LT8708
26
Rev 0
For more information
APPLICATIONS INFORMATION
This Applications Information section provides additional
details for setting up an application using the LT8708. Top-
ics include verifying the power flow conditions, selection
of various external components including the switching
MOSFETs, sensing resistors, filter capacitors, diodes and
the primary inductor among others. In addition, more
information is provided about voltage lockouts, current
monitoring, PCB layout and efficiency considerations.
This section wraps up with a design example to illustrate
the use of the various design equations presented here.
VERIFY THE POWER FLOW CONDITIONS
Due to the configurability of the LT8708, a methodical
approach should be used to verify that power will flow,
as intended, under all relevant conditions. Table 6(a) and
6(b) are provided to help with this verification.
First, using Table 6(a), note which V
IN
and V
OUT
combina-
tions are used in the application. For example, print a copy
of Table 6(a) and highlight or circle the applicable cells.
In Table 6(a):
• V
IN_FBIN
is the V
IN
voltage when FBIN is at 1.205V (typ)
• V
OUT_FBOUT
is the V
OUT
voltage when FBOUT is at
1.207V (typ)
• V
IN_VINHIMON
is the V
IN
voltage when V
INHIMON
at
1.207V (typ)
• V
OUT_VOUTLOMON
is the V
OUT
voltage when V
OUTLOMON
is at 1.207V (typ)
If one or more of the FBIN, FBOUT, VINHIMON and VOUT-
LOMON pins are tied to their inactive states (see Table 5
and the VINHIMON, VOUTLOMON and
RVSOFF
section),
the associated row(s) or column(s) will not apply to the
application. For example, if FBIN is tied to LDO33 to
deactivate that pin function, then the V
IN
< V
IN_FBIN
row
of Table 6(a) is not applicable and no cells in that row
should be circled.
Next, for each cell identified in Table 6(a), check that the
operating condition described in Table 6(b) meets the
application’s requirements.
Table 6. Power Flow Verification Table
6(a)
V
OUT
/V
IN
V
OUT
<
V
OUT_VOUTLOMON
V
OUT
>
V
OUT_VOUTLOMON
&
V
OUT
< V
OUT_FBOUT
V
OUT
>
V
OUT_FBOUT
V
IN
< V
IN_FBIN
No Power
Transfer
B
B
V
IN
> V
IN_FBIN
&
V
IN
<
V
IN_VINHIMON
A
D
C
V
IN
>
V
IN_VINHIMON
A
D
No Power
Transfer
6(b)
MODE =
BURST
MODE = CCM
MODE =
DCM/HCM,
DIR = FWD
MODE = DCM/
HCM, DIR = RVS
A
Power Flows from V
IN
to V
OUT
No Power Flow
B
No Power
Flow
Power Flows
from V
OUT
to V
IN
No Power
Flow
Power Flows
from V
OUT
to V
IN
C
No Power Flow
D
Power Flows from V
IN
to V
OUT
Note: Table 6(a) and Table 6(b) assume that the
RVSOFF
pin is not driven low by an external device.
See the Design Example section for a further example of
using these tables.
OPERATING FREQUENCY SELECTION
The LT8708 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.
