TPS54540
www.ti.com
SLVSBX7 –MAY 2013
f
=
f
f
=
1850 Hz x 610 kHz = 34 kHz
co1
p(mod) x z(mod)
(43)
(44)
f
400 kHz
SW
f
=
f
=
1850 Hz x
= 19 kHz
co2
p(mod) x
2
2
To determine the compensation resistor, R4, use Equation 45. The typical power stage transconductance, gmps,
is 17 A/V. The output voltage, VO, reference voltage, VREF, and amplifier transconductance, gmea, are 3.3 V, 0.8
V and 350 μA/V, respectively. R4 is calculated to be 17 kΩ and a standard value of 16.9 kΩ is selected. Use
Equation 46 to set the compensation zero to the modulator pole frequency. Equation 46 yields 5100 pF for
compensating capacitor C5. 4700 pF is used for this design.
æ
ç
è
ö
÷
ø
æ 2´ p´ f ´ C
ö
÷
ø
V
OUT
æ
ç
è
ö
÷
ø
2´ p´ 30 kHz ´ 130 mF
17 A / V
3.3V
æ
ö
co
OUT
R4 =
x
=
x
= 17 kW
ç
ç
÷
gmps
V
x gmea
0.8 V x 350 mA / V
è
ø
è
REF
(45)
1
1
C5 =
=
= 5100 pF
2´ p´R4 x f
2´ p´16.9 kW x 1850 Hz
p(mod)
(46)
A compensation pole can be implemented if desired by adding capacitor C8 in parallel with the series
combination of R4 and C5. Use the larger value calculated from Equation 47 and Equation 48 for C8 to set the
compensation pole. The selected value of C8 is 47 pF for this design example.
C
x R
ESR
130 mF x 1 mW
OUT
C8 =
=
= 15 pF
R4
(47)
(48)
1
1
C8 =
=
= 47 pF
R4 x f sw x p
16.9 kW x 400 kHz x p
Discontinuous Conduction Mode and Eco-mode Boundary
With an input voltage of 12 V, the power supply enters discontinuous conduction mode when the output current
is less than 560 mA. The power supply enters Eco-mode when the output current is lower than 18 mA. The input
current draw is 241 μA with no load.
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