Datasheet

LTC3536
22
3536fa
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
TOP
C
FF
= 5.4kHz
f
ZERO1
=
1
2πR
FB
C
FB
= 2 • f
ZERO2
=10.8kHz
The new R
FB
value is:
R
FB
=
1
2π • 180pF
( )
• 10.8kHz
= 81.9kΩ ≈80.6kΩ
As consequence the f
POLE2
will move to higher frequency:
f
POLE2
=
1
2πC
POLE
R
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.
applicaTions inForMaTion
GAIN (dB)
120
100
80
60
20
40
0
–20
–40
–60
–100
–80
1 10 100 1k 10k 100k
FREQUENCY (Hz)
V
O
/V
C
1M 10M 100M
–240
–260
–280
3536 F14
–60
–80
–100
–120
–140
–160
–180
–200
–220
PHASE
GAIN
PHASE (DEG)
Figure 14. Complete Loop Bode Plot for Boost Operation Mode Figure 16. FB Resistor Network
Figure 15. Complete Loop Bode Plot for
Buck-Boost Operation Mode
GAIN (dB)
140
120
100
80
40
20
60
0
–20
–40
–60
–120
–100
–80
1 10 100 1k 10k 100k
FREQUENCY (Hz)
V
O
/V
C
1M 10M 100M
PHASE (DEG)
–200
–240
–260
–220
–280
3536 F15
–20
–40
–60
–80
–100
–120
–140
–160
–180
PHASE
GAIN
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 resistnace (R
TOP
//R
BOT
)
from FB to ground should exceed 100k.”
V
OUT
FB
R
TOP
R
BOT
3536 F16
LTC3536