IRS25411PBF View Datasheet(PDF) - International Rectifier
Part Name
Description
MFG CO.
IRS25411PBF Datasheet PDF : 20 Pages
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IRS254(0,1)(S)PbF
the normal operation mode. If already in normal
operation, the IC does not enter UVLO unless the
supply voltage falls below VCCUV--.
Inductance Selection
To maintain tight hysteretic current regulation the
inductor and output capacitor COUT (in parallel with the
LEDs) need to be large enough to maintain the supply to
the load during tHO,ON and avoid significant
undershooting of the load current, which in turn causes
the average current to fall below the desired value.
First, consider 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. Fig. 9 shows how the inductor value
impacts the 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
470uH
325
680uH
1mH
275
1.5mH
225
175
30
80
130
180
Vin (V)
Fig.9 Frequency Response for Chosen Inductances
Iout = 350 mA, Vout = 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 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
accuracy. Fig. 11 shows how the capacitance reduces
the frequency over a range of input voltage. A small
capacitance of 4.7 μF has a large effect on 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 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
380
370
360
350
340
330
30
470uH
680uH
1mH
1.5mH
80
130
180
Vin (V)
Fig.10 Current Regulation for Chosen Inductances
Iout = 350 mA, Vout = 16.8 V
400
380
360
340
320
300
280
260
240
220
200
13
18
23
28
33
Vout (V)
470uH
680uH
1mH
1.5mH
Fig.11 Frequency Response for Chosen Inductances
Iout = 350 mA, Vin = 50 V
345
343
341
339
337
335
333
331
329
327
325
13
18
23
28
33
Vout (V)
470uH
680uH
1mH
1.5mH
Fig.12 Current Regulation for Chosen Inductances
Iout = 350 mA, Vin = 50 V
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