MAX5974A MAXIM [Maxim Integrated Products], MAX5974A Datasheet - Page 22

• Manufacturer Part Number: MAX5974A
• Description: Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

An in-phase tertiary winding is needed to power the bias
circuit when using optocoupler feedback. The voltage
across the tertiary V
where V
ratio from the tertiary to the secondary winding. Select the
turns ratio so that V
(7.5V max) by a margin determined by the holdup time
needed to “ride through” a brownout.
When using coupled-inductor feedback, the power for
the devices can be taken from the coupled inductor dur-
ing the off-time. The voltage across the coupled induc-
tor, V
where V
turns ratio from the coupled output to the main output
winding. Select the turns ratio so that V
above the UVLO shutdown level (7.5V max) by a margin
determined by the holdup time needed to “ride through”
a brownout.
This voltage appears at the input of the devices, less
a diode drop. An RC network consisting of R
C
diode D6.
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
22
Coupled-Inductor Feedback (MAX5974A/MAX5974B)
SNUB
COUPLED
Optocoupler Feedback (MAX5974C/MAX5974D)
is for damping the reverse recovery transients of
OUT
OUT
is the output voltage and N
is the output voltage and N
, during the off-time is:
V
COUPLED
T
V
T
T
is above the UVLO shutdown level
during the on-time is:
=
V
OUT
=
V
OUT
×
N
N
T
S
×
N
N
C
O
T
/N
Bias Circuit
S
C
COUPLED
is the turns
/N
SNUB
O
is the
and
is
During on-time, the coupled output is:
where V
Care must be taken to ensure that the voltage at FB
(equal to V
resistive divider) is not more than 5V:
If this condition is not met, a signal diode should be
placed from GND (anode) to FB (cathode).
Typically, there are two sources of noise emission in a
switching power supply: high di/dt loops and high dV/dt
surfaces. For example, traces that carry the drain current
often form high di/dt loops. Similarly, the heatsink of the
main MOSFET presents a dV/dt source; therefore, mini-
mize the surface area of the MOSFET heatsink as much
as possible. Keep all PCB traces carrying switching cur-
rents as short as possible to minimize current loops. Use
a ground plane for best results.
For universal AC input design, follow all applicable
safety regulations. Offline power supplies can require
UL, VDE, and other similar agency approvals.
V
FB-ON
S
V
COUPLED-ON
is the input supply voltage.
COUPLED-ON
=
V
COUPLED-ON
Layout Recommendations
= −
attenuated by the feedback
(V
S
×
×
(
N
N
R
P
S
FB1
−
R
V
FB2
+
OUT
R
FB2
)
N
N
)
C
O
<
5V