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 max
p m od =
2 Vout Cout
Vref
VO
R1
R3
C1
C2
R2
CO
5pF
RO
gm
ea
COMP
VSENSE
Type 2A
Type 2B
R3
C1
¦
´ ´ p
OUT ESR
1
z =
C R 2
¦
´ ´ p
OUT L
1
p =
C R 2
TPS54418
www.ti.com SLVS946C –MAY 2009–REVISED JULY 2013
(9)
(10)
SMALL SIGNAL MODEL FOR FREQUENCY COMPENSATION
The TPS54418 uses a transconductance amplifier for the error amplifier and readily supports two of the
commonly used frequency compensation circuits. The compensation circuits are shown in Figure 33. The Type 2
circuits are most likely implemented in high bandwidth power supply designs using low ESR output capacitors. In
Type 2A, one additional high frequency pole is added to attenuate high frequency noise.
Figure 33. Types of Frequency Compensation
The design guidelines for TPS54418 loop compensation are as follows:
1. Tthe 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 Co’s ESR.
(17)
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