MAX17497A View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX17497A

AC-DC and DC-DC Peak Current-Mode Converters with Integrated Step-Down Regulator

Maxim Integrated 

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 (VCC)

The devices have an internal linear regulator powered

from the IN pin. The output of the linear regulator is con-

nected to the VCC pin and should be decoupled with a

1FF capacitor to ground for stable operation. The VCC

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-

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

duration of 2ms. See the Programming the Soft-Start of the

Flyback/Boost Converter (SSF) section for more details

on selection of the SSF capacitor.

Maxim Integrated

  12

Share Link: