Datasheet

LT3579/LT3579-1
21
35791fa
For more information www.linear.com/LT3579
3.3µH
10µF
10µF
200k
V
OUT
18V
1A
V
OUT
12V
1.7A
V
IN
5V
35791 F12
SW1 SW2
GATE
FB
V
C
SS
GNDSYNC
CLKOUT
V
IN
RT
SHDN
FAULT
LT3579
SLAVE
10µF
×2
10µF
×3
8k
100k
110k
3.3nF0.1µF
68pF
47pF
4.7µF
4.7µF
86.6k
86.6k
2.2µH
10k
SW1 SW2
GATE CLKOUT
V
C
SS
GNDSYNC
FB
V
IN
RT
FAULT
SHDN
LT3579
MASTER
2.2nF
8k
130k
0.1µF
Figure 12. Synchronize Multiple LT3579s. The External PMOS
Disconnects the Input from Both Power Paths During FAULT Events
CLOCK SYNCHRONIZATION OF ADDITIONAL
REGULATORS
The CLKOUT pin of the LT3579 can synchronize additional
switching regulators and/or additional LT3579s as shown
in Figure 12.
APPLICATIONS INFORMATION
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
=
87.6
f
OSC
–1
Clock Synchronization
An external source can set the operating frequency of the
LT3579 by providing a digital clock signal into the SYNC
pin (R
T
resistor still required). The LT3579 will operate at
the SYNC clock frequency. The LT3579 will revert to its
internal free-running oscillator clock when the SYNC pin is
driven below 0.4V 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 Block Diagram). As a result,
the switching operation of the LT3579 will stop and the
CLKOUT pin will be held at ground.
The duty cycle of the SYNC signal must be between 20%
and 80% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
1. SYNC may not toggle outside the frequency
range of 200kHz-2.5MHz unless it is stopped
below 0.4V to enable the free-running oscillator.
2. The SYNC frequency can always be higher than
the free-running oscillator frequency (as set by the
R
T
resistor), f
OSC
, but should not be less than 25%
below f
OSC
.