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19

RD-0608

s

kHz

HO

V

V

V

V

Cycle

Duty

on

in

out

μ

7

.

2

75

1

*

%

20

%

20

100

*

100

20

100

time

=

=

=

=

∗

=

If the IRS2540/1/01/11 is supplied from the output, a large enough capacitor on VCC is required to maintain

sufficient current while in a disabled state. For this evaluation board where the IC supply comes from the output, a

10uF capacitor is used to ensure continued operation while disabled. The output is capable of dropping to roughly

10V. A “strobe light” effect in the LEDs may be observed if VCC drops too much or if the dimming frequency is

too low.

Enable Duty Cycle Relationship to Light Output

0

10

20

30

40

50

60

70

80

90

100

0

1020304050

60

70

80

90

100

Percentage of Light Output

Fig. 23 Light Output vs Enable Pin Duty Cycle

Fig. 24 IRS2540 Dimming Signals

Since the IRS2540/1/01/11 does not include an onboard oscillator, a soft start feature is not easily implemented. This

is only a concern when operating in the dimming mode. Since PWM dimming is required of LEDs, the output is

essentially turning on and off at a rate of the dimming frequency. In the absence of soft start a large spike of current

would be observed in the load each time the output is turned on. This current spike stresses the load possibly

decreasing LED operating lifetime. The IRPLLED1 includes a jumper setting to define whether or not the board is

being used in the dimming mode. This two position jumper allows the designer to either include or exclude the

resistor Rout, which is in series with the output capacitor. The inclusion of this resistor will sufficiently damp the