lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 19

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
If the synchronization signal powers up in an undetermined
state (V
to the LT3692A as shown in Figure 9. The circuit as shown
will isolate the synchronization signal when the output
voltage is below 90% of the regulated output. The LT3692A
will start up with a switching frequency determined by the
resistor from the RT/SYNC pin to ground.
If the synchronization signal powers up in a low impedance
state (V
and the synchronizing clock. The equivalent resistance
seen from the RT/SYNC pin to ground will set the start-
up frequency.
If the synchronization signal powers up in a high imped-
ance state (Hi-Z), connect a resistor from the RT/SYNC
pin to ground. The equivalent resistance seen from the
RT/SYNC pin to ground will set the start-up frequency.
Figure 10. Timing Diagram RT/SYNC = 1MHz, Duty Cycle = 50%
Figure 9. Synchronous Signal Powered from Regulator’s Output
RT/SYNC
CLKOUT
t
DCLKOSW1
OL
OL
SW1
SW2
, V
), connect a resistor between the RT/SYNC pin
OH
PG1
, Hi-Z), connect the synchronization clock
LT3692A
RT/SYNC
t
t
P
DRTSYNC
/2
V
OUT1
t
P
t
DCLKOSW2
t
P
t
P
/2
t
P
V
SYNCHRONIZATION
/2
CC
CIRCUITRY
t
P
CLK
t
P
3692a F09
3692a F10
Reducing Input Ripple Voltage
Synchronizing the switches to the rising and falling edges
of the synchronization signal provides the unique ability to
reduce input ripple currents in systems where V
are connected to the same supply. Decreasing the input
current ripple reduces the required input capacitance. For
example, the input ripple voltage shown in Figure 12 for
a typical antiphase dual 14.4V to 8.5V and 14.4V to 3.3V
regulator is decreased from a peak of 472mV to 160mV
as shown in Figure 13 by driving the LT3692A with a 71%
duty cycle synchronization signal.
Figure 11. Timing Diagram RT/SYNC = 1MHz, Duty Cycle > 50%
Figure 12. Dual 14.4V/8.5V, 14.4V/3.3V with 180° Phase
Figure 13. Dual 14.4V/8.5V, 14.4V/3.3V with 256° Phase
t
RT/SYNC
DRTSYNCH
CLKOUT
t
SW1
SW2
DCLKOSW1
RIPPLE V
RT/SYNC
RIPPLE V
RT/SYNC
INPUT
INPUT
SW1
SW2
SW1
SW2
t
P
t
t
DRTSYNCH
PON
t
DCLKOSW2
t
P
t
PON
3692a F12
3692a F13
LT3692A
t
P
t
P
IN1
3692a F11
and V
19
3692af
IN2