LT8580
14
8580fa
For more information www.linear.com/LT8580
applicaTions inForMaTion
Using the circuit in Figure 4 as an example, Table 3 shows
the parameters used to generate the bode plot shown in
Figure 6.
In Figure 6, the phase is –126° when the gain reaches 0dB
giving a phase margin of 54°. The crossover frequency is
14kHz, which is more than three times lower than the fre
-
quency of the RHP zero to achieve adequate phase margin.
Diode Selection
Schottky diodes, with their low forward-voltage drops and
fast switching speeds, are recommended for use with the
LT8580. For applications where V
R
(see Tables 4, 5 and
6) < 40V, the Diodes, Inc. SBR1V40LP is a good choice.
Where V
R
> 40V, the Diodes Inc. DFLS1100 works well.
These diodes are rated to handle an average forward
current of 1A.
Oscillator
The operating frequency of the LT8580 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
=
(R
+ 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
=
f
−1
Clock Synchronization
The operating frequency of the LT8580 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 LT8580 will operate at the SYNC clock frequency.
The LT8580 will revert to the internal free-running oscilla
-
tor clock after SYNC is driven low for a few free-running
clock periods.
Figure 6. Bode Plot for Example Boost Converter
FREQUENCY (Hz)
10
60
GAIN (dB)
PHASE (DEG)
80
100
120
100 1k 10k 100k 1M
8580 F06
40
20
0
–20
140
–180
–135
–90
–45
–225
–270
–315
–360
0
54° AT
14kHz
PHASE
GAIN
Table 3. Bode Plot Parameters
PARAMETER VALUE UNITS COMMENT
R
L
60
W
Application Specific
C
OUT
4.7 µF Application Specific
R
ESR
10 mW Application Specific
R
O
300 kW Not Adjustable
C
C
3300 pF Adjustable
C
F
47 pF Optional/Adjustable
C
PL
0 pF Optional/Adjustable
R
C
6.04 kW Adjustable
R1 130 kW Adjustable
R2 14.6 kW Not Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
g
ma
200 µmho Not Adjustable
g
mp
7 mho Not Adjustable
L 15 µH Application Specific
f
S
1.5 MHz Adjustable
