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MAX5021

Current-Mode PWM Controllers for Isolated Power Supplies

Maxim Integrated 

Current-Mode PWM Controllers for Isolated

Power Supplies

85VAC (corresponding to 120VDC) to the 24V wakeup

level results in a resistor value of about 1MΩ. If we

assume RS values between 750kΩ and 1MΩ, then at

the high-line voltage of 265VAC (corresponding to

374VDC) power dissipation will be between 140mW to

190mW. A single 1/4W resistor or a series combination

of two 1/4W resistors is adequate.

Undervoltage Lockout (UVLO)

The device will attempt to start when VIN exceeds the

UVLO threshold of 24V. During startup, the UVLO cir-

cuit keeps the CPWM comparator, ILIM comparator,

oscillator, and output driver shut down to reduce cur-

rent consumption (Functional Diagram). Once VIN

reaches 24V, the UVLO circuit turns on both the CPWM

and ILIM comparators, as well as the oscillator, and

allows the output driver to switch. If VIN drops below

10V, the UVLO circuit will shut down the CPWM com-

parator, ILIM comparator, oscillator, and output driver

returning the MAX5021/MAX5022 to the startup mode.

N-Channel MOSFET Switch Driver

The NDRV pin drives an external N-channel MOSFET.

The NDRV output is supplied by the internal regulator

(VCC), which is internally set to approximately 9V. For

the universal input voltage range, the MOSFET used

must be able to withstand the DC level of the high-line

input voltage plus the reflected voltage at the primary

of the transformer. For most applications that use the

discontinuous flyback topology, this requires a MOS-

FET rated at 600V. NDRV can source/sink 150mA/

250mA peak current, thus select a MOSFET that will

yield acceptable conduction and switching losses.

Internal Oscillator

The internal oscillator switches at 1.048MHz and is

divided down to 262kHz by two D flip-flops. The

MAX5021 inverts the Q output of the last D flip-flop to

provide a duty cycle of 50% (Figure 3). The MAX5022

performs a logic NAND operation on the Q outputs of

both D flip-flops to provide a duty cycle of 75%.

DQ

OSCILLATOR

1.048MHz

Q

DQ

Q

Figure 3. Internal Oscillator

262kHz WITH 50%

(MAX5021)

262kHz WITH 75%

(MAX5022)

Optocoupler Feedback

The MAX5021/MAX5022 do not include an internal error

amplifier and are recommended for use in optocoupler

feedback power supplies. Isolated voltage feedback is

achieved by using an optocoupler and a shunt regula-

tor as shown in the Typical Operating Circuit. The out-

put voltage set point accuracy is a function of the

accuracy of the shunt regulator and resistor divider.

When a TLV431 shunt regulator is used for output volt-

age regulation, the output voltage is set by the ratio of

R4 and R5 (Figure 1). Output voltage is given by the

following equation:

VOUT

= VREF

×

1 +

R4 

R5 

where VREF = 1.24V for the TLV431.

During normal operation, the optocoupler feedback pin

(OPTO) is pulled up through a 6.2kΩ resistor to the

internal supply voltage of 5.25V. When the device is in

UVLO, OPTO is disconnected from the 5.25V regulator

and connected to ground (Functional Diagram). This

helps initial startup by reducing the current consump-

tion of the device.

Current Limit

The current limit is set by a current sense resistor, RCS,

connected between the source of the MOSFET and

ground. The CS input has a voltage trip level (VCS) of

600mV. Use the following equation to calculate the

value of RCS:

RCS

=

VCS

IPRI

where IPRI is the peak current in the primary that flows

through the MOSFET. When the voltage produced by

this current through the current sense resistor exceeds

the current-limit comparator threshold, the MOSFET dri-

ver (NDRV) will quickly terminate the current ON-cycle,

typically within 60ns. In most cases a small RC filter will

be required to filter out the leading-edge spike on the

sense waveform. Set the corner frequency at a

few MHz.

Applications Information

Universal Off-Line Power Supply

Figure 1 shows the design of a 5V/1A isolated power

supply capable of operating from a line voltage of

85VAC to 265VAC. This circuit is implemented in the

MAX5022EVKIT.

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