TPS54360_1212 Datasheet, PDF (28/43 Pages) Texas Instruments – 60 V Input, 3.5 A, Step Down DC-DC Converter with Eco-mode™

TPS54360
SLVSBB4C – AUGUST 2012 – REVISED OCTOBER 2012
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To determine the compensation resistor, R4, use Equation 45. Assume the power stage transconductance,
gmps, is 12 A/V. The output voltage, V_O, reference voltage, V_REF, and amplifier transconductance, gmea, are 5
V, 0.8 V and 350 μA/V, respectively. R4 is calculated to be 13 kΩ which is a standard value. Use Equation 46 to
set the compensation zero to the modulator pole frequency. Equation 46 yields 6404 pF for compensating
capacitor C5. 6800 pF is used for this design.
R4
=
æ
èç
2
´
p
´ f_co ´ C_OUT
gmps
ö
÷ø
x
æ
ç
V_OUT
ö
÷
è ^VREF ^{x gmea} ø
=
æ 2 ´ p ´ 23.9 kHz ´
çè
12 A / V
58.3
mF
ö
÷ø
x
æ
çè
0.8
V
x
5V
350
mA
/
V
ö
÷ø
=
13
kW
(45)
C5 =
1
=
1
= 6404 pF
2 ´ p ´ R4 x f_p(mod) 2 ´ p ´13 kW x 1912 Hz
(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 39 pF for this design example.
C8 = ^{COUT x RESR} = ^{58.3 mF x 2.5 mW} = 11.2 pF
R4
13 kW
(47)
C8 =
1
=
1
= 40.8 pF
R4 x f sw x p 13 kW x 600 kHz x p
(48)
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 300 mA. The power supply enters Eco-mode when the output current is lower than 24 mA. The input
current draw is 270 μA with no load.
28
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