LTC3118_15 Datasheet, PDF (31/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

Applications Information
30
24
BOOST MODE
18
12
6
0
–6
–12
–18
–24
–30
–36
10Hz
GAIN
PHASE
100Hz
BUCK MODE
1kHz
10kHz
FREQUENCY
120
V_OUT / VC
60
0
–60
–120
100kHz
–180
1MHz
36
30
24
18
12
6
0
–6
–12
–18
–24
–30
–36
10Hz
GAIN
PHASE
100Hz
1kHz
10kHz
FREQUENCY
0
–VC/ V_OUTA
–20
–40
–60
–80
–100
100kHz
–120
1MHz
LTC3118
70
60
50
40
30
20
10
0
–10
–20
–30
–40
–50
–60
–70
10Hz
BUCK MODE
BOOST MODE
GAIN
PHASE MARGIN
100Hz
1kHz
10kHz
FREQUENCY
210
V_OUT / V_OUTA
180
120
60
0
–60
–120
100kHz
–180
1MHz
3118 F08
Figure 9. Bode Plot Showing Power Stage Gain (Top), V_A Loop (Center), and Total Loop Gain vs Frequency
These values were verified in the top set of curves in
Figure 9. The resulting power stage crossover frequency
is around 40kHz in buck mode (V_IN > 5V), 20kHz in boost
mode at 3.5V_IN.
boost mode. A pole-zero-pole network will now be added
to the voltage amplifier to increase the DC gain, reduce
the crossover frequency and reduce the overall gain at
high frequencies:
The uncompensated power stage crossover frequency
is higher than the goal of 20kHz. More importantly, the
uncompensated power stage DC gain is low, especially in
V_A
Pole 1 =
1
2π R_EA
C_P1
3118f
For more information www.linear.com/LTC3118
31