LTC1704 Linear Technology, LTC1704 Datasheet - Page 20

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1704/LTC1704B
20
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-
IN
IN
R1
R2
R1
R2
V
REF
FB
V
R3
Figure 10. Type 3 Schematic and Transfer Function
Figure 9. Type 2 Schematic and Transfer Function
REF
FB
C3
–
+
R4
–
+
R4
C2
C2
C1
C1
U
COMP
COMP
U
0
GAIN
0
PHASE
–6dB/OCT
GAIN
–6dB/OCT
PHASE
W
+6dB/OCT
–6dB/OCT
–6dB/OCT
U
1704 F09
1704 F10
FREQ
–90
–180
–270
–360
FREQ
–90
–180
–270
–360
AC SOURCE
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(OUTSW) in degrees. Refer to your SPICE manual for
details of how to generate this plot.
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
V
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 V
nodes don’t corrupt the measurements or damage the
analyzer.
ANALYZER
OUTSW
FROM
ANALYZER
V
Figure 11. Modulator Gain/Phase Measurement Setup
COMP
R2
0.1 F
to FB and a 0.1 F feedback capacitor from COMP
TO
+
10k
10 F
COMP
FB
GND
V
CC
LTC1704
OUTSW
10
RUN/SS
PGND
PV
BOOST
CC
MBR0530T
SW
TG
BG
)/V(COMP) in dB and phase of
5V
1 F
NC
QT
QB
C
L
OUT
+
V
ANALYZER
OUTSW
OUTSW
1704 F11
1704bfa
TO