LTC3783 Datasheet, PDF (15/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

U
OPERATIO
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:
OUTPUT
VOLTAGE
200mV/DIV
INDUCTOR
CURRENT
1A/DIV
LTC3783
L
=
⎛
⎝⎜
V_IN(MIN)
∆I_L • f
⎞
⎠⎟
•
D_MAX
where :
∆I_L
=
χ
• 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 capa-
bility. 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+
χ⎞
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
current 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
MOSFET
DRAIN
VOLTAGE
20V/DIV
1µs/DIV
3783 F07
Figure 7. Discontinuous Mode Waveforms
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 atten-
tion to the BV_DSS specifications for the MOSFETs relative
to the maximum actual switch voltage in the application.
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.
3783f
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