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LT3746
APPLICATIONS INFORMATION
V OUT + V D V IN(MAX) – V OUT
V IN(MAX) + V D
f SW ? Δ I L
Q G(MAX) =
The largest inductor ripple current occurs at the highest
V IN . To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L ≥ ?
The inductor DC or RMS current rating must be greater
than the maximum output load current I OUT(MAX) and its
saturation current should be higher than the maximum
inductor current I L(MAX) . To achieve high efficiency, the DCR
resistance should be less than 0.1Ω, and the core material
should be intended for high frequency applications.
Power MOSFET Selection
Important parameters for the external P-channel MOSFET
M1 include drain-to-source breakdown voltage (V (BR)DSS) ,
maximum continuous drain current (I D(MAX) ), maximum
gate-to-source voltage (V GS(MAX) ), total gate charge (Q G ),
drain-to-source on resistance (R DS(ON) ), reverse transfer
capacitance (C RSS ). The MOSFET V (BR)DSS specification
should exceed the maximum voltage across the source to
the drain of the MOSFET, which is V IN(MAX) plus V D . The
I D(MAX) should exceed the peak inductor current, I L(MAX) .
Since the gate driver circuit is supplied by the internal 6.8V
V IN referenced regulator, the V GS(MAX) rating should be
at least 10V.
Each switching cycle the MOSFET is switched off and on,
a packet of gate charge Q G is transferred from the V IN pin
to the GATE pin, and then from the GATE pin to the CAP
pin. The resulting dQ G /dt is a current that must be sup-
plied to the C CAP capacitor by the internal regulator. The
maximum 20mA current capability of the internal regulator
limits the maximum Q G(MAX) it can deliver to:
20mA
f SW
Therefore, the Q G at V GS = 6.8V from the MOSFET data
sheet should be less than Q G(MAX) .
For maximum efficiency, both R DS(ON) and C RSS should
be minimized. Lower R DS(ON) means less conduction loss
while lower C RSS reduces transition loss. Unfortunately,
R DS(ON) is inversely related to C RSS . Thus balancing the
conduction loss with the transition loss is a good criterion
in selecting a MOSFET. For applications with higher V IN
voltages (≥24V) a lower C RSS is more important than a
low R DS(ON) .
Catch Diode Selection
The catch diode D1 carries load current during the switch
off time. Important parameters for the catch diode includes
peak repetitive reverse voltage (V RRM ), forward voltage
(V F ), and maximum average forward current (I F(AV) ). The
diode V RRM specification should exceed the maximum
reverse voltage across it, i.e., V IN(MAX) . A fast switching
schottky diode with lower V F should be used to yield lower
power loss and higher efficiency.
In continuous conduction mode, the average current
conducted by the catch diode is calculated as:
I D(AVG) = I OUT ? (1–D)
The worst-case condition for the diode is when V OUT is
shorted to ground with maximum V IN and maximum I OUT
at present. In this case, the diode must safely conduct
the maximum load current almost 100% of the time. To
improve efficiency and to provide adequate margin for
short circuit operation, a schottky diode rated to at least
the maximum output current is recommended.
C IN , C VCC , and C CAP Capacitor Selection
A local input bypass capacitor C IN 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 voltage rating, bulk capacitance, and RMS
current capability. The capacitor voltage rating must be
greater than V IN(MAX) . The bulk capacitance determines the
input supply ripple voltage and the RMS current capability
is used to keep from overheating the capacitor.
For more information www.linear.com/3746
3746fa
21
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