LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 26

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3810
With the gain/phase plot in hand, a loop crossover fre-
quency can be chosen. Usually the curves look something
like Figure 13. Choose the crossover frequency about 25%
of the switching frequency for maximum bandwidth. Al-
though it may be tempting to go beyond f
that signifi cant phase shift occurs at half the switching
frequency that isn’t modeled in the above H(s) equation
and PSPICE code. Note the gain (GAIN, in dB) and phase
(PHASE, in degrees) at this point. The desired feedback
amplifi er gain will be –GAIN to make the loop gain at 0dB
at this frequency. Now calculate the needed phase boost,
assuming 60° as a target phase margin:
If the required BOOST is less than 60°, a Type 2 loop can
be used successfully, saving two external components.
BOOST values greater than 60° usually require Type 3
loops for satisfactory performance.
Finally, choose a convenient resistor value for R1 (10k
is usually a good value). Now calculate the remaining
values:
TYPE 2 Loop:
26
BOOST = – (PHASE + 30°)
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10
absolute gain)
R
R2 =
C2 =
C1= C2 K
K = tan
B
=
2 • f • G • K • R1
2 • f • C1
V
OUT
V
(GAIN/20)
(
REF
K
BOOST
2
(R1)
1
V
2
1
REF
)
(this converts GAIN in dB to G in
+ 45°
SW
/4, remember
TYPE 3 Loop:
SPICE or mathematical software can be used to generate
the gain/phase plots for the compensated power supply to
do a sanity check on the component values before trying
them out on the actual hardware. For software, use the
following transfer function:
T(s) = A(s)H(s)
R
R2 =
R3 =
C2 =
C3 =
C1= C2 K 1
K = tan
B
=
Figure 13. Transfer Function of Buck Modulator
K 1
2 • f • G • R1
2 • f • C1
2 f K • R3
V
R1
OUT
V
(
2
REF
0
K
1
BOOST
1
(R1)
V
)
REF
4
+ 45°
FREQUENCY (Hz)
PHASE
GAIN
3810 F13
0
–90
–180
3810fb