Datasheet

LTC3122
18
3122fa
For more information www.linear.com/LTC3122
Figure 7. Boost Converter with Phase Lead
Figure 8. Bode Plot Showing Phase Lead
3122 F06
V
IN
SD
PWM/SYNC
RT
V
CC
V
OUT
CAP
FB
V
C
LTC3122
SGND PGND
R
C
127k
C
C
220pF
SW
L1
3.3µH
R2
113k
1.02M
C
F
33pF
C
VCC
4.7µF
C1
100nF
C
OUT
22µF
V
OUT
12V
800mA
R
T
57.6k
V
IN
5V
C
IN
4.7µF
ONOFF
PWMBURST
R
PL
604k
C
PL
10pF
applicaTions inForMaTion
The circuit in Figure 7 shows the same application as
that in Figure 5 with Type III compensation. This is ac-
complished by adding C
PL
and R
PL
and adjusting C
C
, C
F
,
and R
C
accordingly. Table 4 shows the parameters used
to generate the bode plot shown in Figure 8.
Table 4. Bode Plot Parameters for Type III Compensation
PARAMETER VALUE UNITS COMMENT
V
IN
5 V App Specific
V
OUT
12 V App Specific
R
L
15 Ω App Specific
C
OUT
22 µF App Specific
R
ESR
5 mΩ App Specific
L 3.3 µH App Specific
ƒ
OSC
1 MHz Adjustable
R1 113 kΩ Adjustable
R2 1020 kΩ Adjustable
g
ma
95 µS Fixed
R
O
10 MΩ Fixed
g
mp
3.4 S Fixed
η
80 % App Specific
R
C
127 kΩ Adjustable
C
C
220 pF Adjustable
C
F
33 pF Adjustable
R
PL
604 kΩ Adjustable
C
PL
10 pF Adjustable
From Figure 8, the phase margin is still optimized at 60°
and the crossover frequency remains 15kHz. Adding C
PL
and R
PL
provides some feedforward signal in Burst Mode
operation, leading to lower output voltage ripple.
FREQUENCY (kHz)
0.01
GAIN (dB)
PHASE (deg)
170
150
110
70
50
130
90
30
10
–10
–30
180
140
100
60
20
–20
–60
–100
–140
–180
–220
100.1
3122 F08
10001 100
GAIN
PHASE