Datasheet

10
100 1k 10k 100k 1M
FREQUENCY
-80
-60
-40
-20
0
20
40
60
80
dB
-180
-90
0
90
180
RHP-Zero
gm-Zero
gm-Pole
RC-Pole
Vi = 5V
Vo = 12V
Io = 500 mA
Co = 10 PF
Lo = 5 PH
REF
V
¨
¨
©
§
OUT
V
=
2
R
1
¸
¸
¹
·
- x
1
R
LOAD
R
O
V
3
C
2
R
1
R
FB
V
LM2735
www.ti.com SNVS485F –JUNE 2007–REVISED APRIL 2013
Figure 22. Setting Vout
A good value for R1 is 10kΩ.
(9)
COMPENSATION
The LM2735 uses constant frequency peak current mode control. This mode of control allows for a simple
external compensation scheme that can be optimized for each application. A complicated mathematical analysis
can be completed to fully explain the LM2735’s internal & external compensation, but for simplicity, a graphical
approach with simple equations will be used. Below is a Gain & Phase plot of a LM2735 that produces a 12V
output from a 5V input voltage. The Bode plot shows the total loop Gain & Phase without external compensation.
Figure 23. LM2735 Without External Compensation
One can see that the Crossover frequency is fine, but the phase margin at 0dB is very low (22°). A zero can be
placed just above the crossover frequency so that the phase margin will be bumped up to a minimum of 45°.
Below is the same application with a zero added at 8 kHz.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM2735