Datasheet

LT3579/LT3579-1
31
35791fa
For more information www.linear.com/LT3579
APPENDIX
The current mode zero (Z3) is a right half plane zero
which can be an issue in feedback control design, but is
manageable with proper external component selection.
Using the circuit in Figure 20 as an example, Table 8 shows
the parameters used to generate the Bode plot shown in
Figure 19.
Table 8. Bode Plot Parameters
PARAMETER VALUE UNITS COMMENT
R
L
7 Ω Application Specific
C
OUT
30 µF Application Specific
R
ESR
2 mΩ Application Specific
R0 305 kΩ Not Adjustable
C
C
2200 pF Adjustable
C
F
47 pF Optional/Adjustable
C
PL
0 pF Optional/Adjustable
R
C
8 kΩ Adjustable
R1 130 kΩ Adjustable
R2 14.6 kΩ Not Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
g
ma
250 µmho Not Adjustable
g
mp
28 mho Not Adjustable
L 2.2 µH Application Specific
f
OSC
1.0 MHz Adjustable
From Figure 19, the phase is –134° when the gain reaches
0dB giving a phase margin of 46°. The crossover frequency
is 8kHz, which is more than three times lower than the
frequency of the RHP zero Z3 to achieve adequate phase
margin.
Figure 19. Bode Plot for Example Boost Converter
Figure 20. 5V to 12V Boost Converter
FREQUENCY (Hz)
10
–20
GAIN (dB)
PHASE (DEG)
60
80
100
120
40
20
0
140
–360
–315
–180
–135
–90
–45
–225
–270
0
100 10k 100k 1M
35791 F19
1k
PHASE
GAIN
46° AT
8kHz
V
OUT
12V
1.7A
C
OUT
10µF
×3
L1
2.2µH
D1
V
IN
5V
V
IN
SW1 SW2
35791 F20
LT3579
86.6k
8k
130k
100k
SHDN
FAULT
FB
GATE
CLKOUT
VC
SYNC GND SS
RT
2.2nF
0.1µF
47pF
C
IN
22µF