LT3580
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.
Table 3. Bode Plot Parameters
PARAMETER
R
L
C
OUT
R
ESR
R
O
C
C
C
F
C
PL
R
C
R1
R2
V
OUT
V
IN
g
ma
g
mp
L
f
S
VALUE
21.8
10
10
305
1000
0
0
10
130
14.6
12
5
230
7
4.2
1.2
UNITS
Ω
μF
mΩ
kΩ
pF
pF
pF
kΩ
kΩ
kΩ
V
V
μmho
mho
μH
MHz
COMMENT
Application Specific
Application Specific
Application Specific
Not Adjustable
Adjustable
Optional/Adjustable
Optional/Adjustable
Adjustable
Adjustable
Not Adjustable
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Application Specific
Adjustable
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
T
to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency is
calculated using the following formula:
f
OSC
=
91.9
(R
T
+
1)
where f
OSC
is in MHz and R
T
is in kΩ. Conversely, R
T
(in kΩ) can be calculated from the desired frequency
(in MHz) using:
R
T
=
91.9
−
1
f
OSC
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.
180
160
140
120
GAIN (dB)
100
80
60
40
20
0
–20
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
3580 F05
Clock Synchronization
The operating frequency of the LT3580 can be synchronized
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-running oscillator
clock after SYNC is driven low for a few free-running clock
periods.
Driving SYNC high for an extended period of time effectively
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:
0
–20
–40
PHASE
–60
PHASE (DEG)
–80
–100
40 AT
10kHz
GAIN
–120
–140
–160
–180
–200
Figure 5. Bode Plot for Example Boost Converter
3580fg
12
LINER [ LINEAR TECHNOLOGY ]
