LT3580 LINER [Linear Technology], LT3580 Datasheet - Page 13

• Manufacturer Part Number: LT3580
• Description: Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start, and Synchronization
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Using the circuit in Figure 14 as an example, Table 3 shows
the parameters used to generate the bode plot shown in
Figure 5. Note that R2 is 14.6k ||14.6k = 7.3k which is the
effective small signal resistance looking into the FB pin
of the LT3580.
Table 3. Bode Plot Parameters
PARAMETER
R
C
R
R
C
C
R
R1
R2
V
V
V
g
g
L
f
In Figure 5, the phase is –140° when the gain reaches 0dB
giving a phase margin of 40°. The crossover frequency
is 10kHz, which is more than three times lower than the
frequency of the RHP zero to achieve adequate phase
margin.
S
ma
mp
OUT
C
PL
REF
OUT
IN
L
ESR
O
C
Figure 5. Bode Plot for Example Boost Converter
160
140
120
100
–20
180
60
40
20
80
0
10
VALUE
1.215
1000
21.8
305
130
230
7.3
4.2
1.2
GAIN
100
10
10
10
12
0
5
7
FREQUENCY (Hz)
1k
PHASE
UNITS
10k
μmho
mho
MHz
40° AT
mΩ
10kHz
kΩ
kΩ
kΩ
kΩ
μH
μF
pF
pF
Ω
V
V
V
100k
3580 F05
Application Specifi c
Application Specifi c
Application Specifi c
Optional/Adjustable
Application Specifi c
Application Specifi c
Application Specifi c
1M
Not Adjustable
Not Adjustable
Not Adjustable
Not Adjustable
Not Adjustable
COMMENT
Adjustable
Adjustable
Adjustable
Adjustable
0
–20
–40
–60
–80
–100
–120
–140
–160
–180
–200
Diode Selection
Schottky diodes, with their low forward voltage drops and
fast switching speeds, are recommended for use with the
LT3580. The Microsemi UPS120 is a very good choice.
Where the input-to-output voltage differential exceeds 20V,
use the UPS140 (a 40V diode). These diodes are rated to
handle an average forward current of 1A.
Oscillator
The operating frequency of the LT3580 can be set by the
internal free-running oscillator. When the SYNC pin is
driven low (< 0.4V), the frequency of operation is set by a
resistor from R
capacitor resides inside the IC. The oscillator frequency
is calculated using the following formula:
where f
(in kΩ) can be calculated from the desired frequency (in
MHz) using:
Clock Synchronization
The operating frequency of the LT3580 can be synchro-
nized to an external clock source. To synchronize to the
external source, simply provide a digital clock signal into
the SYNC pin. The LT3580 will operate at the SYNC clock
frequency. The LT3580 will revert to the internal free-run-
ning oscillator clock after SYNC is driven low for a few
free-running clock periods.
Driving SYNC high for an extended period of time effec-
tively stops the operating clock and prevents latch SR1
from becoming set (see the Block Diagram). As a result,
the switching operation of the LT3580 will stop.
The duty cycle of the SYNC signal must be between 35%
and 65% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
R
f
OSC
T
=
=
OSC
91.9
f
OSC
(R
91.9
is in MHz and R
T
+ 1)
T
1
to ground. An internally trimmed timing
T
is in kΩ. Conversely, R
LT3580
13
3580fc
T