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ILD8150 80 V high side buck LED driver IC with hybrid dimming
Circuit description
2.3
Component selection
2.3.1
Current Sense (CS) resistor
The average LED current is determined by the value of the CS resistor R
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+��
, V
= 360 mV. Hence, the proper value of R
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CS
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2
2.3.2
Inductor and switching frequency selection
The inductor L has to maintain a constant current in the LEDs, so that the circuit operates in CCM. The inductor
value L is related to the switching frequency f
Where V
is the input voltage, V
IN
accordingly, and where ��
f
should be chosen so that it never runs in an audible range less than 20 kHz across all conditions. Lower
SW
inductance leads to smaller inductor size, but on the other hand it leads to higher switching losses, which has
to dissipate the IC and efficiency decrease. An optimal frequency in which switching losses are not so critical
should be found.
The saturation current (I
/2.
∆I
OUT
Because output ripple is very low it is recommended to use iron powder magnetics. The core should be a closed
magnetic shape or shielded so as not to affect the IC's sensitive inputs, such as CS.
2.3.3
Diode D selection
Once the internal MOSFET turns off, the residual inductor energy is discharged through the diode D into the
output capacitor and the LED load. Typically, a Schottky diode is used to reduce losses caused by the diode
forward voltage and reverse recovery times.
The first parameter to consider when selecting a diode is its maximum reverse voltage V
must be higher than the maximum input voltage V
The other two parameters defining the diode are average and RMS forward currents I
Where D is the duty cycle of the switching waveform and is given by:
Application Note
Power dissipated on the resistor also has to be considered:
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, as shown below:
SW
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is the output voltage, V
OUT
≈ ��
+ ��
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) of the chosen inductor has to be higher than the peak LED current I
sat
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∙ √1 − �� ∙ √ 1 +
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= ��
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cs
is given by:
CS
360 ����
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=
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2
= ��
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and V
are high and low CS thresholds
CSH
CSL
, t
is the internal delay (less than 120 ns).
CSSW
of the circuit, V
more than V
IN
BR
= ��
∙ ( 1 − �� )
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1
∙ (
12
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10 of 28
. The mean CS threshold voltage is
−��
)
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Pk
. This voltage rating
BR
.
IN
and I
D,AVG
D,RMS
2
∆��
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)
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+
= I
LED,AVG
:
V 1.0
2019-01-23
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