LTC3783 Datasheet, PDF (18/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

LTC3783
U
OPERATIO
Layout section for more information on component place-
ment). Lab breadboards generally suffer from excessive
series inductance (due to inter-component wiring), and
these parasitics can make the switching waveforms look
significantly worse than they would be on a properly
designed PC board.
The output capacitor in a boost regulator experiences high
RMS ripple currents. The RMS output capacitor ripple
current is:
I_RMS(COUT)
I_OUT(MAX) •
V_OUT – V_IN(MIN)
V_IN(MIN)
Note that the ripple current ratings from capacitor manu-
facturers are often based on only 2000 hours of life. This
makes it advisable to further derate the capacitor or to
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be placed in parallel
to meet size or height requirements in the design.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical than
the output capacitor, due to the fact that the inductor is in
series with the input, and hence, the input current wave-
form is continuous (see Figure 10). The input voltage
source impedance determines the size of the input capaci-
tor, which is typically in the range of 10µF to 100µF. A low
ESR capacitor is recommended, although it is not as
critical as for the output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I_RMS(CIN)
0.3
•
V_IN(MIN)
L•f
• D_MAX
I_L
I_IN
Figure 10. Inductor and Input Currents
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to the
input of the converter, and solid tantalum capacitors can
fail catastrophically under these conditions. Be sure to
specify surge-tested capacitors!
Boost Converter Design Example
The design example given here will be for the circuit shown
in Figure 1. The input voltage is 12V, and the output voltage
is 25V at a maximum load current of 0.7A (1A peak).
1. The duty cycle is:
D = ^{VOUT + VD – VIN} = ^{25 + 0.4 – 12} = 53%
V_OUT + V_D
25 + 0.4
2. The operating frequency is chosen to be 1MHz to
maximize the PWM dimming range. From Figure 2, the
resistor from the FREQ pin to ground is 6k.
3. An inductor ripple current of 40% of the maximum load
current is chosen, so the peak input current (which is also
the minimum saturation current) is:
I_IN(PEAK)
=
⎛⎝⎜₁₊
χ⎞
2 ⎠⎟
•
I_OUT(MAX)
1– D_MAX
=
1.2 •
0.7
1– 0.53
=
1.8A
The inductor ripple current is:
∆_IL
=
χ
•
I_OUT(MAX)
1− D_MAX
=
0.4
•
0.7
1− 0.53
=
0.6A
And so the inductor value is:
L
=
V_IN(MIN)
∆I_L • f
• D_MAX
=
12V _{• 0.53}
0.6A • 1MHz
=
11µH
4. R_SENSE should be:
R_SENSE
=
0.5
• V_SENSE(MAX)
I_IN(PEAK)
=
0.5 • 150mV
1.8A
=
42mΩ
3783f
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