MAX5942A/MAX5942B
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
14 ______________________________________________________________________________________
Thermal Dissipation
During classification mode, if the PSE applies the maxi-
mum DC voltage, the maximum voltage drop from GND
to V
RCL
will be 13V. If the maximum classification cur-
rent of 42mA flows through the MAX5942A/
MAX5942B, then the maximum DC power dissipation
will be close to 546mW, which is slightly higher than the
maximum DC power dissipation the IC can handle.
However, according to the IEEE 802.3af standard, the
duration of the classification mode is limited to 75ms
(max). The MAX5942A/MAX5942B handle the maxi-
mum classification power dissipation for the maximum
duration time without sustaining any internal damage. If
the PSE violates the IEEE 802.3af standard by exceed-
ing the 75ms maximum classification duration, it may
cause internal damage to the IC.
PWM Controller
Current-Mode Control
The MAX5942A/MAX5942B offer current-mode control
operation with added features such as leading-edge
blanking with dual internal path that only blanks the
sensed current signal applied to the input of the PWM
comparator. The current-limit comparator monitors the
CS pin at all times and provides cycle-by-cycle current
limit without being blanked. The leading-edge blanking
of the CS signal prevents the PWM comparator from
prematurely terminating the on cycle. The CS signal
contains a leading-edge spike that is the result of the
MOSFET gate charge current, capacitive and diode
reverse recovery current of the power circuit. Since this
leading-edge spike is normally lower than the current-
limit comparator threshold, current limiting is not
blanked and cycle-by-cycle current limiting is provided
under all conditions.
Use the MAX5942A in discontinuous flyback applica-
tions where wide line voltage and load current variation
are expected. Use the MAX5942B for single-transistor
forward converters where the maximum duty cycle
must be limited to less than 50%.
Under certain conditions, it may be advantageous to
use a forward converter with greater than 50% duty
cycle. For those cases, use the MAX5942A. The large
duty cycle results in much lower operating primary
RMS currents through the MOSFET switch and in most
cases a smaller output filter inductor. The major disad-
vantage to this is that the MOSFET voltage rating must
be higher and that slope compensation must be provid-
ed to stabilize the inner current loop. The MAX5942A
provides internal slope compensation.
Internal Regulators
The internal regulators of the MAX5942A/MAX5942B
enable initial startup without a lossy startup resistor and
regulate the voltage at the output of a tertiary (bias)
winding to provide power for the IC. At startup, V+ is reg-
ulated down to V
CC
to provide bias for the device. The
V
DD
regulator then regulates from the output of the ter-
tiary winding to V
CC
. This architecture allows the tertiary
winding to have only a small filter capacitor at its output,
thus eliminating the additional cost of a filter inductor.
When designing the tertiary winding, calculate the num-
ber of turns so the minimum reflected voltage is always
higher than 12.7V. The maximum reflected voltage
must be less than 36V.
To reduce power dissipation, the high-voltage regulator
is disabled when the V
DD
voltage reaches 12.7V. This
greatly reduces power dissipation and improves effi-
ciency. If V
CC
falls below the undervoltage lockout
threshold (V
CC
= 6.6V), the low-voltage regulator is dis-
abled, and soft-start is reinitiated. In undervoltage lock-
out, the MOSFET driver output (NDRV) is held low.
If the input voltage range is between 13V and 36V, V+
and V
DD
may be connected to the line voltage, provid-
ed that the maximum power dissipation is not exceed-
ed. This eliminates the need for a tertiary winding.
PWM Controller Undervoltage Lockout,
Soft-Start, and Shutdown
The soft-start feature of the MAX5942A/MAX5942B
allows the load voltage to ramp up in a controlled man-
ner, thus eliminating output voltage overshoot.
While the part is in undervoltage lockout, the capacitor
connected to the SS_SHDN pin is discharged. Upon
coming out of undervoltage lockout, an internal current
source starts charging the capacitor to initiate the soft-
start cycle. Use the following equation to calculate total
soft-start time:
where C
SS
is the soft-start capacitor as shown in Figure 5.
Operation begins when V
SS_SHDN
ramps above 0.6V.
When soft-start has completed, V
SS_SHDN
is regulated
to 2.4V, the internal voltage reference. Pull V
SS_SHDN
below 0.25V to disable the controller.
Undervoltage lockout shuts down the controller when
V
CC
is less than 6.6V. The regulators for V+ and the ref-
erence remain on during shutdown.
