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IRS254(0,1)(S)PbF
operation, the IC does not enter UVLO unless the
supply voltage falls below V CCUV- - .
Inductance Selection
To maintain tight hysteretic current regulation the
inductor and output capacitor C OUT (in parallel with
the LEDs) need to be large enough to maintain the
supply to the load during t HO,ON and avoid significant
undershooting of the load current, which in turn
add capacitance no longer has a significant effect on
the operating frequency or current regulation, as can
be seen in Figs. 13 and 14.
400
390
causes the average current to fall below the desired
value.
First, we are going to look at the effect of the
inductor when there is no output capacitor to clearly
demonstrate the impact of the inductor. In this case,
the load current is identical to the inductor current.
380
370
360
350
340
330
470uH
680uH
1mH
1.5mH
Fig. 9 shows how the inductor value impacts the
30
80
130
180
frequency over a range of input voltages. As can be
seen, the input voltage has a great impact on the
frequency and the inductor value has the greatest
impact at reducing the frequency for smaller input
voltages.
425
375
Vin (V)
Fig.10 Current Regulation for Chosen Inductances
I out = 350 mA, V out = 16.8 V
400
380
360
325
275
225
470uH
680uH
1mH
1.5mH
340
320
300
280
260
240
470uH
680uH
1mH
1.5mH
220
175
200
30
80
130
180
13
18
23
28
33
Vin (V)
Fig.9 Frequency Response for Chosen Inductances
I out = 350 mA, V out = 16.8 V
Fig. 10 shows how the variation in load current
increases over a span of input voltages, as the
inductance is decreased. Fig. 11 shows the variation
Vout (V)
Fig.11 Frequency Response for Chosen Inductances
I out = 350 mA, V in = 50 V
345
343
of frequency over different output voltages and
different inductance values. Finally Fig. 12 shows
how the load current variation increases with lower
inductance over a range of output voltages.
The output capacitor can be used simultaneously to
achieve the target frequency and current control
341
339
337
335
333
331
329
327
470uH
680uH
1mH
1.5mH
accuracy. Fig. 11 shows how the capacitance
reduces the frequency over a range of input voltage.
A small capacitance of 4.7 μ F has a large effect on
325
13
18
23
Vout (V)
28
33
reducing the frequency. Fig. 12 shows how the
current regulation is also improved with the output
capacitance. There is a point at which continuing to
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Fig.12 Current Regulation for Chosen Inductances
I out = 350 mA, V in = 50 V
Page 10
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