LT8582
20
8582f
APPLICATIONS INFORMATION
SWITCHING FREQUENCY
There are several considerations in selecting the operat-
ing frequency of the converter. The first is staying clear
of sensitive frequency bands, which cannot tolerate any
spectral noise. For example, in RF communication prod-
ucts with a 455kHz IF, switching above 600kHz is desired.
Communication products with sensitivity to 1.1MHz would
require to set the switching frequency to 1.5MHz or higher.
Also, like any other switching regulator, harmonics of much
higher frequency than the switching frequency are also
produced. The second consideration is the physical size
of the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade-off is efficiency, since the switching losses due
to inductor AC loss, NPN base drive (see Thermal Calcula-
tions), Schottky diode charge and other capacitive loss
terms increase proportionally with frequency.
Oscillator Timing Resistor (R
T
)
The operating frequency of the LT8582 can be set by the
internal free running oscillator. When the SYNC pin for a
channel is driven low (< 0.4V), the oscillator frequency
for that channel is set by a resistor from the RT pin to
ground. The oscillator frequency is calculated using the
following formula:
f
OSC
=
81.6
R
T
+ 1
where f
OSC
is in MHz and R
T
is in k. Conversely, R
T
(in k) can be calculated from the desired frequency (in
MHz) using:
R
T
=
81.6
f
OSC
–1
Clock Synchronization
The operating frequency of each channel of the LT8582
can be set by an external source by simply providing
a clock into the SYNC pin for that channel (R
T
resistor
still required). The LT8582 will revert to its internal free
running oscillator clock (set by the R
T
resistor) when the
SYNC pin is driven below 400mV for several free running
clock periods.
Driving the SYNC pin of a channel high for an extended
period of time effectively stops the oscillator for that chan-
nel. As a result, the switching operation for that channel of
the LT8582 will stop and the CLKOUT pin of that channel
will be pulled low.
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 to 2.5MHz.
(2) The SYNC frequency can be higher than the free run-
ning oscillator frequency (as set by the R
T
resistor),
f
OSC
, but should not be less than 25% below f
OSC
.
Clock Synchronization of Additional Regulators
The CLKOUT pins of the LT8582 can be used to synchro-
nize additional switching regulators or other channels of
LT8582s, as shown in the Typical Application figure on
the front page.
The frequency of channel 1 of the LT8582 is set by the
external R
T
resistor. The SYNC pin of channel 2 of the
LT8582 is driven by the CLKOUT pin of channel 1 of the
LT8582. Channel 1’s CLKOUT pin has a 50% duty cycle
intended for driving SYNC2 and is 180° out of phase for
reduced input ripple or multiphase topologies.
Note that the RT pin of channel 2 of the LT8582 must have
a resistor tied to ground. It takes a few clock cycles for the
CLKOUT signal to begin oscillating and it is preferable for
all LT8582 channels to have the same internal free running
frequency. Therefore, in general, use the same value R
T
resistor for all of the synchronized LT8582s.
EVENT BASED SEQUENCING
The PG pin may be used to sequence other ICs since it
is pulled low as long as the LT8582 is enabled and the
magnitude of the output voltage is below regulation (refer
to the Block Diagram). Since the PG pin is an open drain
output, it can be used to pull the SHDN pin of another IC
low until the output of one of the channels of the LT8582
