MAX15004_11 Datasheet, PDF (20/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
An additional small LC filter may be necessary to sup-
press the remaining low-energy high-frequency spikes.
The LC filter also helps attenuate the switching frequen-
cy ripple. Care must be taken to avoid any compensa-
tion problems due to the insertion of the additional LC
filter. Design the LC filter with a corner frequency at more
than a decade higher than the estimated closed-loop,
unity-gain bandwidth to minimize its effect on the phase
margin. Use 1μF to 10μF low-ESR ceramic capacitors
and calculate the inductance using following equation:
L
≤
4 × 10³
1
×
fc²
×C
where f_C = estimated converter closed-loop unity-gain
frequency.
SEPIC Converter
The MAX15004A/B/MAX15005A/B can be configured
for SEPIC conversion when the output voltage must be
lower and higher than the input voltage when the input
voltage varies through the operating range. The duty-
cycle equation:
V_{O =} D
V_IN 1− D
indicates that the output voltage is lower than the input
for a duty cycle lower than 0.5 while V_OUT is higher
than the input at a duty cycle higher than 0.5. The
inherent advantage of the SEPIC topology over the
boost converter is a complete isolation of the output
from the source during a fault at the output. For the
MAX15004/MAX15005, the SEPIC converter output can
be fed back to V_CC (Figure 6), so that the controller can
function even during cold-crank input voltage (≤ 2.5V).
Use a Schottky diode (D_VIN) in the V_IN path to avoid
backfeeding the input source. A current-limiting resistor
(R_VCC) is also needed from the output to V_CC depend-
ing upon the converter output voltage. The total V_CC
current sink must be limited to 25mA. See the Selecting
V_CC Resistor (R_VCC) section to calculate the optimum
value of the V_CC resistor.
The SEPIC converter design includes sizing of induc-
tors, a MOSFET, series capacitance, and the rectifier
diode. The inductance is determined by the allowable
ripple current through all the components mentioned
above. Lower ripple current means lower peak and RMS
currents and lower losses. The higher inductance value
needed for a lower ripple current means a larger-sized
inductor, which is a more expensive solution. The induc-
tors L1 and L2 can be independent, however, winding
them on the same core reduces the ripple currents.
Calculate the maximum duty cycle using the following
equation and choose the RT and CT values accordingly
for a given switching frequency (see the Oscillator
Frequency/External Synchronization section).
D_MAX
=
⎡
⎢
⎣
V_IN−MIN
V_OUT + V_D
+ V_OUT + V_D − (V_DS
+
⎤
V_CS
)
⎥
⎦
where V_D is the forward voltage of the Schottky diode,
V_CS (0.305V) is the current-sense threshold of the
MAX15004/MAX15005, and V_DS is the voltage drop
across the switching MOSFET during the on-time.
Inductor Selection in SEPIC Converter
Use the following equations to calculate the inductance
values. Assume both L1 and L2 are equal and that the
inductor ripple current (ΔI_L) is equal to 20% of the input
current at nominal input voltage to calculate the induc-
tance value.
L
=
L₁
=
L2
=
⎡
⎢
⎣
V_IN−MIN × D_MAX
2 × f_OUT × ΔI_L
⎤
⎥
⎦
ΔI_L
=
⎡
⎢
⎣
0.2
× I_OUT−MAX × D_MAX
(1− D_MAX ) × η
⎤
⎥
⎦
where f_OUT is the converter switching frequency and η
is the targeted system efficiency. Use the coupled
inductors MSD-series from Coilcraft or PF0553-series
from Pulse Engineering, Inc. Make sure the inductor
saturating current rating (I_SAT) is 30% higher than the
peak inductor current calculated using the following
equation. Use the current-sense resistor calculated
based on the I_LPK value from the equation below (see
the Current Limit section).
I_LPK
=
⎡
⎢
⎣
I_OUT−MAX × D_MAX
(1− D_MAX ) × η
+ I_OUT−MAX
⎤
+ ΔI_{L ⎥}
⎦
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