OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LTC3703EG#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34
LTC3703
19
3703fc
approach its ESR and the rolloff due to the capacitor will
stop, leaving 6dB/octave and 90° of phase shift (Figure 11).
Type 2 loops work well in systems where the ESR zero
in the LC roll-off happens close to the LC pole, limiting
the total phase shift due to the LC. The additional phase
compensation in the feedback amplifier allows the 0dB
point to be at or above the LC pole frequency, improving
loop bandwidth substantially over a simple Type 1 loop.
It has limited ability to compensate for LC combinations
where low capacitor ESR keeps the phase shift near 180°
for an extended frequency range. LTC3703 circuits using
conventional switching grade electrolytic output capaci-
tors can often get acceptable phase margin with Type 2
compensation.
“Type 3” loops (Figure 14) use two poles and two zeros to
obtain a 180° phase boost in the middle of the frequency
band. A properly designed Type 3 circuit can maintain
acceptable loop stability even when low output capacitor
ESR causes the LC section to approach 180° phase shift
well above the initial LC roll-off. As with a Type 2 circuit,
the loop should cross through 0dB in the middle of the
phase bump to maximize phase margin. Many LTC3703
circuits using
low ESR tantalum or OS-CON output capaci-
tors need Type 3 compensation to obtain acceptable phase
margin with a high bandwidth feedback loop.
applicaTions inForMaTion
GAIN (dB)
3703 F11
A
V
0
PHASE
–6dB/OCT
–12dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
Figure 11. Transfer Function of Buck Modulator
So far, the AC response of the loop is pretty well out
of the user’s control. The modulator is a fundamental
piece of the LTC3703 design and the external L and C are
usually chosen based on the regulation and load current
requirements without considering the AC loop response.
The feedback amplifier, on the other hand, gives us a
handle with which to adjust the AC response. The goal is
to have 180° phase shift at DC (so the loop regulates) and
something less than 360° phase shift at the point that the
loop gain falls to 0dB. The simplest strategy is to set up
the feedback amplifier as an inverting integrator, with the
0dB frequency lower than the LC pole (Figure 12). This
“Type 1” configuration is stable but transient response is
less than exceptional if the LC pole is at a low frequency.
GAIN (dB)
3703 F12
0
PHASE
–6dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
R
B
R1
FB
C1
IN
OUT
+
–
V
REF
Figure 12. Type 1 Schematic and Transfer Function
Figure 13 shows an improved “Type 2” circuit that uses
an additional pole-zero pair to temporarily remove 90°
of phase shift. This allows the loop to remain stable with
90° more phase shift in the LC section, provided the loop
reaches 0dB gain near the center of the phase “bump.”
GAIN (dB)
3703 F13
0
PHASE
–6dB/OCT
–6dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
R
B
V
REF
R1
R2
FB
C2
IN
OUT
+
–
C1
Figure 13. Type 2 Schematic and Transfer Function
GAIN (dB)
3703 F14
0
PHASE
–6dB/OCT
+6dB/OCT –6dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
R
B
V
REF
R1
R2
FB
C2
IN
OUT
+
–
C1
C3
R3
Figure 14. Type 3 Schematic and Transfer Function
LTC3703
20
3703fc
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
performance with similar power components, but can be
way off if even one major power component is changed
significantly. Applications that require optimized transient
response will require recalculation of the compensation
values specifically for the circuit in question. The underlying
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
OUT
to FB and a 0.1µF feedback capacitor from COMP
to FB. Choose the bias resistor, R
B
, as required to set the
desired output voltage. Disconnect R
B
from ground and
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 capacitor.
Make sure the analyzer’s input is AC coupled so that the
DC voltages present at both the COMP and V
OUT
nodes
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
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.
*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 ;MOSFET rdson
*inductor
lext sw out1 10u ;inductor value
rl out1 out 0.015 ;inductor series R
*output cap
cout out out2 540u ;capacitor value
resr out2 0 0.01 ;capacitor ESR
*3703 internals
emod mod 0 value = {57*v(comp)}
;3703multiplier
vstim comp 0 0 ac 1 ;ac stimulus
.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 ris-
ing 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:
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.
applicaTions inForMaTion
LTC3703
21
3703fc
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)
f = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in
absolute gain)
TYPE 2 Loop:
K = tan
BOOST
2
+ 45°
C2=
1
2π • f •G•K •R1
C1= C2 K
2
−1
( )
R2=
K
2π • f •C1
R
B
=
V
REF
(R1)
V
OUT
− V
REF
TYPE 3 Loop:
K = tan
2
BOOST
4
+ 45°
C2=
1
2π • f •G•R1
C1= C2 K − 1
( )
R2=
K
2π • f •C1
R3=
R1
K − 1
C3=
1
2πf K •
R3
R
B
=
V
REF
(R1)
V
OUT
− V
REF
Boost Converter Design
The following sections discuss the use of the LTC3703
as a step-up (boost) converter. In boost mode, the
LTC3703 can step-up output voltages as high as 80V.
These sections discuss only the design steps specific to
a boost converter. For the design steps common to both
a buck and a boost, see the applicable section in the buck
mode section. An example of a boost converter circuit
is shown in the Typical Applications section. To operate
the LTC3703 in boost mode, the INV pin should be tied
to the V
CC
voltage (or a voltage above 2V). Note that in
boost mode, pulse-skip operation and the line feedforward
compensation are disabled.
For a boost converter, the duty cycle of the main switch is:
D=
V
OUT
– V
IN
V
OUT
For high V
OUT
to V
IN
ratios, the maximum V
OUT
is limited
by the LTC3703’s maximum duty cycle which is typically
93%. The maximum output voltage is therefore:
V
OUT(MAX)
=
V
IN(MIN)
1–D
MAX
≅ 14V
IN(MIN)
Boost Converter: Inductor Selection
In a boost converter, the average inductor current equals
the average input current. Thus, the maximum average
inductor current can be calculated from:
I
L(MAX)
=
I
O(MAX)
1−D
MAX
=I
O(MAX)
•
V
O
V
IN(MIN)
Similar to a buck converter, choose the ripple current to
be 20% to 40% of I
L(MAX)
. The ripple current amplitude
then determines the inductor value as follows:
L =
V
IN(MIN)
∆I
L
• f
•D
MAX
The minimum required saturation current for the inductor is:
I
L(SAT)
> I
L(MAX)
+ ∆I
L
/2
Boost Converter: Power MOSFET Selection
For information about choosing power MOSFETs for a
boost converter, see the Power MOSFET Selection section
for the buck converter, since MOSFET selection is similar.
applicaTions inForMaTion
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34

LTC3703EG#TRPBF

Mfr. #:
Buy LTC3703EG#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 100V Step-Down DC/DC Controller
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet