LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 20

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

LTC3703
APPLICATIO S I FOR ATIO
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 signifi-
cantly. Applications that require optimized transient re-
sponse will require recalculation of the compensation
values specifically for the circuit in question. The underly-
ing 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
parasitic values are known. Measurement will give more
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 LTC3703
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 LTC3703, 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
FB. Choose the bias resistor (R
desired output voltage. Disconnect R
connect it to a signal generator or to the source output of
a network analyzer 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 capaci-
tor. Make sure the analyzer’s input is AC coupled so that
the DC voltages present at both the COMP and V
don’t corrupt the measurements or damage the analyzer.
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
how to generate this plot.
20
OUT
OUT
in degrees. Refer to your SPICE manual for details of
to FB and a 0.1µF feedback capacitor from COMP to
U
OUT
)/V(COMP) in dB and phase of
U
B
) as required to set the
W
B
from ground and
U
OUT
nodes
*3703 modulator gain/phase
*2003 Linear Technology
*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 10u
rl out1 out 0.015
*output cap
cout out out2 540u
resr out2 0 0.01
*3703 internals
emod mod 0 value = {57*v(comp)}
vstim comp 0 0 ac 1
.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 11. 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 at 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:
BOOST = – (PHASE + 30°)
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10
absolute gain)
(GAIN/20)
(this converts GAIN in dB to G in
;MOSFET rdson
;inductor value
;inductor series R
;capacitor value
;capacitor ESR
;3703multiplier
;ac stimulus
3703fa