Rainbow-electronics MAX5974D Manual do Utilizador descarregar pdf (Página 22)

MAX5974A/MAX5974B/MAX5974C/MAX5974D

Active-Clamped, Spread-Spectrum,

Current-Mode PWM Controllers

During on-time, the coupled output is:

S C

COUPLED-ON OUT

P O

N N

V (V V )

N N

−= − ×

where V

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

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.

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