MAX5942BCSE Maxim Integrated, MAX5942BCSE Datasheet - Page 18

• Manufacturer Part Number: MAX5942BCSE
• Description: Power Switch ICs - POE / LAN
• Manufacturer: Maxim Integrated

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 MAX5942B 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 (MAX5942A only) to determine when to
switch the N-channel MOSFET off. In normal operation,
the N-channel MOSFET turns off when:
where I
MOSFET, V
output voltage of the internal amplifier, and V
ramp function starting at zero and slewing at 26mV/µs
(MAX5942A only). When using the MAX5942A in a for-
ward-converter configuration, the following condition
must be met to avoid control-loop subharmonic oscilla-
tions:
where k = 0.75 to 1, and N
turns on the secondary and primary side of the trans-
former, respectively. L is the output filter inductor. This
makes the output inductor current downslope as refer-
enced across R
tion. The controller responds to transients within one
cycle when this condition is met.
NDRV drives an N-channel MOSFET. NDRV sources
and sinks large transient currents to charge and dis-
charge the MOSFET gate. To support such switching
transients, bypass V
average current as a result of switching the MOSFET is
the product of the total gate charge and the operating
frequency. It is this current plus the DC quiescent cur-
rent that determines the total operating current.
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
18
PWM Comparator and Slope Compensation
______________________________________________________________________________________
I
PRIMARY
PRIMARY
N
N
REF
S
P
×
N-Channel MOSFET Gate Driver
is the 2.4V internal reference, V
k R
SENSE
×
is the current through the N-channel
×
R
SENSE
SENSE
CC
equal to the slope compensa-
L
with a ceramic capacitor. The
>
×
S
V
V
and N
EA
OUT
- V
REF
=
P
26
are the number of
- V
mV s
SCOMP
/
µ
SCOMP
EA
is the
is a
The following is a general procedure for designing a
forward converter using the MAX5942B:
1) Determine the requirements.
2) Set the output voltage.
3) Calculate the transformer primary to secondary
4) Calculate the reset to primary winding turns ratio.
5) Calculate the tertiary to primary winding turns
6) Calculate the current-sense resistor value.
7) Calculate the output inductor value.
8) Select the output capacitor.
The circuit in Figure 5 was designed as follows:
1) 30V ≤ V
2) To set the output voltage, calculate the values of
3) The turns ratio of the transformer is calculated based
50mV. Turn-on threshold is set at 38.6V.
resistors R1 and R2 according to the following
equation:
where V
ulator, and R
Figures 5 and 6.
on the minimum input voltage and the lower limit of
the maximum duty cycle for the MAX5942B (44%).
To enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V, use the
MAX5942B with the 50% maximum duty cycle.
Calculate the turns ratio according to the following
equation:
winding turns ratio.
ratio.
where:
N
turns and N
S
/N
P
= Turns ratio (N
IN
REF
≤ 67V, V
P
is the reference voltage of the shunt reg-
Applications Information
N
N
R R
V
V
is the number of primary turns).
1
S
P
OUT
1
REF
and R
//
≥
V
OUT
=
2
=
OUT
<<
V
D
V
SS SHDN
MAX
REF
2
S
= 5V, I
50
+
_
are the resistors shown in
is the number of secondary
(
⎡
⎢
⎣
k
1
V
×
Ω
D1
+
V
IN_MIN
R
OUT
R
×
=
Design Example
2
1
D
2 4
⎤
⎥
⎦
MAX
.
= 10A, V
V
)
RIPPLE
≤