LTC3536_15 Datasheet, PDF (21/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Applications Information
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.
81
72
63
54
45
36
27
18
9
0
–9
–18
–27
–36
–45
–54
1
GAIN
10 100
V_O/V_C
PHASE
1k 10k 100k 1M
FREQUENCY (Hz)
60
50
40
30
20
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
10M 100M
3536 F11
Figure 11. Compensated Error Amplifier Bode Plot
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
140
120
100
80
60
40
20
0
–20
–40
–60
–80
–100
1
V_O/V_C
–40
–60
–80
PHASE
–100
–120
–140
GAIN
–160
–180
–200
–220
–240
–260
–280
10 100 1k 10k 100k 1M 10M 100M
FREQUENCY (Hz)
3536 F12
Figure 12. Complete Loop Bode Plot for Boost Operation Mode
100
V_O/V_C
0
80
–30
60
PHASE
40
GAIN
20
–60
–90
–120
0
–150
–20
–180
–40
–210
–60
–240
–80
1
–270
10 100 1k 10k 100k 1M 10M 100M
FREQUENCY (Hz)
3536 F13
Figure 13. Complete Loop Bode Plot for
Buck-Boost Operation Mode
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.
3536fa
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