Datasheet
LTC3536
21
3536fa
Figure 11. Compensated Error Amplifier Bode Plot
Figure 12. Complete Loop Bode Plot for Boost Operation Mode
A Bode plot of the error amplifier with the designed com-
pensation component values is shown in Figure 11. The
Bode plot confirms that the peak phase occurs at 37.8kHz
and the phase boost at that point is about 60°. In addition,
the gain at the peak phase frequency is 2dB which is close
to the design target.
applicaTions inForMaTion
Figure 13. Complete Loop Bode Plot for
Buck-Boost Operation Mode
GAIN (dB)
81
72
63
54
45
36
27
18
9
0
–9
–18
–27
–36
–45
–54
1 10 100 1k 10k 100k
FREQUENCY (Hz)
1M 10M 100M
–50
–60
–70
–80
–90
3536 F11
60
50
40
30
20
10
0
–10
–20
–30
–40
PHASE
GAIN
V
O
/V
C
PHASE (DEG)
The final step in the design process is to compute the Bode
plot for the entire loop using the designed compensation
network and confirm its phase margin and crossover
frequency. The complete loop Bode plot for this example
is shown in Figure 12. The loop crossover frequency is
37.8kHz which matches the design target and the phase
margin is approximately 60°.
The Bode plot for the complete loop should be checked
overall operating conditions and for variations in compo-
nent values to ensure that sufficient phase margin and
gain margin exists in all cases. The stability of the loop
should also be confirmed via time domain simulation and
by evaluating the transient response of the converter in
the actual circuit.
In this example the V
IN
varies from 1.8V to 5.5V. In buck-
boost operation (when 0.85 • V
OUT
< V
IN
< V
OUT
/0.85) the
Bode plot of the complete loop shows a phase margin of
GAIN (dB)
140
120
100
80
60
40
20
0
–20
–40
–60
–80
–100
1 10 100 1k 10k 100k
FREQUENCY (Hz)
V
O
/V
C
1M 10M 100M
–240
–260
–280
3536 F12
–40
–60
–80
–100
–120
–140
–160
–180
–200
–220
GAIN
PHASE
PHASE (DEG)
40° for V
IN
= V
OUT
= 3.3V. In fact in this mode of operation
the DC gain increase and often make this the most critical
region to compensate.
GAIN (dB)
100
80
60
40
20
0
–20
–40
–60
–80
1 10 100 1k 10k 100k
FREQUENCY (Hz)
V
O
/V
C
1M 10M 100M
–240
–270
3536 F13
0
–30
–60
–90
–120
–150
–180
–210
PHASE
GAIN
PHASE (DEG)