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MAX17498A
(Rev.:2012)

AC-DC and DC-DC Peak Current-Mode Converters for Flyback/Boost Applications

Maxim Integrated 

MAX17498A / MAX17498B / MAX17498C

AC-DC and DC-DC Peak Current-Mode Converters

for Flyback/Boost Applications

Soft-Start

The devices implement soft-start operation for the

flyback /boost converter. A capacitor connected to the

SS pin programs the soft-start period for the flyback/

boost converter. The soft-start feature reduces the input

inrush current. These devices allow the end user to select

between voltage soft-start usually preferred in nonisolat-

ed applications and current soft-start, which is useful in

isolated applications to get a monotonic rise in the output

voltage. See the Programming Soft-Start of the Flyback/

Boost Converter (SS) section.

Spread-Spectrum Factory Option

For EMI-sensitive applications, a spread-spectrum-

enabled version of the device can be requested from

the factory. The frequency-dithering feature modulates

the switching frequency by Q10% at a rate of 4kHz.

This spread-spectrum-modulation technique spreads

the energy of switching-frequency harmonics over a

wider band while reducing their peaks, helping to meet

stringent EMI goals.

Applications Information

Startup Voltage and Input Overvoltage-

Protection Setting (EN/UVLO, OVI)

The devices’ EN /UVLO pin serves as an enable /disable

input, as well as an accurate programmable input UVLO

pin. The devices do not commence startup operation

unless the EN/UVLO pin voltage exceeds 1.23V (typ).

The devices turn off if the EN/UVLO pin voltage falls

below 1.17V (typ). A resistor-divider from the input DC

bus to ground can be used to divide down and apply a

fraction of the input DC voltage (VDC) to the EN/UVLO

pin. The values of the resistor-divider can be selected

so that the EN/UVLO pin voltage exceeds the 1.23V (typ)

turn-on threshold at the desired input DC bus voltage. The

same resistor-divider can be modified with an additional

resistor (ROVI) to implement input overvoltage protection

in addition to the EN/UVLO functionality as shown in

Figure 2. When voltage at the OVI pin exceeds 1.23V (typ),

the devices stop switching and resume switching opera-

tions only if voltage at the OVI pin falls below 1.17V (typ).

For given values of startup DC input voltage (VSTART),

and input overvoltage-protection voltage (VOVI), the

resistor values for the divider can be calculated as fol-

lows, assuming a 24.9kI resistor for ROVI:

REN=

ROVI ×

 VSVTOAVRI T



− 1 kΩ



where ROVI is in kI while VSTART and VOVI are in volts.

R SUM=

ROVI

+ REN×

VSTART

 1.23

− 1 kΩ

where REN and ROVI are in kI. In universal AC input

applications, RSUM might need to be implemented as

equal resistors in series (RDC1, RDC2, RDC3) so that

voltage across each resistor is limited to its maximum

operation voltage.

R= DC1

R= DC1

R= DC1

RSUM kΩ

3

For low-voltage DC-DC applications based on the

MAX17498B/MAX17498C, a single resistor can be used

in the place of RSUM, as the voltage across it is

approximately 40V.

VDC

RDC1

RSUM RDC2

RDC3

EN/UVLO

REN

MAX17498A

MAX17498B

OVI

MAX17498C

ROVI

Figure 2. Programming EN/UVLO and OVI

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