LT8708
39
Rev 0
For applications with high input or output voltages
(typically >40V) avoid Schottky diodes with excessive
reverse-leakage currents, particularly at high tempera-
tures. Some ultra-low V
F
diodes will trade-off increased
high temperature leakage current for reduced forward
voltage. Diodes D1 and D2 can have reverse voltages in
excess of V
IN
and D3 and D4 can have reverse voltages in
excess of V
OUT
. The combination of high reverse voltage
and current can lead to self-heating of the diode. Besides
reducing efficiency, this can increase leakage current
which increases temperatures even further. Choose pack-
ages with lower thermal resistance (
θ
JA
) to minimize self
heating of the diodes.
TOPSIDE MOSFET DRIVER SUPPLY
(C
B1
, D
B1
, C
B2
, D
B2
)
The top MOSFET drivers (TG1 and TG2) are driven digitally
between their respective SW and BOOST pin voltages.
The BOOST voltages are biased from floating booststrap
capacitors C
B1
and C
B2
, which are normally recharged
through external silicon diodes D
B1
and D
B2
when the
respective top MOSFET is turned off. The capacitors are
charged to about 6.3V (about equal to GATEV
CC
) forcing the
V
BOOST1-SW1
and V
BOOST2-SW2
voltages to be about 6.3V.
The boost capacitors C
B1
and C
B2
need to store about 100
times the gate charge required by the top switches M1 and
M4. In most applications, a 0.1μF to 0.47μF, X5R or X7R
dielectric capacitor is adequate. The bypass capacitance
from GATEV
CC
to GND should be at least 10 times the C
B1
or C
B2
capacitance.
Top Driver: Boost Cap Charge Control Block
When the LT8708 operates exclusively in the boost or buck
region, M1 or M4 respectively may be “on” continuously.
This prevents the respective bootstrap capacitor, C
B1
or
C
B2
, from being recharged through the silicon diode, D
B1
or D
B2
. The Boost Cap Charge Control block (see Figure 1)
keeps the appropriate bootstrap capacitor charged in these
cases. In the boost region, when M1 is always on, current
is drawn, as needed, from the CSNOUT and/or BOOST2
pins to charge the C
B1
capacitor. In the buck region, when
M4 is always on, current is drawn, as needed, from the
CSNIN and/or BOOST1 pins to charge the C
B2
capacitor.
Because of this function, CSPIN and CSNIN should be
connected across R
SENSE1
in series with the M1 drain.
Connect both pins to the M1 drain if they are not being
used. Also, CSPOUT and CSNOUT should be connected
across R
SENSE2
in series with the M4 drain or connect
both to the M4 drain if not being used.
Top Driver: Boost Diodes D
B1
and D
B2
Although Schottky diodes have the benefit of low forward
voltage drops, they can exhibit high reverse current leak-
age and have the potential for thermal runaway under high
voltage and temperature conditions. Silicon diodes are
thus recommended for diodes D
B1
and D
B2
. Make sure
that D
B1
and D
B2
have reverse breakdown voltage ratings
higher than V
IN(MAX)
and V
OUT(MAX)
and have less than
1mA of reverse-leakage current at the maximum operating
junction temperature. Make sure that the reverse-leakage
current at high operating temperatures and voltages won’t
cause thermal runaway of the diode.
In some cases it is recommended that up to 5Ω of resistance
is placed in series with D
B1
and D
B2
. The resistors reduce
surge currents in the diodes and can reduce ringing at the
SW and BOOST pins of the IC. Since SW pin ringing is
highly dependent on PCB layout, SW pin edge rates and
the type of diodes used, careful measurements directly
at the SW pins of the IC are recommended. If required, a
single resistor can be placed between GATEV
CC
and the
common anodes of D
B1
and D
B2
(as in the front page
application) or by placing separate resistors between the
cathodes of each diode and the respective BOOST pins.
Excessive resistance in series with D
B1
and D
B2
can reduce
the BOOST-SW capacitor voltage when the M2 or M3 on-
times are very short and should be avoided.
VINHIMON, VOUTLOMON AND
RVSOFF
During reverse conduction, current and power are drawn
from V
OUT
and delivered to V
IN
. This has the potential to
draw V
OUT
lower than desired or drive V
IN
higher than
desired, depending on the supplies and loads. The VIN-
HIMON and VOUTLOMON pins are used to detect either
of these conditions and disable reverse conduction by
pulling
RVSOFF
low.
APPLICATIONS INFORMATION
