LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 25

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

APPLICATIONS INFORMATION
Feedback Component Selection
Selecting the R and C values for a typical Type 2 or Type 3
loop is a nontrivial task. The applications shown in this
data sheet show typical values, optimized for the power
components shown. They should give acceptable perfor-
mance with similar power components, but can be way off if
even one major power component is changed signifi cantly.
Applications that require optimized transient response will
require recalculation of the compensation values specifi -
cally for the circuit in question. The underlying mathematics
are complex, but the component values can be calculated
in a straightforward manner if we know the gain and phase
of the modulator at the crossover frequency.
Modulator gain and phase can be obtained in one of
three ways: measured directly from a breadboard, or if
the appropriate parasitic values are known, simulated or
generated from the modulator transfer function. Mea-
surement will give more accurate results, but simulation
or transfer function can often get close enough to give
a working system. To measure the modulator gain and
phase directly, wire up a breadboard with an LTC3810
and the actual MOSFETs, inductor and input and output
capacitors that the fi nal design will use. This breadboard
should use appropriate construction techniques for high
speed analog circuitry: bypass capacitors located close
to the LTC3810, no long wires connecting components,
appropriately sized ground returns, etc. Wire the feedback
amplifi er with a 0.1μF feedback capacitor from I
and a 10k to 100k resistor from V
bias resistor (R
voltage. Disconnect R
a signal generator or to the source output of a network
analyzer to inject a test signal into the loop. Measure the
gain and phase from the I
positive terminal of the output capacitor. Make sure the
analyzer’s input is AC coupled so that the DC voltages
present at both the I
measurements or damage the analyzer.
B
) as required to set the desired output
TH
B
and V
from ground and connect it to
TH
pin to the output node at the
OUT
nodes don’t corrupt the
OUT
to FB. Choose the
TH
to FB
If breadboard measurement is not practical, a SPICE
simulation can be used to generate approximate gain/
phase curves. Plug the expected capacitor, inductor
and MOSFET values into the following SPICE deck and
generate an AC plot of V
phase in degrees. Refer to your SPICE manual for details
of how to generate this plot.
*3810 modulator gain/phase
*2006 Linear Technology
*this fi le simulates a simplifi ed model of
*the LTC3810 for generating a v(out)/v(ith)
*bode plot
.param rdson=.0135 ;MOSFET rdson
.param Vrng=2
.param vsnsmax={0.173*Vrng-0.026}
.param Imax={vsnsmax/rdson}
.param DL=4
*inductor current
gl out 0 value={(v(ith)-1.2)*Imax/1.2+DL/2}
*output cap
cout out out2 270u ;capacitor value
resr out2 0 0.018 ;capacitor ESR
*load
Rout out 0 2 ; load resistor
vstim ith 0 0 ac 1 ;ac stimulus
.ac dec 100 100 10meg
.probe
.end
Mathematical software such as MATHCAD or MATLAB
can also be used to generate plots using the following
transfer function of the modulator:
H(s) =
0.7 for ground
s = j2 f
1.2 • R
V
SENSE(MAX)
DS(ON)
;use 1.4 for INTVCC and
;inductor ripple current
•
OUT
1+ s • R
1+ s • R
/ V
ITH
ESR
with gain in dB and
L
• C
LTC3810
• C
OUT
OUT
• R
25
L
3810fb
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