LTC3703-5
20
37035fa
if even one major power component is changed signifi-
cantly. Applications that require optimized transient re-
sponse will require recalculation of the compensation
values specifically for the circuit in question. The underly-
ing 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 measured directly from
a breadboard or can be simulated if the appropriate
parasitic values are known. Measurement will give more
accurate results, but simulation can often get close enough
to give a working system. To measure the modulator gain
and phase directly, wire up a breadboard with an LTC3703-5
and the actual MOSFETs, inductor and input and output
capacitors that the final design will use. This breadboard
should use appropriate construction techniques for high
speed analog circuitry: bypass capacitors located close to
the LTC3703-5, no long wires connecting components,
appropriately sized ground returns, etc. Wire the feedback
amplifier as a simple Type 1 loop, with a 10k resistor from
V
OUT
to FB and a 0.1µF feedback capacitor from COMP to
FB. Choose the bias resistor (R
B
) as required to set the
desired output voltage. Disconnect R
B
from ground and
connect it to a signal generator or to the source output of
a network analyzer (Figure 14) to inject a test signal into
the loop. Measure the gain and phase from the COMP pin
to the output node at the positive terminal of the output
capacitor. Make sure the analyzer’s input is AC coupled so
that the DC voltages present at both the COMP and V
OUT
APPLICATIO S I FOR ATIO
WUUU
nodes don’t corrupt the measurements or damage the
analyzer.
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 gener-
ate an AC plot of V(V
OUT
)/V(COMP) in dB and phase of
V
OUT
in degrees. Refer to your SPICE manual for details of
how to generate this plot.
*3703-5 modulator gain/phase
*2003 Linear Technology
*this file written to run with PSpice 8.0
*may require modifications for other
SPICE simulators
*MOSFETs
rfet mod sw 0.02 ;MOSFET rdson
*inductor
lext sw out1 10u ;inductor value
rl out1 out 0.015 ;inductor series R
*output cap
cout out out2 540u ;capacitor value
resr out2 0 0.01 ;capacitor ESR
*3703-5 internals
emod mod 0 value = {43*v(comp)}
;3703-5multiplier
vstim comp 0 0 ac 1 ;ac stimulus
.ac dec 100 1k 1meg
.probe
.end
With the gain/phase plot in hand, a loop crossover fre-
quency can be chosen. Usually the curves look something
like Figure 10. Choose the crossover frequency in the
rising or flat parts of the phase curve, beyond the external
LC poles. Frequencies between 10kHz and 50kHz usually
work well. Note the gain (GAIN, in dB) and phase (PHASE,
in degrees) at this point. The desired feedback amplifier
gain will be –GAIN to make the loop gain at 0dB at this
frequency. Now calculate the needed phase boost, assum-
ing 60° as a target phase margin:
BOOST = –(PHASE + 30°)
If the required BOOST is less than 60°, a Type 2 loop can
be used successfully, saving two external components.
V
IN
TG
SW
BG
INV
MODE/SYNC
COMP
FB
RUN/SS
LTC3703-5
V
CC
C
IN
5V V
IN
M1
V
OUT
TO
ANALYZER
V
COMP
TO
ANALYZER
AC
SOURCE
FROM
ANALYZER
L
EXT
M2
10µF
DRV
CC
f
SET
0.1µF
R
B
BOOST
GND
BGRTN
+
+
10k
NC
C
OUT
37035 F14
+
Figure 14. Modulator Gain/Phase Measurement Set-Up
