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ILD8150 80 V high side buck LED driver IC with hybrid dimming
Circuit description
The diode must be chosen so that its respective current ratings are higher than these values.
2.3.4
Input capacitor C
The input current of the buck regulator is identical to the current through the MOSFET, i.e. it is pulsating and
thus causes a ripple voltage at the input. A capacitor on the V
current when the switch is conducting. The RMS current through the input capacitor I
be calculated from average and RMS input currents I
Hence,
Use low-ESR capacitors, especially under high switching frequency applications. With low-ESR capacitors, the
input voltage ripple can be estimated by:
The input capacitor must be chosen so that it is capable of withstanding the calculated RMS current rate and
reducing input voltage ripples to an acceptable level.
2.3.5
Output capacitor C
Due to the relatively low output ripple current, a capacitor in parallel to the LEDs is not needed in many
applications. Due to the non-linear I-V characteristics of LEDs, it is very difficult to estimate the ripple voltage
with and without an output capacitor. A generally accepted model is the approximation of LED V-I
characteristics by a voltage source V
model differential resistance. Both parameters need to be determined from the LED datasheet. As a rule a V
3 V and a differential resistance R
are very reasonable values for a typical white high-power LED. Consequently, for 17 LEDs in series this would
lead to a V
of 51 V and a total R
FD
A meaningful output capacitor should therefore have an impedance at f
than R
:
D
2.3.6
Bootstrap capacitor C
Integrated high-side MOSFET is driven by the gate driver. The bootstrap capacitor is defined as:
Where Q
is internal MOSFET gate charge 2.5 nC, ∆��
G
2.4
Layout consideration
An optimized PCB layout leads to better performance, reliability and lower cost. Certain layout guidelines must
be kept in mind while routing the PCB. The power components include the internal switch, Schottky diode,
Application Note
selection
IN
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selection
OUT
that models the forward voltage of the LED with a series resistor R
FD
of 0.4 (this parameter is being improved with every new LED generation)
D
of 6.8 . The ripple voltage without capacitor is then approximated as:
D
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selection
BOOT
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11 of 28
pin reduces this ripple voltage by providing
IN
and I
as:
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2
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, which is at least five to ten times lower
SW
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is the bootstrap capacitor voltage ripple.
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2019-01-23
to
D
of
FD
V 1.0
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