LT3845A
16
3845afa
APPLICATIONS INFORMATION
Note that R
DS(ON)
has a large positive temperature
dependence. The MOSFET manufacturer’s data sheet
contains a curve, R
DS(ON)
vs Temperature.
In the main MOSFET, transition losses are proportional
to V
IN
2
and can be considerably large in high voltage
applications (V
IN
> 20V). Calculate the maximum transition
losses:
P
TRAN(TOP)
= k • V
IN
2
• I
OUT(MAX)
• C
RSS
• f
SW
where k is a constant inversely related to the gate
driver current, approximated by k = 2 for LT3845A
applications.
The total maximum power dissipations of the MOSFET
are:
P
TOP(TOTAL)
= P
COND(MAIN)
+ P
TRAN(MAIN)
P
BOT(TOTAL)
= P
COND(SYNC)
To achieve high supply efficiency, keep the total power
dissipation in each switch to less than 3% of the total
output power. Also, complete a thermal analysis to ensure
that the MOSFET junction temperature is not exceeded.
T
J
= T
A
+ P
(TOTAL)
• q
JA
where q
JA
is the package thermal resistance and T
A
is the
ambient temperature. Keep the calculated T
J
below the
maximum specified junction temperature, typically 150°C.
Note that when V
IN
is high and f
SW
is high, the transition
losses may dominate. A MOSFET with higher R
DS(ON)
and lower C
RSS
may provide higher efficiency. MOSFETs
with higher voltage V
DSS
specification usually have higher
R
DS(ON)
and lower C
RSS
.
Choose the MOSFET V
DSS
specification to exceed the
maximum voltage across the drain to the source of the
MOSFET, which is V
IN(MAX)
plus any additional ringing
on the switch node. Ringing on the switch node can be
greatly reduced with good PCB layout and, if necessary,
an RC snubber.
In some applications, parasitic FET capacitances couple
the negative going switch node transient onto the bottom
gate drive pin of the LT3845A, causing a negative voltage
in excess of the Absolute Maximum Rating to be imposed
on that pin. Connection of a catch Schottky diode from
this pin to ground will eliminate this effect. A 1A current
rating is typically sufficient of the diode.
The internal V
CC
regulator is capable of sourcing up to
40mA limiting the maximum total MOSFET gate charge,
Q
G
, to 35mA/f
SW
. The Q
G
vs V
GS
specification is typically
provided in the MOSFET data sheet. Use Q
G
at V
GS
of 8V.
If V
CC
is back driven from an external supply, the MOSFET
drive current is not sourced from the internal regulator
of the LT3845A and the Q
G
of the MOSFET is not limited
by the IC. However, note that the MOSFET drive current
is supplied by the internal regulator when the external
supply back driving V
CC
is not available such as during
start-up or short circuit.
The manufacturer’s maximum continuous drain current
specification should exceed the peak switch current,
I
OUT(MAX)
+ DI
L
/2.
During the supply start-up, the gate drive levels are set by
the V
CC
voltage regulator, which is approximately 8V. Once
the supply is up and running, the V
CC
can be back driven
by an auxiliary supply such as V
OUT
. It is important not
to exceed the manufacturer’s maximum V
GS
specification.
A standard level threshold MOSFET typically has a V
GS
maximum of 20V.
Input Capacitor Selection
A local input bypass capacitor is required for buck
converters because the input current is pulsed with fast
rise and fall times. The input capacitor selection criteria are
based on the bulk capacitance and RMS current capability.
The bulk capacitance will determine the supply input ripple
voltage. The RMS current capability is used to prevent
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple, DV
IN
:
C
IN(BULK)
=
I
OUT(MAX)
• V
OUT
DV
IN
• f
SW
• V
IN(MIN)
