Datasheet

ESR OUT
R C
C2 =
R3
´
´
L OUT
R C
C1 =
R3
¦
´ ´ p
OUT L
1
p =
C R 2
OUT OUT
ea ref ps
2 × c V C
R3 =
gm V gm
p ¦ ´ ´
´ ´
¦
¦ ¦ ´
C
sw
= p mod
2
¦ ¦ ´ ¦
C
= p mod z mod
ESR OUT
1
z mod =
2 R C
¦
p ´ ´
OUT
OUT OUT
I max
p mod =
2 V C
¦
p ´ ´
TPS54218
SLVS974B –SEPTEMBER 2009–REVISED JULY 2013 www.ti.com
The design guidelines for TPS54218 loop compensation are as follows:
1. The modulator pole, fpmod, and the esr zero, fz1 must be calculated using Equation 11 and Equation 12.
Derating the output capacitor (C
OUT
) may be needed if the output voltage is a high percentage of the
capacitor rating. Use the capacitor manufacturer information to derate the capacitor value. Use Equation 13
and Equation 14 to estimate a starting point for the crossover frequency, fc. Equation 13 is the geometric
mean of the modulator pole and the esr zero and Equation 14 is the mean of modulator pole and the
switching frequency. Use the lower value of Equation 13 or Equation 14 as the maximum crossover
frequency.
(11)
(12)
(13)
(14)
2. R3 can be determined by
(15)
Where is the gm
ea
amplifier gain (225 μA/V), gm
ps
is the power stage gain (13 A/V).
3. Place a compensation zero at the dominant pole . C1 can be determined by
(16)
4. C2 is optional. It can be used to cancel the zero from te C
OUT
ESR.
(17)
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