LTC3536_15 Datasheet, PDF (22/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Applications Information
In order to improve the stability also in buck-boost mode
of operation, the two compensation zeros could be move
to different frequency:
f_ZERO2 =
1
2πR_TOPC_FF
= 5.4kHz
f_ZERO1 =
1
2πR_FBC_FB
=
2•
f_ZERO2
= 10.8kHz
The new R_FB value is:
R_FB
=
2π
•
1
(180pF)
•
10.8kHz
=
81.9kΩ
≈
80.6kΩ
As consequence the f_POLE2 will move to higher frequency:
f_POLE2 =
1
2πC_POLER_FB
= 532kHz
As shown from Figures 14 and 15, the stability is now
improved for the buck-boost region (V_IN = 3V) and remains
good for the boost region (V_IN = 1.8V).
In buck mode there is no right-half plane zero and the
stability is normally achieved.
120
100
80
60
40
20
0
–20
–40
–60
–80
–100
1
V_O/V_C
–60
–80
–100
GAIN
PHASE
–120
–140
–160
–180
–200
–220
–240
–260
–280
10 100 1k 10k 100k 1M 10M 100M
FREQUENCY (Hz)
3536 F14
Figure 14. Complete Loop Bode Plot for Boost Operation Mode
140
120
100
80
60
40
20
0
–20
–40
–60
–80
–100
–120
1
V_O/V_C
–20
–40
–60
–80
–100
GAIN
PHASE
–120
–140
–160
–180
–200
–220
–240
–260
–280
10
100
1k
10k 100k 1M 10M 100M
FREQUENCY (Hz)
3536 F15
Figure 15. Complete Loop Bode Plot for
Buck-Boost Operation Mode
Output Voltage Programming
The output voltage is set via the external resistor divider
comprised of resistors R_TOP and R_BOT. The resistor divider
values determine the output regulation voltage according to:
V_OUT
=
0.6


1+
R_TOP
R_BOT


V
In addition to setting the output voltage, the value of R_TOP
is instrumental in controlling the dynamics of the compen-
sation network. When changing the value of this resistor,
care must be taken to understand the impact this will have
on the compensation network. As noted in the Input and
Peak Current Limit section, “for current limit feature to
be most effected, the Thevenin resistance (R_TOP//R_BOT)
from FB to ground should exceed 100k.”
V_OUT
LTC3536
FB
R_TOP
R_BOT
3536 F16
Figure 16. FB Resistor Network
3536fa
22