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6

RD-0608

Fig. 6 Vbus = 100V, L1 = 470uH, COUT = 33uF

3. Frequency selection

The frequency in the IRS2540/1/01/11 application is free running and maintains current regulation by quickly

adapting to changes in input and output voltages. There is no need for additional external components to set the

frequency as seen in other controllers. The frequency is determined by L1 and COUT, as well as the input/output

voltages and load current. The selection of the frequency is a trade-off between system efficiency, current control

regulation, size, and cost.

The higher the frequency, the smaller and lower the cost of L1 and COUT, the higher the ripple, the higher the FET

switching losses, which becomes the driving factor as VBUS increases to higher voltages, the higher the component

stresses and the harder it is to control the output current.

4. Output L1 and COUT selection

In order to maintain good output current regulation with minimum ripple, the value of L1 needs to be adequately

sized. COUT is optional and can also be used for energy storage, which allows the converter to run at a lower

frequency. Excess ripple at the output results in high peak currents that are undesirable for driving high power LEDs

and also negates the ability of the system to accurately regulate the average current.

First, the effect of the inductor in a configuration with no output capacitor will be considered to clearly demonstrate

the impact of the inductor. In this case, the load current is identical to the inductor current. Fig. 7 shows how the

inductor value influences the frequency over a range of input voltages. As can be seen, the input voltage also has a

great effect on the frequency. The inductor value reduces the frequency for lower input voltages.

0

200

400

600

800

1000

1200

1400

1600

10

15

20

25

30

Vout (V)

350mA

700mA

1A

1.5A

±0.25%

±0.13%

±0.17%

±0.1%