Datasheet

LT3579/LT3579-1
22
35791fa
For more information www.linear.com/LT3579
APPLICATIONS INFORMATION
The frequency of the master LT3579 is set by the external
R
T
resistor. The SYNC pin of the slave LT3579 is driven
by the CLKOUT pin of the master LT3579. Note that the
RT pin of the slave LT3579 must have a resistor tied to
ground. It takes a few clock cycles for the CLKOUT signal
to begin oscillating, and it’s preferable for all LT3579s to
have the same internal free-running frequency. Therefore,
in general, use the same value R
T
resistor for all of the
synchronized LT3579s.
Also, the FAULT pins can be tied together so that a fault
condition from one LT3579 causes all of the LT3579s to
enter fault, until the fault condition disappears.
2-Phase Converters using LT3579-1
The CLKOUT pin on the LT3579-1 is ~180° out of phase
with the internal oscillator, which allows two LT3579-1s to
operate in parallel for a high current, high power output.
The advantage of multiphase converters is that the ripple
current flowing into the output node is divided by the
number of phases or ICs used to generate the output
voltage. The V
IN
, SHDN, FAULT, FB, and V
C
pins of all the
LT3579-1s should be connected together. Figure 13 shows
a typical application of a 2-phase 12V to 24V boost with
output disconnect.
Use the following equations to calculate the FB resistor
for 2-phase converters:
R
FB
=
V
OUT
–1.215V
2 • 83.3µA






; Boost or SEPIC
Multiphase Converter
R
FB
=
|V
OUT
| + 9mV
2 • 83.3µA






; Inverting Multiphase
Converter
4.7µH
4.7µH
V
PWR
12V
V
IN
5V
V
OUT
24V
3.7A,
89W
V
OUT1
V
OUT1
V
PWR
10µF
4.7µF
10µF
4.7µF
×2
0.22µF
4.7µF
4.7µF
×2
137k
35791 F13
SW1 SW2
V
IN
CLKOUT
GATE
V
C
SSGND
FB
FAULT
SYNC
SHDN
RT
LT3579-1
MASTER
SW1 SW2
V
IN
CLKOUT
GATE
V
C
SSGND
FB
FAULT
SYNC
SHDN
RT
LT3579-1
SLAVE
86.6k
6.4k
7k
2.2nF
47pF
0.22µF
5k 86.6k
21.5k500k
100k
4.7µF
×2
Figure 13. 2-Phase Converters Using LT3579-1
Note that the CLKOUT pin on the LT3579-1 runs at a fixed
duty cycle of ~50%. If monitoring the die temperature is
desired, the slave IC can be a LT3579.
It is possible to use the LT3579-1 in a multiphase converter
of more than 2 phases. Consult the LTC Applications
Engineering Department for more information.