Datasheet

LT3745
21
3745f
applicaTions inForMaTion
Inductor Selection
The critical parameters for selection of an inductor are
inductance value, DC or RMS current, saturation current,
and DCR resistance. For a given input and output voltage,
the inductor value and switching frequency will determine
the peak-to-peak ripple current, ∆I
L
. The ∆I
L
value usually
ranges from 20% to 50% of the maximum output load
current, I
OUT(MAX)
. Lower values of ∆I
L
require larger and
more costly inductors; higher values of ∆I
L
increase the
peak currents and the inductor core loss. An inductor
current ripple of 30% to 40% offers a good compromise
between inductor performance and inductor size and cost.
However, for high duty cycle applications, a ∆I
L
value of
~20% should be used to prevent sub-harmonic oscillation
due to insufficient slope compensation.
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 ≥
V
OUT
+ V
D
V
IN(MAX)
+ V
D
•
V
IN(MAX)
– V
OUT
f
SW
• ∆I
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 supplied to
the C
CAP
capacitor by the internal regulator. The maximum
22mA current capability of the internal regulator limits the
maximum Q
G(MAX)
it can deliver to:
Q
G(MAX)
=
22mA
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.