MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
The MAX17497B is intended for implementing a noniso-
lated flyback/boost converter with an on-board 60V rated
n-channel MOSFET. The IN pin of the MAX17497B has
a maximum operating voltage of 36V. The MAX17497B
implements rising and falling thresholds on the IN pin that
assume power-supply startup schemes, typical of lower
voltage DC-DC applications, down to an input voltage of
4.5V DC. Therefore, flyback/boost converters with a 4.5V
to 36V supply voltage range can be implemented with
the MAX17497B. See the Startup Operation section for
more details on power-supply startup schemes for both
devices. The on-board synchronous step-down regulator
is rated for a 16V (max) operating input voltage.
Linear Regulator (V CC )
The devices have an internal linear regulator powered
from the IN pin. The output of the linear regulator is con-
nected to the V CC pin and should be decoupled with a
1 F F capacitor to ground for stable operation. The V CC
regulator output supplies the operating current for the
devices. The maximum operating voltage of the IN pin
is 29V for the MAX17497A and 36V for the MAX17497B.
Configuring the Power Stage (LXF)
The devices use an internal nMOSFET to implement
internal current sensing for current-mode control and
overcurrent protection of the flyback/boost converter.
To facilitate this, the drain of the internal nMOSFET is
connected to the source of the external MOSFET in
the MAX17497A application. The gate of the external
MOSFET is connected to the IN pin. Ensure by design
that the IN pin voltage does not exceed the maximum
operating gate-voltage rating of the external MOSFET.
The external MOSFET gate-source voltage is controlled
by the switching action of the internal nMOSFET, while
also sensing the source current of the external MOSFET.
In the MAX17497B application, the LXF pin is directly
connected to either the flyback transformer primary wind-
ing or to the boost-converter inductor.
Maximum Duty Cycle
The MAX17497A/MAX17497B offer a maximum duty cycle
greater than 90%. Both devices can be used to implement both
flyback and boost converters involving large input-to-out-
Maxim Integrated
put voltage ratios in DC-DC applications. The on-board
synchronous step-down regulator has a maximum duty
cycle of 85% and is internally compensated for stable
operation.
RESETN Power-Good Signal
The devices include a RESETN signal that serves as a
power-good signal to the system. RESETN is an open-
drain signal and requires a pullup resistor to the pre-
ferred supply voltage. The RESETN signal monitors both
the flyback/boost output and the synchronous step-down
regulator output, pulling high when both outputs are at
95% (typ) of their regulation values. The RESETN signal
pulls low when either of the outputs fall below 92% (typ)
of their regulation values.
Sequencing
The MAX17497A is typically configured such that the
output of the flyback converter serves as the input source
to the integrated synchronous step-down regulator.
Because the synchronous step-down regulator has a 6.5V
input UVLO threshold, the 3.3V output always comes up
after the output of the flyback converter. Figure 2 shows
the sequencing of the MAX17497A outputs configured
as described above. The sequencing for the devices is
identical when the MAX17497B is configured as either
a flyback or boost output generating the input supply
voltage for the integrated step-down regulator. The step-
down regulator can also operate from an independent 7V
to 16V DC supply. In this case, the step-down regulator
starts up when its INB pin voltage exceeds 7V, provided
that the EN/UVLO pin voltage is greater than 1.23V (typ).
Soft-Start
The devices implement soft-start operation for the flyback/
boost converter, as well as the synchronous step-down
regulator. A capacitor connected to the SSF pin programs
the soft-start period for the flyback/boost converter, while
the step-down regulator has a fixed internal digital soft-
start scheme. The step-down regulator includes soft-start
on selection of the SSF capacitor.
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