Datasheet

LT3844
13
3844fb
APPLICATIONS INFORMATION
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 effi ciency. MOSFETs
with higher voltage V
DSS
specifi cation usually have higher
R
DS(ON)
and lower C
RSS
.
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.
The internal V
CC
regulator is capable of sourcing up to
40mA which limits the maximum total MOSFET gate
charge, Q
G
, to 40mA/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 LT3844 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.
Step-Down Converter: Rectifi er Selection
The rectifi er diode (D1 on the Functional Diagram) in a
buck converter generates a current path for the inductor
current when the main power switch is turned off. The
rectifi er is selected based upon the forward voltage, re-
verse voltage and maximum current. A Schottky diode is
recommended. Its low forward voltage yields the lowest
power loss and highest effi ciency. The maximum reverse
voltage that the diode will see is V
IN(MAX)
.
In continuous mode operation, the average diode cur-
rent is calculated at maximum output load current and
maximum V
IN
:
II
VV
V
DIODE AVG OUT MAX
IN MAX OUT
IN MAX
() ( )
()
()
=
−
To improve effi ciency and to provide adequate margin
for short-circuit operation, a diode rated at 1.5 to 2
times the maximum average diode current, I
DIODE(AVG)
,
is recommended.
Step-Down Converter: 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 keep from
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple, ΔV
IN
:
C
IV
Vf V
IN BULK
OUT MAX OUT
IN SW IN MIN
()
()
()
•
••
=
Δ
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:
II
VVV
V
CIN RMS OUT
OUT IN OUT
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