23
LTC1703
1703fa
V(OUT) in degrees. Refer to your SPICE manual for details
of how to generate this plot.
*1703 modulator gain/phase
*
©
1999 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 1u ;inductor value
rl out1 out 0.005 ;inductor series R
*output cap
cout out out2 1000u ;capacitor value
resr out2 0 0.01 ;capacitor ESR
*1703 internals
emod mod 0 laplace {v(comp)} =
+ {5*exp(–s*909e–9)} ;5 -> 3.3 for 3.3 VCC
*emod mod 0 comp 0 5 ;use if above lines fail
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 8. 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 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.
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). Note that channel 1 includes R1 and
R
B
internally as part of the VID DAC circuitry. R1 is fixed
at 10kΩ and R
B
varies depending on the VID code
selected.
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 LTC1703
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 LTC1703, 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 12) 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
nodes don’t corrupt the measurements or damage the
analyzer.
BOOST2
TG
SW
BG
FCB
FAULT
COMP
FB
RUN/SS
1/2 LTC1703
V
CC
10Ω
MBR0530T
C
IN
5V
QT
1µF
V
OUT
TO
ANALYZER
V
COMP
TO
ANALYZER
AC
SOURCE
FROM
ANALYZER
L
EXT
QB
10µF
0.1µF
R
B
PV
CC
SGND
PGND
+
+
10k
NC
C
OUT
1703 F12
+
Figure 12. Modulator Gain/Phase Measurement Set-Up
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
APPLICATIO S I FOR ATIO
WUUU
