Datasheet

MAX5941A/MAX5941B
IEEE 802.3af-Compliant Power-Over-Ethernet
Interface/PWM Controller for Power Devices
______________________________________________________________________________________ 17
PWM Comparator and Slope Compensation
An internal 275kHz oscillator determines the switching
frequency of the controller. At the beginning of each
cycle, NDRV switches the N-channel MOSFET on.
NDRV switches the external MOSFET off after the maxi-
mum duty cycle has been reached, regardless of the
feedback.
The MAX5941B uses an internal ramp generator for
slope compensation. The internal ramp signal is reset
at the beginning of each cycle and slews at 26mV/µs.
The PWM comparator uses the instantaneous current,
the error voltage, the internal reference, and the slope
compensation (MAX5941A only) to determine when to
switch the N-channel MOSFET off. In normal operation,
the N-channel MOSFET turns off when:
where I
PRIMARY
is the current through the N-channel
MOSFET, V
REF
is the 2.4V internal reference, and
V
SCOMP
is a ramp function starting at zero and slewing
at 26mV/µs (MAX5941A only). When using the
MAX5941A in a forward-converter configuration, the fol-
lowing condition must be met to avoid control-loop sub-
harmonic oscillations:
IRV-V-V
PRIMARY SENSE OPTO REF SCOMP
×>
MAX5941A
V
DD
V+GND
NDRV
CS
V-
OUT
V
CC
OPTO
V
IN
V
OUT
OPTOCOUPLER
C
DD
C
CC
100Ω
R
SENSE
R
1
R
2
C
OUT
C
IN
N
T
N
P
N
S
M1
4.7nF
250VAC
TLV431
220Ω
UVLO
RCL
GATE
SS_SHDN
PGOOD
PGOOD
V
EE
C
SS
R
DISC
25.5kΩ
R
CL
Figure 6. Flyback Converter