MAX17498A(2012) Ver la hoja de datos (PDF) - Maxim Integrated

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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

Startup Operation

The MAX17498A is optimized for implementing an offline

single-switch flyback converter and has a 20V IN UVLO

wake-up level with hysteresis of 15V (min). In offline appli-

cations, a simple cost-effective RC startup circuit is used.

When the input DC voltage is applied, the startup resis-

tor (RSTART) charges the startup capacitor (CSTART),

causing the voltage at the IN pin to increase towards the

wake-up IN UVLO threshold (20V typ). During this time,

the MAX17498A draws a low startup current of 20µA (typ)

through RSTART. When the voltage at IN reaches the

wake-up IN UVLO threshold, the MAX17498A commenc-

es switching operations and drives the internal n-channel

MOSFET whose drain is connected to the LX pin. In this

condition, the MAX17998A draws 1.8mA current from

CSTART, in addition to the current required to switch the

gate of the external nMOSFET. Since this current cannot

be supported by the current through RSTART, the volt-

age on CSTART starts to drop. When suitably configured,

as shown in Figure 10, the external nMOSFET is

switched by the LX pin and the flyback/forward con-

verter generates an output voltage (VOUT) bootstrapped

to the IN pin through the diode (D2). If VOUT exceeds

the sum of 5V and the drop across D2 before the volt-

age on CSTART falls below 5V, then the IN voltage is

sustained by VOUT, allowing the MAX17498A to continue

operating with energy from VOUT. The large hysteresis

(15V typ) of the MAX17498A allows for a small startup

capacitor (CSTART). The low startup curent (20µA typ)

allows the use of a large start resistor (RSTART), thus reducing

power dissipation at higher DC bus voltages. Figure 3 shows

the typical RC startup scheme for the MAX17498A.

RSTART might need to be implemented as equal, multiple

resistors in series (RIN1, RIN2, and RIN3) to share the

VDC

VOUT

D1

VDC

RIN1

COUT

RSTART RIN2

VOUT

RIN3

D2

CSTART

CVCC

MAX17498A

IN

LX

LDO

VCC

applied high DC voltage in offline applications so that

the voltage across each resistor is limited to the maximum

continuous operating-voltage rating. RSTART and CSTART

can be calculated as:

C START

= IIN +  QGA1T0E6×

fsw







×

t SS

10

µF

where IIN is the supply current drawn at the IN pin

in mA, QGATE is the gate charge of the external

nMOSFET used in nC, fSW is the switching frequency

of the converter in Hz, and tSS is the soft-start time

programmed for the flyback/forward converter in ms.

See the Programming Soft-Start of the Flyback /Boost

Converter (SS) section.

= R START

(VSTART − 10) × 50 kΩ

1+ CSTART

where CSTART is the startup capacitor in µF.

For designs that cannot accept power dissipation in the

startup resistors at high DC input voltages in offline appli-

cations, the startup circuit can be set up with a current

source instead of a startup resistor as shown in Figure 4.

VDC

RIN1

RSTART RIN2

RIN3

VDC

VOUT

D1

COUT

IN

VOUT

D2

CSTART

RISRC

CVCC

MAX17498A

LX

IN

LDO

VCC

Figure 3. MAX17498A RC-Based Startup Circuit

Figure 4. MAX17498A Current Source-Based Startup Circuit

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