85VAC (corresponding to 120VDC) to the 24V wakeup
level results in a resistor value of about 1MΩ. If we
assume R
S
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 V
IN
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 V
IN
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 V
IN
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
(V
CC
), 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%.
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:
where V
REF
= 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, R
CS
,
connected between the source of the MOSFET and
ground. The CS input has a voltage trip level (V
CS
) of
600mV. Use the following equation to calculate the
value of R
CS
:
where I
PRI
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.
