Datasheet
R
fb1
- 1
V
OUT
0.6
R
fb2
=
www.ti.com
Evaluation Board Component Selection
9.10 Tracking
The LM27402 evaluation board is setup with a tracking connection (SS/TK). The SS/TK terminal is also
the soft-start pin. If a voltage source is connected to the SS/TK connection of the board, the output can be
controlled up to 1.5V (voltage set by the feedback resistors). The LM27402 will stop tracking when the
SS/TK voltage exceeds 0.6V. Please refer to the datasheet for more details of the tracking function.
9.11 Compensation and Feedback, R
fb1
, R
c1
, R
c2
, C
c1
, C
c2
, C
c3
In order for the LM27402 to regulate, the feedback loop must be closed and compensated. The LM27402
employs voltage mode control to regulate the output voltage. Voltage mode control requires the LC
complex double pole caused by L
out
and C
o1
- C
o5
to be compensated to reduce the likelihood of oscillation.
The evaluation board incorporates type III compensation which adds three poles and two zeros to the
open loop transfer function. The evaluation board is conservatively compensated to grant the user the
freedom to make small changes to the powertrain circuitry while maintaining adequate stability. Please
refer to the LM27402 High Performance Synchronous Buck Controller with DCR Current Sensing
(SNVS615) data sheet for the type III compensator design equations. The compensation components
include R
fb1
, R
fb2
, R
c1
, R
c2
, C
c1
, C
c2
, C
c3
.
9.12 R
fb1
and R
fb2
The resistors labeled R
fb1
and R
fb2
create a voltage divider from V
OUT
to FB and FB to GND that is used to
set the nominal output voltage of the regulator. Nominally, the output of the LM27402 evaluation board is
set to 1.5V using resistor values of R
fb1
= 20.0 kΩ and R
fb2
= 13.3 kΩ. If a different output voltage is
required, the value of R
fb2
can be adjusted according to the equation:
(15)
R
fb1
does not need to be changed from its value of 20.0 kΩ.
9
SNVA406B–May 2010–Revised May 2013 AN-1992 LM27402 Evaluation Board
Submit Documentation Feedback
Copyright © 2010–2013, Texas Instruments Incorporated