MAX15004_11 Datasheet, PDF (15/27 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
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 (V_CS-LOW), calculate the
value of R1 using the following equation.
R1 = ^{4.75 × RCS}
0.290 − V_CS − Low
VIN
REG5
MAX15004A/B
MAX15005A/B
R1
N
RCS
0.3V
CURRENT-LIMIT
COMPARATOR
CCS
RS
Figure 4. Reducing Current-Sense Threshold
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 V_CC (see Figure 5) 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 boost converter
output to V_CC depending upon the boost converter out-
put voltage. The total current sink into V_CC must be lim-
ited to 30mA. Use the equations in the following
sections to calculate R_VCC, inductor (L_MIN), input
capacitor (C_IN), and output capacitor (C_OUT) 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 (I_OMIN).
L _MIN
=
2×
V_IN2 × D × η
f_OUT × V_OUT × I_OMIN
where:
D = ^{VOUT + VD − VIN}
V_OUT + V_D − V_DS
and
I_OMIN = (0.1× I_O) to (0.25 × I_O)
The higher value of I_OMIN reduces the required induc-
tance; however, it increases the peak and RMS currents
in the switching MOSFET and inductor. Use I_OMIN from
10% to 25% of the full load current. The V_D is the for-
ward voltage drop of the external Schottky diode, D is
the duty cycle, and V_DS is the voltage drop across the
external switch. Select the inductor with low DC resis-
tance and with a saturation current (I_SAT) rating higher
than the peak switch current limit of the converter.
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