LTC1702ACGN#TR-SMI Linear Technology, LTC1702ACGN#TR-SMI Datasheet - Page 23

• Manufacturer Part Number: LTC1702ACGN#TR-SMI
• Manufacturer: Linear Technology

Specifications of LTC1702ACGN#TR-SMI

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS
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
V(OUT) in degrees. Refer to your SPICE manual for details
of how to generate this plot.
*1702A modulator gain/phase
*
*this file written to run with PSpice 8.0
*may require modifications for other SPICE
simulators
*MOSFETs
rfet mod sw 0.02
*inductor
lext sw out1 1u
rl out1 out 0.005
*output cap
cout out out2 1000u
resr out2 0 0.01
*1702A internals
emod mod 0 comp 0 5
vstim comp 0 0 ac 1
.ac dec 100 1k 1meg
.probe
.end
ANALYZER
ANALYZER
©
SOURCE
2001 Linear Technology
V
FROM
COMP
Figure 12. Modulator Gain/Phase Measurement Set-Up
TO
AC
10µF
0.1µF
R
+
B
10k
NC
COMP
FB
RUN/SS
SGND PGND
U
1/2 LTC1702A
V
CC
10Ω
OUT
BOOST2
PV
FAULT
INFORMATION
)/V(COMP) in dB and phase of
U
CC
FCB
MBR0530T
SW
BG
TG
;MOSFET rdson
;inductor value
;inductor series R
;capacitor value
;capacitor ESR
;3.3 for 3.3V supply
;ac stimulus
5V
W
QT
QB
+
1µF
C
IN
L
EXT
1702A F12
U
+
C
V
TO
ANALYZER
OUT
OUT
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:
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)
Type 2 Loop:
BOOST = – (PHASE + 30°)
K Tan
C
C
R
R
1
2
2
B
=
=
=
=
(GAIN/20)
=
C K
2
2
V
2
πƒ
πƒ
OUT
( )
V
⎛
⎜
⎝
K
REF
BOOST
GKR
2
C
1
1
–
–
(this converts GAIN in dB to G in absolute
( )
2
R
1
V
1
1
REF
+
45
°
⎞
⎟
⎠
LTC1702A
23
1702afa