LTC1704B View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1704B
Linear Technology
'LTC1704B' PDF : 28 Pages View PDF
LTC1704/LTC1704B
APPLICATIO S I FOR ATIO
C2
IN
R4
C1
R1
FB –
R2
VREF +
–6dB/OCT
GAIN
COMP 0
PHASE
–6dB/OCT
FREQ
–90
–180
–270
–360
1704 F09
Figure 9. Type 2 Schematic and Transfer Function
IN
C2
C3 R4
C1
R1 R3
FB –
R2
VREF +
–6dB/OCT
+6dB/OCT
GAIN
COMP 0
PHASE
–6dB/OCT
FREQ
–90
–180
–270
–360
1704 F10
Figure 10. Type 3 Schematic and Transfer Function
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
significantly. Applications that require optimized tran-
sient response will need to recalculate the compensation
values specifically for the circuit in question. The under-
lying 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 para-
sitic values are known. Measurement will give more accu-
rate 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 LTC1704 and
the actual MOSFETs, inductor, and input and output capaci-
tors that the final design will use. This breadboard should
use appropriate construction techniques for high speed
analog circuitry: bypass capacitors located close to the
LTC1704, no long wires connecting components, appro-
priately sized ground returns, etc. Wire the feedback am-
plifier as a simple Type 1 loop, with a 10k resistor from
VOUTSW to FB and a 0.1µF feedback capacitor from COMP
to FB. Choose the bias resistor (R2) as required to set the
desired output voltage. Disconnect R2 from ground and
connect it to a signal generator or to the source output of
a network analyzer (Figure 11) 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 ca-
pacitor. Make sure the analyzer’s input is AC coupled so that
the DC voltages present at both the COMP and VOUTSW
nodes don’t corrupt the measurements or damage the
analyzer.
10Ω
5V
+
10µF
MBR0530T
VCOMP TO
ANALYZER
0.1µF
R2
AC SOURCE
FROM
ANALYZER
VCC
COMP
PVCC
TG
BOOST
FB LTC1704 SW
10k
BG
RUN/SS
GND PGND
1µF
NC
QT
L
QB +
COUT
VOUTSW TO
ANALYZER
1704 F11
Figure 11. Modulator Gain/Phase Measurement Setup
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(VOUTSW)/V(COMP) in dB and phase of
V(OUTSW) in degrees. Refer to your SPICE manual for
details of how to generate this plot.
1704bfa
20
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