Datasheet

23
LTC1702A
1702afa
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). Now calculate the remaining values:
(K is a constant used in the calculations)
ƒ = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in absolute
gain)
Type 2 Loop:
K Tan
BOOST
C
GKR
CCK
R
K
C
R
VR
VV
B
REF
OUT REF
=+°
⎛
⎝
⎜
⎞
⎠
⎟
=
πƒ
=
()
=
πƒ
=
()
2
45
2
1
21
12 1
2
21
1
2
–
–
APPLICATIONS INFORMATION
WUU
U
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.
*1702A modulator gain/phase
*
©
2001 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
*1702A internals
emod mod 0 comp 0 5 ;3.3 for 3.3V supply
vstim comp 0 0 ac 1 ;ac stimulus
.ac dec 100 1k 1meg
.probe
.end
BOOST2
TG
SW
BG
FCB
FAULT
COMP
FB
RUN/SS
1/2 LTC1702A
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
1702A F12
+
Figure 12. Modulator Gain/Phase Measurement Set-Up