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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

The negative output voltage of the converter can be

programmed by selecting the correct values for the

resistor-divider connected from VOUT, the flyback /boost

output to REF with the midpoint of the divider connected

to the EA+ pin (Figure 8). With R1 selected in the range

of 20kI to 50kI, R2 can be calculated as:

R2

= R1× V1O.2U2T





kΩ

where R1 is in kI.

Current-Limit Programming (LIM)

The devices include a robust overcurrent-protection

scheme that protects the device under overload and

short-circuit conditions. For the flyback/boost con-

verter, the devices include a cycle-by-cycle peak

current limit that turns off the driver whenever the

current into the LX pin exceeds an internal limit that is

programmed by the resistor connected from the LIM

pin to GND. The devices include a runaway current limit

that protects the device under high-input-voltage short-

circuit conditions when there is insufficient output voltage

available to restore the inductor current built up dur-

ing the on period of the flyback/boost converter. Either

eight consecutive occurrences of the peak current-

limit event or one occurrence of the runaway current limit

trigger a hiccup mode that protects the converter by

immediately suspending switching for a period of time

(tRSTART). This allows the overload current to decay due

to power loss in the converter resistances, load, and

the output diode of the flyback/boost converter before

soft-start is attempted again. The resistor at the LIM pin

for a desired current limit (IPK) can be calculated as:

RLIM =50 × IPK kΩ

where IPK is expressed in amperes.

For a given peak current-limit setting, the runaway

current limit is typically 20% higher. The peak current-

limit-triggered hiccup operation is disabled until the end

of soft-start, while the runaway current-limit-triggered hic-

cup operation is always enabled.

Programming Slope Compensation (SLOPE)

Since the MAX17498A/MAX17498C operate at a maxi-

mum duty cycle of 49%, in theory they do not require slope

compensation for preventing subharmonic instability that

occurs naturally in continuous-mode peak current-mode-

controlled converters operating at duty cycles greater

than 50%. In practice, the MAX17498A/MAX17498C

require a minimum amount of slope compensation to

provide stable, jitter-free operation. The MAX17498A/

VOUT

RU

EA-

MAX17498A

MAX17498B

RB

MAX17498C

Figure 7. Programming the Positive Output Voltage

REF

MAX17498A

EA-

MAX17498B

MAX17498C

VOUT

R1

R2

REA-

EA+

Figure 8. Programming the Negative Output Voltage

MAX17498C allow the user to program this default value

of slope compensation simply by connecting the SLOPE

pin to VCC. It is recommended that discontinuous-mode

designs also use this minimum amount of slope compen-

sation to provide noise immunity and jitter-free operation.

The MAX17498B flyback/boost converter can be

designed to operate in either discontinuous mode or

to enter into the continuous-conduction mode at a spe-

cific heavy-load condition for a given DC input voltage.

In the continuous-conduction mode, the flyback/boost

converter needs slope compensation to avoid subhar-

monic instability that occurs naturally over all specified

load and line conditions in peak current-mode-controlled

converters operating at duty cycles greater than 50%.

A minimum amount of slope signal is added to the

sensed current signal even for converters operating

below 50% duty to provide stable, jitter-free operation.

The SLOPE pin allows the user to program the necessary

slope compensation by setting the value of the resistor

(RSLOPE) connected from SLOPE pin to ground.

RSLOPE =0.5 × SE kΩ

where the slope (SE) is expressed in millivolts per micro-

second.

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