
DS(on)
is increased 30% (1.3 × 0.008) to allow for
MOSFET heating. Using Equation 15 to calculate R
ILIM
.
(69)
12. Calculate loop compensation values
Calculate the DC modulator gain (A
MOD
) from Equation 19:
(70)
Calculate the output filter L-C
O
poles and C
O
ESR zeros from Equation 20 and Equation 21:
(71)
and
(72)
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 4.93 kHz.
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 29:
(73)
And also from Equation 29:
(74)
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