Rainbow Electronics MAX5974D - Manual (Page 22)

MAX5974A/MAX5974B/MAX5974C/MAX5974D
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
22
During on-time, the coupled output is:
S C
COUPLED-ON OUT S
P O
N N
V (V V )
N N
− = − ×
where V S is the input supply voltage.
Care must be taken to ensure that the voltage at FB
(equal to V
COUPLED-ON attenuated by the feedback
resistive divider) is not more than 5V:
( )
FB2
FB-ON COUPLED-ON
FB1 FB2
R
V V 5V
R R
= × <
+
If this condition is not met, a signal diode should be
placed from GND (anode) to FB (cathode).
Layout Recommendations
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.