Datasheet

´
OUT
Resr C
C2 =
R3
´
L OUT
R C
C1 =
R3
¦
´ ´ p
OUT L
1
p =
C R 2
p ¦ ´ ´
´ ´
OUT
ea ps
2 × c Vo C
R3 =
gm Vref gm
¦
¦ ¦ ´
C
sw
= p mod
2
¦ ¦ ´ ¦
C
= p mod z mod
¦
p ´ ´
1
z m od =
2 Resr Cout
¦
p ´ ´
Iout m ax
p mod =
2 Vout Cout
Vref
VO
R1
R3
C1
C2
R2
CO
5pF
RO
gm
ea
COMP
VSENSE
Type 2A
Type 2B
R3
C1
TPS54618
SLVSAE9D –NOVEMBER 2010–REVISED DECEMBER 2013 www.ti.com
Figure 37. Types of Frequency Compensation
The design guidelines for TPS54618 loop compensation are as follows:
1. The modulator pole, fpmod, and the esr zero, fz1 must be calculated using Equation 15 and Equation 16.
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 17
and Equation 18 to estimate a starting point for the crossover frequency, fc. Equation 17 is the geometric
mean of the modulator pole and the esr zero and Equation 18 is the mean of modulator pole and the
switching frequency. Use the lower value of Equation 17 or Equation 18 as the maximum crossover
frequency.
(15)
vertical spacer
(16)
vertical spacer
(17)
vertical spacer
(18)
vertical spacer
2. R3 can be determined by
(19)
vertical spacer
Where is the gm
ea
amplifier gain (245 μA/V), gm
ps
is the power stage gain (25 A/V).
3. Place a compensation zero at the dominant pole . C1 can be determined by
(20)
vertical spacer
4. C2 is optional. It can be used to cancel the zero from Co’s ESR.
(21)
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