MAX15005 Maxim Integrated Products, MAX15005 Datasheet - Page 18

• Manufacturer Part Number: MAX15005
• Description: (MAX15004 / MAX15005) 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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The V
to provide enough average current from V
the external MOSFET (I
clamped internally to 10.4V and capable of sinking
30mA current. The V
limit the V
output voltage. Maintain the V
feeding the power from V
output voltage of V
following equation:
See Figure 5 and the Power Dissipation section for the
values of I
The choice of the conversion topology is the first stage
in power-supply design. The topology selection criteria
include input voltage range, output voltage, peak cur-
rents in the primary and secondary circuits, efficiency,
form factor, and cost.
For an output power of less than 50W and a 1:2 input
voltage range with small form factor requirements, the
flyback topology is the best choice. It uses a minimum
of components, thereby reducing cost and form factor.
The flyback converter can be designed to operate
either in continuous or discontinuous mode of opera-
tion. In discontinuous mode of operation, the trans-
former core completes its energy transfer during the
off-cycle, while in continuous mode of operation, the
next cycle begins before the energy transfer is com-
plete. The discontinuous mode of operation is chosen
for the present example for the following reasons:
• It maximizes the energy storage in the magnetic
• Simplifies the dynamic stability compensation design
• Higher unity-gain bandwidth.
A major disadvantage of discontinuous mode operation
is the higher peak-to-average current ratio in the primary
and secondary circuits. Higher peak-to-average current
means higher RMS current, and therefore, higher loss
and lower efficiency. For low-power converters, the
advantages of using discontinuous mode easily surpass
the possible disadvantages. Moreover, the drive capabil-
ity of the MAX15004/MAX15005 is good enough to drive
a large switching MOSFET. With the presently available
MOSFETs, power output of up to 50W is easily achiev-
4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
18
component, thereby reducing size.
(no right-half plane zero).
______________________________________________________________________________________
CC
external supply series resistor should be sized
SUPPLY
CC
R
sink current below 30mA at the highest
VCC
and I
OUT
CC
=
(
DRIVE
Selecting V
I
DRIVE
, calculate the R
resistor must be high enough to
SUPPLY
OUT
(
V
OUT
.
) and I
to V
CC
+
Flyback Converter
−
I
DRIVE
8
CC
voltage to 8V while
CC
SUPPLY
)
. For a regulated
Resistor (R
)
VCC
OUT
. The V
using the
to drive
CC
VCC
is
)
able with a discontinuous mode flyback topology using
the MAX15004/MAX15005 in automotive applications.
Step-by-step transformer specification design for a dis-
continuous flyback example is explained below.
Follow the steps below for the discontinuous mode
transformer:
Step 1) Calculate the secondary winding inductance
Step 2) Calculate primary winding inductance for suffi-
Step 3) Calculate the secondary and bias winding
Step 4) Calculate the RMS current in the primary and
Step 5) Consider proper sequencing of windings and
Step 1) As discussed earlier, the core must be dis-
charged during the off-cycle for discontinuous mode
operation. The secondary inductance determines the
time required to discharge the core. Use the following
equations to calculate the secondary inductance:
where:
D
V
I
Step 2) The rising current in the primary builds the
energy stored in the core during on-time, which is then
released to deliver the output power during the off-time.
Primary inductance is then calculated to store enough
energy during the on-time to support the maximum out-
put power.
D
OUT
D
OFFMIN
MAX
= secondary diode forward voltage drop.
= maximum output rated current.
= Maximum D.
for guaranteed core discharge within a mini-
mum off-time.
cient energy to support the maximum load.
turns ratios.
estimate the secondary RMS current.
transformer construction for low leakage.
= minimum D
L
D
S
OFF
L
D
≤
P
=
(
= =
V
=
t
OUT
ON
t
V
2
2
ON
INMIN
×
×
t
t
OFF
ON
+
P
I
OFF
+
OUT
+
OUT
t t
V
OFF
t
.
D
OFF
2
)
×
×
×
×
D
f
f
OUT MAX
(
OUT MAX
MAX
D
OFFMIN
Transformer Design
(
(
2
×
)
η
)
)
2
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