Datasheet

LT8705
20
8705fb
For more information www.linear.com/LT8705
applicaTions inForMaTion
The first page shows a typical LT8705 application circuit.
After the switching frequency is selected, external compo-
nent selection continues with the selection of R
SENSE
and
the inductor value. Next, the power MOSFETs are selected.
Finally, C
IN
and C
OUT
are selected. The following examples
and equations assume continuous conduction mode un-
less otherwise specified. The circuit can be configured
f
o
r operation up to an input and/or output voltage of 80V.
Operating Frequency Selection
The LT8705 uses a constant frequency architecture
between 100kHz and 400kHz. The frequency can be set
using the internal oscillator or can be synchronized to an
external clock source. Selection of the switching frequency
is a trade-off between efficiency and component size.
Low frequency operation increases efficiency by reducing
MOSFET switching losses, but requires more inductance
and/or capacitance to maintain low output ripple voltage.
For high power applications, consider operating at lower
frequencies to minimize MOSFET heating from switching
losses. The switching frequency can be set by placing an
appropriate resistor from the RT pin to ground and tying
the SYNC pin low. The frequency can also be synchronized
to an external clock source driven into the SYNC pin. The
following sections provide more details.
Internal Oscillator
The operating frequency of the LT8705 can be set using
the internal free-running oscillator. When the SYNC pin
is driven low (<0.5V), the frequency of operation is set
by the value of a resistor from the RT pin to ground. An
internally trimmed timing capacitor resides inside the IC.
The oscillator frequency is calculated using the following
formula:
f
OSC
=
43,750
R
T
+1




kHz
where f
OSC
is in kHz and R
T
is in kΩ. Conversely, R
T
(in
kΩ) can be calculated from the desired frequency (in
kHz) using:
R
T
=
43,750
f
OSC
–1




kΩ
SYNC Pin and Clock Synchronization
The operating frequency of the LT8705 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 LT8705 will operate at the SYNC clock frequency.
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
100kHz to 400kHz unless it is stopped low to enable
the free-running oscillator.
2. The SYNC pin frequency can always be higher than the
free-running oscillator set frequency, f
OSC
, but should
not be less than 25% below f
OSC
.
After SYNC begins toggling, it is recommended that
switching activity is stopped before the SYNC pin stops
toggling. Excess inductor current can result when SYNC
stops toggling as the LT8705 transitions from the external
SYNC clock source to the internal free-running oscillator
clock. Switching activity can be stopped by driving either
the SWEN or SHDN pin low.
CLKOUT Pin and Clock Synchronization
The CLKOUT pin can drive up to 200pF and toggles at the
LT8705’s internal clock frequency whether the internal clock
is synchronized to the SYNC pin or is free-running based
on the external R
T
resistor. The rising edge of CLKOUT is
approximately 180° out of phase from the internal clock’s
rising edge or the SYNC pin’s rising edge if it is toggling.
CLKOUT toggles only in normal mode (see Figure 2).
The CLKOUT pin can be used to synchronize other de
-
vices to the LT8705’s switching frequency. For example,
t
h
e CLKOUT pin can be tied to the SYNC pin of another
LT8705 regulator which will operate approximately 180°
out of phase of the master LT8705 due to the CLKOUT
phase shift. The frequency of the master LT8705 can be
set by the external R
T
resistor or by toggling the SYNC
pin. CLKOUT will begin oscillating after the master LT8705
enters normal mode (see Figure 2). Note that the RT pin
of the slave LT8705 must have a resistor tied to ground.
In general, use the same value R
T
resistor for all of the
synchronized LT8705s.