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CN5815 Datasheet(PDF) 8 Page - Shanghai Consonance Electronics Incorporated

Part No. CN5815
Description  4.5V to 32V Input, Constant Current Boost HB LED Driver
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Maker  CONSONANCE [Shanghai Consonance Electronics Incorporated]
Homepage  http://www.consonance-elec.com/
Logo CONSONANCE - Shanghai Consonance Electronics Incorporated

CN5815 Datasheet(HTML) 8 Page - Shanghai Consonance Electronics Incorporated

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8
REV 1.0
over current protection and current mode control purposes:
and
For design purposes, some margin should be applied to the actual value of the inductor current sense resistor RSW.
As a starting point, the actual resistor chosen should be 80% or less that the value calculated in the above 2
equations.
Current Sense Filtering
In most cases, a small filter placed on the ISW pin improves performance of the converter. These are the
components R4 and C4 in Figure 1. The time constant of this filter should be approximately 100ns. R4 should be
less than 2KΩ.
Freewheeling Diode Selection
For better efficiency and less power dissipation, a low forward voltage schottky diode should be used as the
freewheeling diode (D1 in Figure 1), the diode must have a breakdown voltage that is a few volts higher than the
LED voltage. The diode’s average current should be higher than the maximum LED current, the diode’s peak
current should be higher than the inductor’s peak current estimated by the following equation:
MOSFET Selection
The CN5815 drives an external N-channel MOSFET. The voltage stress on the MOSFET ideally equals the sum
of the LED voltage and the forward drop of the output diode. In practice, voltage overshoot and ringing occur
due to action of circuit parasitic elements during the turn-off transition. The MOSFET voltage rating should be
selected with the necessary margin to accommodate this extra voltage stress. A voltage rating of 1.3 x VLED
provides the necessary design margin in most cases.
The RMS current flowing through the MOSFET is given as:
So the power dissipation of the MOSFET is given as:
Where, Rdson is the on-resistance of the MOSFET.
The MOSFET’s power rating and on-resistance should be chosen based on the above equation.
Output Capacitor Selection
In a boost LED driver, the output capacitor requirements are demanding due to the fact that the current
waveform is pulsed. The choice of component is driven by the acceptable ripple voltage which is affected by the
ESR, ESL and bulk capacitance. The total output ripple voltage is:


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