www.irf.com

25

RD-0608

10.Other Design Considerations

Filtering

The RC filter at the IFB pin is used to remove high frequency transients associated with the switching. The corner

frequency of this filter was left high enough to prevent any further distortion of the feedback signal.

The input filter is a low-pass filter. Its main objective is to prevent ringing of comparable frequency on Vbus. Exact

values of capacitance and inductance are not of critical importance, so long as adequate filtering is accomplished. In

addition to the electrolytic capacitor that is used for filtering on the bus there is also a small ceramic for decoupling

of high frequency noise. Ceramic capacitors typically have low ESR such that they are more ideal for high

frequency filtering.

The IRS2540/1/01/11 was specifically designed to handle low frequency ripples on VBUS. Its capability to handle

such ripple makes it ideal for an offline rectified waveform. However if high voltage (on the order of 5V-10V) high

frequency oscillations (greater than or close to the operating frequency) are present on VBUS, it is recommended to

implement an input filter. If these high frequency signals are present on VBUS the IRS2540/1/01/11 will still

continue to regulate the current through the load, however abnormal switching of LO and HO may be observed.

This poses a problem in terms of switching losses. As previously discussed, the application may need to control the

operating frequency to optimize the system efficiency. Excessive high frequency ripple at the DC bus could cause

the frequency to become unstable, since this system relies on a self-oscillating principle and is sensitive to noise.

Careful attention to the PCB layout is also necessary for this system to operate correctly. If filters on IFB and VCC

are not placed correctly, high frequency ripple will couple to the IFB input and interfere with the operation of the

control loop. Also if the load current is 1A or 1.5A, when HO turns on the load immediately tries to draw this

current. Since the circuit supply is not usually close by, the capacitance of the input wire is not enough to

compensate for this large pull of current and this can result in oscillations or change in potential on the input line.

To alleviate the circuit of such potential problems it is advisable to implement an input filter. The input filter will

also greatly improve the EMC performance.

EMC performance

The IRPLLED1 evaluation board has not been EMC tested. Input and Output filters can be used to reduce the

conducted emissions to below the limits of the applicable EMC standard as needed. Inductors may require a

powdered Iron core rather than Ferrite, which can handle a much larger current before saturating. If EMC is of

critical importance, it may be beneficial to use a MOSFET for the upper switching element and a diode for the

lower switch. The reverse recovery time for a diode is inherently shorter than that of a MOSFET and this can help

in reducing transients observed in the switching elements resulting in better EMC performance.

Layout Considerations

It is very important when laying out the PCB for the IRS2540/1/01/11 based ballast to consider the following points:

1. CVCC2 and CF must be as close to IC1 as possible.

2. The feedback path should be kept to a minimum without crossing any high frequency lines.

3. COUT should be as close to the main inductor as possible.

4. All traces that form the nodes VS and VB should be kept as short as possible.

5. It is essential that all signal and power grounds should be kept separated from each other to prevent noise

from entering the control environment. Signal and power grounds should be connected together at one point

only, which must be at the COM pin of the IRS2540/1/01/11. The IRS2540/1/01/11 is very sensitive to noise

and will not operate if these guidelines are not followed!