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MAX15004 Datasheet(PDF) 15 Page - Maxim Integrated Products

Part No. MAX15004
Description  4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX15004 Datasheet(HTML) 15 Page - Maxim Integrated Products

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4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
______________________________________________________________________________________
15
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (OUT) quickly ter-
minates the on-cycle. In most cases, a short-time
constant RC filter is required to filter out the leading-
edge spike on the sense waveform. The amplitude and
width of the leading edge depends on the gate capaci-
tance, drain capacitance (including interwinding capac-
itance), and switching speed (MOSFET turn-on time).
Set the RC time constant just long enough to suppress
the leading edge. For a given design, measure the lead-
ing spike at the highest input and rated output load to
determine the value of the RC filter.
The low 305mV current-limit threshold reduces the
power dissipation in the current-sense resistor. The cur-
rent-limit threshold can be further reduced by adding
an offset to the CS input from REG5 voltage. Do not
reduce the current-limit threshold below 150mV as it
may cause noise issues. See Figure 4. For a new value
of the current-limit threshold (VCS-LOW), calculate the
value of R1 using the following equation.
Applications Information
Boost Converter
The MAX15004A/B/MAX15005A/B can be configured
for step-up conversion. The boost converter output can
be fed back to VCC (see Figure 5) so that the controller
can function even during cold-crank input voltage
(
≤ 2.5V). Use a Schottky diode (DVIN) in the VIN path to
avoid backfeeding the input source. A current-limiting
resistor (RVCC) is also needed from the boost converter
output to VCC depending upon the boost converter out-
put voltage. The total current sink into VCC must be lim-
ited to 30mA. Use the equations in the following
sections to calculate RVCC, inductor (LMIN), input
capacitor (CIN), and output capacitor (COUT) when
using the converter in boost operation.
Inductor Selection in Boost Configuration
Using the following equation, calculate the minimum
inductor value so that the converter remains in continu-
ous mode operation at minimum output current (IOMIN).
where:
and
The higher value of IOMIN reduces the required induc-
tance; however, it increases the peak and RMS currents
in the switching MOSFET and inductor. Use IOMIN from
10% to 25% of the full load current. The VD is the for-
ward voltage drop of the external Schottky diode, D is
the duty cycle, and VDS is the voltage drop across the
external switch. Select the inductor with low DC resis-
tance and with a saturation current (ISAT) rating higher
than the peak switch current limit of the converter.
I
(0.1 I ) to (0.25
I
OMIN
O
O
× )
D
VV
V
OUT
D
IN
=
+
V
VV
V
OUT
D
DS
+
L
VD
2f
V
I
MIN
IN
OUT
OUT
OMIN
=
××
××
×
2
η
R
R
V
Low
CS
CS
1
475
0 290
=
×
.
.
Figure 4. Reducing Current-Sense Threshold
MAX15004A/B
MAX15005A/B
REG5
0.3V
CURRENT-LIMIT
COMPARATOR
RS
RCS
CCS
VIN
N
R1


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