Detailed Description

The MAX5021/MAX5022 are current-mode PWM con-

trollers that have been specifically designed for use in

isolated power supplies. An undervoltage lockout cir-

cuit (UVLO) with a large hysteresis (14V) along with

very low startup and operating current result in high-

efficiency, universal input power supplies. Both devices

can be used in power supplies capable of operating

from a universal 85VAC to 265VAC line or the telecom

voltage range of -36VDC to -72VDC.

Power supplies designed with these devices use a

Figure 1). During this initial period while the voltage is

less than the UVLO start threshold, the IC typically con-

sumes only 50µA of quiescent current. This low startup

current and the large UVLO hysteresis combined with

the use of a ceramic capacitor C2 keeps the power dis-

the universal AC input voltage (265VAC).

The MAX5021/MAX5022 include a cycle-by-cycle cur-

rent limit which turns off the gate drive to the external

MOSFET during an overcurrent condition. If the output

on the secondary side of transformer T1 is shorted, the

tertiary winding voltage will drop below the 10V thresh-

old causing the UVLO circuit to turn off the gate drive to

the external power MOSFET, thus re-initiating the start-

up sequence.

Startup

voltage is applied, C2 charges through the startup

the internal reference and regulator begin charging C3

(see Figure 1). The bias current consumed by the

device during this period is only 50µA; the remaining

input current charges C2 and C3. Charging of C3 stops

while the voltage across C2 continues rising until it

the UVLO threshold, NDRV begins switching the

MOSFET, transferring energy to the secondary and ter-

tiary outputs. If the voltage on the tertiary output builds

to higher than 10V (UVLO lower threshold), then startup

has been accomplished and sustained operation

will commence.

the IC goes back into UVLO. In this case, increase the

value of C2 and/or use a MOSFET with a lower gate-

charge requirement.

Startup Time Considerations

ly after wakeup. The size of C2 will determine the number

of cycles available for startup. Large values for C2 will

increase the startup time, but will also supply more gate

charge, allowing for more cycles after wakeup. If the

Current-Mode PWM Controllers for Isolated

Power Supplies

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5

Pin Description

PIN

SOT23

PDIP

µMAX

NAME

FUNCTION

18

CS

Current Sense Connection for PWM Regulation and Overcurrent Protection. The current-limit

comparator threshold is internally set to 0.6V.

2

7

GND

Power-Supply Ground

3

6

NDRV

External N-Channel MOSFET Gate Connection

43

capacitor to GND.

52

IC Supply. Decouple with a 10nF or larger capacitor to GND. Connect a startup resistor

See Typical Operating Circuit.

6

1

OPTO

Optocoupler Transistor Collector Connection. Connect emitter of optocoupler to GND.

The OPTO has an internal pullup resistor with a typical value of 6.2k

Ω.

—

4, 5

N.C.

No Connection. Do not make connections to these pins.