LT3845
16
3845fd
APPLICATIONS INFORMATION
Choose the MOSFET V
DSS
specifi cation 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 LT3845, 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 suffi cient 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
specifi cation 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 LT3845 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
specifi cation should exceed the peak switch current,
I
OUT(MAX)
+ ΔI
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
specifi cation.
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 convert-
ers 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, ΔV
IN
:
C
IN(BULK)
=
I
OUT(MAX)
•V
OUT
ΔV
IN
•f
SW
•V
IN
MIN
ΔV
IN
is typically chosen at a level acceptable to the user.
100mV to 200mV is a good starting point. Aluminum elec-
trolytic capacitors are a good choice for high voltage, bulk
capacitance due to their high capacitance per unit area.
The capacitor’s RMS current is:
I
CIN(RMS)
=I
OUT
V
OUT
(V
IN
–V
OUT
)
(V
IN
)
2
If applicable, calculate it at the worst case condition,
V
IN
= 2V
OUT
. The RMS current rating of the capacitor
is specifi ed by the manufacturer and should exceed the
calculated I
CIN(RMS)
. Due to their low ESR (Equivalent
Series Resistance), ceramic capacitors are a good choice
for high voltage, high RMS current handling. Note that the
ripple current ratings from aluminum electrolytic capacitor
manufacturers are based on 2000 hours of life. This makes
it advisable to further derate the capacitor or to choose a
capacitor rated at a higher temperature than required.
The combination of aluminum electrolytic capacitors and
ceramic capacitors is an economical approach to meet-
ing the input capacitor requirements. The capacitor volt-
age rating must be rated greater than V
IN(MAX)
. Multiple
capacitors may also be paralleled to meet size or height
requirements in the design. Locate the capacitor very close
to the MOSFET switch and use short, wide PCB traces to
minimize parasitic inductance.
Output Capacitor Selection
The output capacitance, C
OUT
, selection is based on the
design’s output voltage ripple, ΔV
OUT
and transient load
requirements. ΔV
OUT
is a function of ΔI
L
and the C
OUT
ESR. It is calculated by:
V
OUT
= I
L
•ESR+
1
(8 • f
SW
•C
OUT
)
