Application Note
10 of 28
V 1.0
2019-01-23
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
cs
. The mean CS threshold voltage is
𝑉
𝐶𝑆
=
𝑉
𝐶𝑆𝐻
+𝑉
𝐶𝑆𝐿
2
, V
CS
= 360 mV. Hence, the proper value of R
CS
is given by:
𝑅
𝐶𝑆
=
360 𝑚𝑉
𝐼
𝐿𝐸𝐷,𝐴𝑉𝐺
Power dissipated on the resistor also has to be considered:
𝑃
𝑑𝑖𝑠
= 𝑅
𝐶𝑆
𝐼
𝐿𝐸𝐷,𝐴𝑉𝐺
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
SW
, as shown below:
𝐿 =
𝑅
𝐶𝑆
(𝑉
𝑂𝑈𝑇
−𝑉
𝐼𝑁
𝑡
𝑑𝑒𝑙𝑎𝑦
𝑓
𝑆𝑊
)
𝑓
𝑆𝑊
(𝑉
𝐶𝑆𝐻
−𝑉
𝐶𝑆𝐿
)
−
𝑅
𝐶𝑆
𝑉
𝑂𝑈𝑇
2
𝑉
𝐼𝑁
𝑓
𝑆𝑊
(𝑉
𝐶𝑆𝐻
−𝑉
𝐶𝑆𝐿
)
Where V
IN
is the input voltage, V
OUT
is the output voltage, V
CSH
and V
CSL
are high and low CS thresholds
accordingly, and where
𝑡
𝑑𝑒𝑙𝑎𝑦
≈ 𝑡
𝐶𝑆𝑆𝑊
+ 𝑅
𝑓𝑙𝑡𝑟
𝐶
𝑓𝑙𝑡𝑟
, t
CSSW
is the internal delay (less than 120 ns).
f
SW
should be chosen so that it never runs in an audible range less than 20 kHz across all conditions. Lower
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
sat
) of the chosen inductor has to be higher than the peak LED current
I
Pk
= I
LED,AVG
+
∆I
OUT
/2.
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
BR
. This voltage rating
must be higher than the maximum input voltage V
IN
of the circuit, V
BR
more than V
IN
.
The other two parameters defining the diode are average and RMS forward currents I
D,AVG
and I
D,RMS
:
𝐼
𝐷,𝐴𝑉𝐺
= 𝐼
𝐿𝐸𝐷,𝐴𝑉𝐺
∙ (1 − 𝐷)
𝐼
𝐷,𝑅𝑀𝑆
= 𝐼
𝐿𝐸𝐷,𝐴𝑉𝐺
∙ √1 − 𝐷 ∙ √1 +
1
12
∙ (
∆𝐼
𝑂𝑈𝑇
𝐼
𝐿𝐸𝐷,𝐴𝑉𝐺
)
2
Where D is the duty cycle of the switching waveform and is given by:
𝐷 =
𝑉
𝑂𝑈𝑇
𝑉
𝐼𝑁
