Datasheet

V
REF
V
IN
V
OUT
Compensation
Network
+
_
Modulator
Filter
Current
Feedback
Network
+
_
V
C
( )
( )
ON
SW
TPS5429 x
K t
IN OUT
6
f
FM
V V
19.7 e 95 10
L
´
-
=
æ ö
æ ö
-
ç ÷
´ + ´ ´
ç ÷
ç ÷
ç ÷
è ø
è ø
( )
4
IN
C
6
IN
LOAD
V FM 2 10
f
V FM 95 10
1
2 R
-
-
´ ´ ´
=
æ ö
æ ö
´ ´ ´
ç ÷
ç ÷
+
ç ÷
ç ÷
´
ç ÷
ç ÷
è ø
è ø
( )
( )
C
EA
CO LOAD OUT
f
K 20 log
1 2 f 2 R C
æ ö
ç ÷
= - ´
ç ÷
+ ´ p ´ ´ ´ ´
è ø
TPS54290, TPS54291, TPS54292
SLUS973 –OCTOBER 2009 www.ti.com
Figure 16. Small Signal Equivalent Block Diagram
To determine the components necessary for compensating the feedback loop, the controller frequency response
characteristics must be understood and the desired crossover frequency selected. The best results are obtained
if 10% of the switching frequency is used as this closed loop crossover frequency. In some cases, up to 20% of
the switching frequency is also possible.
With the output filter components selected, the next step is to calculate the DC gain of the modulator. For
TPS5429x:
(5)
where
• K = 5.6 ×10
5
for TPS54290
• K = 1.5 × 10
6
for TPS54291
• K = 3.6 × 10
6
for TPS54292
The overall DC gain of the converter control-to-output transfer function is approximated Equation 6.
(6)
The next step is to find the desired gain of the error amplifier at the desired crossover frequency. Assuming a
single-pole roll-off, us Equation 6 to evaluate the following expression at the desired crossover frequency.
(7)
where
• ƒ
CO
is the desired crossover frequency
14 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s) :TPS54290 TPS54291 TPS54292