AL8807AMP-13 Ver la hoja de datos (PDF) - Diodes Incorporated.

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AL8807AMP-13

HIGH EFFICIENCY LOW EMI WIDE ANALOG DIMMING RANGE 36V 1A/1.3A BUCK LED DRIVER

Diodes Incorporated. 

AL8807A

Application Information (cont.)

EMI and Layout Considerations

The AL8807A is a switching regulator with fast edges and measures small differential voltages; as a result of this care has to be taken with

decoupling and layout of the PCB.To help with these effects the AL8807A has been developed to minimise radiated emissions by controlling the

switching speeds of the internal power MOSFET.

The rise and fall times are controlled to get the right compromise between power dissipation due to switching losses and radiated EMI. The turn-

on edge (falling edge) dominates the radiated EMI which is due to an interaction between the Schottky diode (D1), Switching MOSFET and PCB

tracks. After the Schottky diode reverse recovery time of around 5ns has occurred; the falling edge of the SW pin sees a resonant loop between

the Schottky diode capacitance and the track inductance, LTRACK, See Figure 41.

The tracks from the SW pin to the Anode of the Schottky diode, D1,

and then from D1’s cathode to the decoupling capacitors C1 should

be as short as possible.

There is an inductance internally in the AL8807A this can be assumed

to be around 1nH. For PCB tracks a figure of 0.5nH per mm can be

used to estimate the primary resonant frequency. If the track is

capable of handling 1A increasing the thickness will have a minor

effect on the inductance and length will dominate the size of the

inductance.

The resonant frequency of any oscillation is determined by the

combined inductance in the track and the effective capacitance of the

Schottky diode.

Figure 41 PCB Loop Resonance

An example of good layout is shown in Figure 42 - the stray track inductance should be less than 5nH.

VIN

SET

SW GND CTRL

Place D1 anode , SW pin and

Inductor as close as possible to

minimize ringing

Figure 42 Recommended PCB Layout

Recommendations for minimising radiated EMI and other transients and thermal considerations are:

1. The decoupling capacitor (C1) has to be placed as close as possible to the VIN pin and D1 Cathode.

2. The freewheeling diode’s (D1) anode, the SW pin and the inductor have to be placed as close as possible to each other to avoid ringing.

3. The Ground return path from C1 must be a low impedance path with the ground plane as large as possible.

4. The LED current sense resistor (R1) has to be placed as close as possible to the VIN and SET pins.

5. The majority of the conducted heat from the AL8807A is through the GND pin 2. A maximum earth plane with thermal vias into a second

earth plane will minimise self-heating.

6. To reduce emissions via long leads on the supply input and LEDs low RF impedance capacitors (C2 and C5) should be used at the point

the wires are joined to the PCB.

AL8807A

Document number: DS35990 Rev. 2 - 2

15 of 20

www.diodes.com

March 2013

© Diodes Incorporated

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