Datasheet

LTC3783
15
3783fb
OPERATION
Boost Converter: Inductor Selection
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value can be determined using the following
equation:
L =
V
IN(MIN)
∆I
L
• f






• D
MAX
where :
∆I
L
=
c • I
OUT(MAX)
1– D
MAX
Remember that most boost converters are not short-circuit
protected. Under a shorted output condition, the inductor
current is limited only by the input supply capability. For
applications requiring a step-up converter that is short-
circuit protected, please refer to the applications section
covering SEPIC converters.
The minimum required saturation current of the inductor
can be expressed as a function of the duty cycle and the
load current, as follows:
I
L(SAT)
> 1+
c
2






•
I
OUT(MAX)
1– D
MAX
The saturation current rating for the inductor should be
checked at the minimum input voltage (which results in the
highest inductor current) and maximum output current.
Boost Converter: Operating in Discontinuous Mode
Discontinuous mode operation occurs when the load cur-
rent is low enough to allow the inductor current to run out
during the off-time of the switch, as shown in Figure 7.
Once the inductor current is near zero, the switch and diode
capacitances resonate with the inductance to form damped
ringing at 1MHz to 10MHz. If the off-time is long enough,
the drain voltage will settle to the input voltage.
Depending on the input voltage and the residual energy in
the inductor, this ringing can cause the drain of the power
MOSFET to go below ground where it is clamped by the body
diode. This ringing is not harmful to the IC and it has not
been shown to contribute significantly to EMI. Any attempt
to damp it with a snubber will degrade the efficiency.
Boost Converter: Power MOSFET Selection
The power MOSFET can serve two purposes in the LTC3783:
it represents the main switching element in the power path,
and its R
DS(ON)
can represent the current sensing element
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
BV
DSS
, the threshold voltage V
GS(TH)
, the on-resistance
R
DS(ON)
versus gate-to-source voltage, the gate-to-source
and gate-to-drain charges Q
GS
and Q
GD
, respectively, the
maximum drain current I
D(MAX)
and the MOSFET’s thermal
resistances
θ
JC
and θ
JA
.
The gate drive voltage is set by the 7V INTV
CC
low drop
regulator. Consequently, 6V rated MOSFETs are required
in most high voltage LTC3783 applications. If low input
voltage operation is expected (e.g., supplying power
from a lithium-ion battery or a 3.3V logic supply), then
sublogic-level threshold MOSFETs should be used. Pay
close attention to the BV
DSS
specifications for the MOSFETs
relative to the maximum actual switch voltage in the ap-
plication. Many logic-level devices are limited to 30V or
less, and the switch node can ring during the turn-off of
the MOSFET due to layout parasitics. Check the switching
waveforms of the MOSFET directly across the drain and
source terminals using the actual PC board layout for
excessive ringing.
OUTPUT
VOLTAGE
200mV/DIV
INDUCTOR
CURRENT
1A/DIV
1µs/DIV
3783 F07
MOSFET
DRAIN
VOLTAGE
20V/DIV
Figure 7. Discontinuous Mode Waveforms