Datasheet
¦
´ ´ p
OUT ESR
1
z =
C R 2
¦
´ ´ p
OUT L
1
p =
C R 2
ps L
Adc = gm R´
s
1+
2 × z
vo
= Adc
vc
s
1+
2 × p
æ ö
ç ÷
p ¦
è ø
´
æ ö
ç ÷
p ¦
è ø
VO
R
L
VC
fp
fz
Adc
gm
ps
R
ESR
C
OUT
TPS54618
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SLVSAE9D –NOVEMBER 2010–REVISED DECEMBER 2013
SIMPLE SMALL SIGNAL MODEL FOR PEAK CURRENT MODE CONTROL
Figure 35 is a simple small signal model that can be used to understand how to design the frequency
compensation. The TPS54618 power stage can be approximated to a voltage controlled current source (duty
cycle modulator) supplying current to the output capacitor and load resistor. The control to output transfer
function is shown in Equation 11 and consists of a dc gain, one dominant pole and one ESR zero. The quotient
of the change in switch current and the change in COMP pin voltage (node c in Figure 35) is the power stage
transconductance. The gm for the TPS54618 is 25 A/V. The low frequency gain of the power stage frequency
response is the product of the transconductance and the load resistance as shown in Equation 12. As the load
current increases and decreases, the low frequency gain decreases and increases, respectively. This variation
with load may seem problematic at first glance, but the dominant pole moves with load current [see Equation 13].
The combined effect is highlighted by the dashed line in the right half of Figure 36. As the load current
decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover frequency the same
for the varying load conditions which makes it easier to design the frequency compensation.
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vertical spacer
Figure 36. Simple Small Signal Model and Frequency Response for Peak Current Mode Control
(11)
(12)
(13)
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(14)
SMALL SIGNAL MODEL FOR FREQUENCY COMPENSATION
The TPS54618 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 37. 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.
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