LTC3703EGN-5#PBF Linear Technology, LTC3703EGN-5#PBF Datasheet - Page 20
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LTC3703EGN-5#PBF
Manufacturer Part Number
LTC3703EGN-5#PBF
Description
IC BUCK/BOOST SYNC ADJ 5A 16SSOP
Manufacturer
Linear Technology
Type
Step-Down (Buck), Step-Up (Boost)r
Datasheet
1.LTC3703EGN-5PBF.pdf
(32 pages)
Specifications of LTC3703EGN-5#PBF
Internal Switch(s)
No
Synchronous Rectifier
Yes
Number Of Outputs
1
Voltage - Output
0.8 ~ 55.8 V
Current - Output
5A
Frequency - Switching
100kHz ~ 600kHz
Voltage - Input
9.3 ~ 60 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
16-SSOP
Primary Input Voltage
60V
No. Of Outputs
1
Output Voltage
60V
Output Current
5A
No. Of Pins
16
Operating Temperature Range
-40°C To +85°C
Msl
MSL 1 - Unlimited
Rohs Compliant
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
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ANALYZER
ANALYZER
LTC3703-5
APPLICATIO S I FOR ATIO
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 LTC3703-5
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 LTC3703-5, 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 (Figure 14) 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
20
SOURCE
OUT
V
FROM
COMP
Figure 14. Modulator Gain/Phase Measurement Set-Up
TO
AC
to FB and a 0.1µF feedback capacitor from COMP to
0.1µF
10µF
R
B
+
10k
NC
DRV
f
COMP
FB
RUN/SS
SET
GND BGRTN
V
5V
LTC3703-5
U
CC
MODE/SYNC
CC
BOOST
INV
SW
V
BG
U
TG
IN
B
) as required to set the
V
IN
W
M1
M2
+
B
37035 F14
from ground and
C
IN
L
EXT
+
U
C
V
TO
ANALYZER
OUT
OUT
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
V
how to generate this plot.
*3703-5 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
*3703-5 internals
emod mod 0 value = {43*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 10. 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.
OUT
BOOST = – (PHASE + 30°)
in degrees. Refer to your SPICE manual for details of
OUT
)/V(COMP) in dB and phase of
;MOSFET rdson
;inductor value
;inductor series R
;capacitor value
;capacitor ESR
;3703-5multiplier
;ac stimulus
37035fa