Datasheet

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Y(s) +
1 * K
FILT
(s)
L
R
LDC
s ) 1
(dimensionless)
(27)
K
CS
+ gm
CSA
20 kW (dimensionless)
(28)
+
+
L
ISNS
+
X2
+
FB
COMP
690 mV
1 V AC
gm
CSA
C
OUT
C
FLT
R
2
R
LOAD
R
1
R
BIAS
UDG−04149
R
ESR
C
2
C
1
R
FLT
R
LDC
V
IN
VO
R
EA
20 kΩ
V
OUT
V
X
Compensating the Loop (Type II)
f
C
v
f
SW
5
(Hz)
(29)
TPS40100
SLUS601–MAY 2005
APPLICATION INFORMATION (continued)
K
CS
is the gain of the current sense amplifier in the current feedback loop:
where (for Equation 24 through Equation 28)
• V
IN
is the input voltage (V)
• K
PWM
is the gain of the pulse width modulator and is equal to 2
• R
LOAD
is the equivalent load resistance (Ω)
• R
LDC
is the DC inductor resistance (Ω)
• L is the output filter inductance (H)
• C
OUT
is the output filter capacitance (F)
• R
ESR
is the equivalent series resistance of the output filter capacitor (Ω)
• gm
CSA
is the gain of the current sense amplifier (S)
• 20 kΩ is the impedance the current sense amplifier works against (from block diagram)
A computer aided math tool is highly recommended for use in evaluating these equations.
Figure 6. Averaged Model for a Converter Based on the TPS40100
The first step is to select a target loop crossover frequency. Choosing the crossover frequency too high
contributes to making the converter pulse skip. A balance of crossover frequency and amount of current
feedback must be maintained to avoid pulse skipping. A suggested maximum loop crossover frequency is one
fifth-of the switching frequency.
where
• f
C
is the loop crossover frequency
• f
SW
is the switching frequency
16