Diodes AL8806Q - Manual (Page 11)

AL8806Q
Document number: DS36905 Rev. 1 - 2
11 of 13
www.diodes.com
April 2014
© Diodes Incorporated
AL8806Q
Application Information
(cont.)
EMI and PCB Layout Considerations
The AL8806Q 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 AL8807Q 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, L
TRACK
, See figure 6.
Figure 6 PCB Loop Resonance
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 AL8807Q 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.
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 V
IN
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 V
IN
and SET pins.
5. The majority of the conducted heat from the AL8807Q 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