Datasheet

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.
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
).
where:
and
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.
I (0.1 I ) to (0.25 I
OMIN O O
=× ×)
D
VVV
OUT D IN
=
+
−
VVVV
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
R
S
R
CS
C
CS
V
IN
N
R1
MAX15004A/B/MAX15005A/B
Maxim Integrated
15