Datasheet

1
sw
f p
=
´ ´
C6
R4
Re´
=
o
C sr
C6
R4
1
2 fp
=
´ ´ ´
p
C5
R4 mod
2 f
gmps gmea
p
æ ö
æ ö
´ ´ ´
= ´
ç ÷
ç ÷
´
è ø
è ø
co out out
ref
C V
R4
V
2
sw
p
f
f f= ´
co
mod
p z
f f f= ´
co
mod mod
1
z mod =
2 Resr × Cout
¦
´ p ´
TPS54040
SLVS918A –MARCH 2009–REVISED SEPTEMBER 2013 www.ti.com
(42)
(43)
(44)
To determine the compensation resistor, R4, use Equation 45. Assume the power stage transconductance,
gmps, is 1.9A/V. The output voltage, V
O
, reference voltage, VREF, and amplifier transconductance, gmea, are
5V, 0.8V and 92μA/V, respectively. R4 is calculated to be 77.1 kΩ, use the nearest standard value of 76.8kΩ.
Use Equation 46 to set the compensation zero to the modulator pole frequency. Equation 46 yields 2754 pF for
compensating capacitor C5, a 2700 pF is used on the board.
(45)
(46)
Use the larger value of Equation 47 and Equation 48 to calculate the C6, to set the compensation pole.
Equation 48 yields 5.9pF so the nearest standard of 5.6pF is used.
(47)
(48)
Discontinuous Mode and Eco Mode Boundary
With an input voltage of 34V, the power supply enters discontinuous mode when the output current is less than
50mA. The power supply enters EcoMode when the output current is lower than 30mA.
The input current draw at no load is 235μA.
APPLICATION CURVES
Figure 51. Load Transient Figure 52. Startup With VIN
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