LTC1873EG#TR Linear Technology, LTC1873EG#TR Datasheet - Page 22

• Manufacturer Part Number: LTC1873EG#TR
• Description: IC REG SW 2PH DUAL SYNC 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1873EG#TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 750kHz
• Duty Cycle: 93%
• Voltage - Supply: 3 V ~ 7 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-SSOP
• Frequency-max: 750kHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1873
loop should cross through 0dB in the middle of the phase
bump to maximize phase margin. Many LTC1873 circuits
using low ESR tantalum or OS-CON output capacitors
need type 3 compensation to obtain acceptable phase
margin with a high bandwidth feedback loop.
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 com-
ponents 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
need to recalculate 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.
22
GAIN
(dB)
Figure 11a. Type 3 Amplifier Schematic Diagram
Figure 11b. Type 3 Amplifier Transfer Function
0
IN
GAIN
PHASE
–6dB/OCT
C3
U
R1
R
B
R3
+6dB/OCT
U
V
REF
R2
+
–
C2
C1
W
–6dB/OCT
1873 F11a
OUT
PHASE
0
–90
–180
–270
1873 F11b
(DEG)
U
ANALYZER
ANALYZER
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 LTC1873
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 LTC1873, 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 12) 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
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
SOURCE
OUT
V
FROM
COMP
Figure 12. Modulator Gain/Phase Measurement Set-Up
TO
AC
to FB and a 0.1 F feedback capacitor from COMP to
10 F
0.1 F
R
+
B
10k
NC
COMP
FB
RUN/SS
SGND PGND
V
1/2 LTC1873
CC
10
OUT
BOOST2
PV
FAULT
CC
FCB
)/V(COMP) in dB and phase of
MBR0530T
SW
BG
TG
B
) as required to set the
5V
QT
QB
+
B
from ground and
1 F
C
L
IN
EXT
1873 F12
+
C
V
TO
ANALYZER
OUT
OUT
OUT