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CN5815 Datasheet(PDF) 7 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/
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CN5815 Datasheet(HTML) 7 Page - Shanghai Consonance Electronics Incorporated

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CONSONANCE
www.consonance-elec.com
7
REV 1.0
The inductor value should meet the requirement of the following equation:
Where, fSW is the switching frequency in Hz, typical value is 330KHz.
Inductor Current Sense Resistor Selection
The CN5815 is current mode controller and use a resistor in series with the source terminal of external
N-channel MOSFET to perform cycle-by-cycle inductor current sense for both the current mode control and over
current protection. The inductor current sense resistor is shown in Figure 1 as RSW. The DRV pin will become
low and turn off the external N-channel MOSFET if the voltage at the ISW pin exceeds the current limit
threshold voltage VSW(oc) from the electrical specifications table.
So the value of RSW should meet the requirement of the following equation:
The CN5815 adopts peak current mode control to regulate the output voltage, which needs a compensation slope
to prevent the device from sub-harmonic oscillation. In CN5815, the compensation slope is applied in a fixed
amount. At ISW pin, the compensation slope is:
Se=5.5×10
4 V/S
To ensure that the converter does not enter into sub-harmonic oscillation, the compensation slope Se must be at
least half of the down slope of the current sense signal at ISW pin. Since the compensation slope is fixed in the
CN5815, this places a constraint on the selection of the inductor current sense resistor.
The down slope of the inductor current sense signal at ISW pin is:
Where,
Se is the compensation slope applied to ISW pin in V/S
m2 is the down slope of the inductor current sense waveform seen at ISW pin in V/s
RSW is the inductor current sense resistor at ISW pin in ohm(Ω)
VLED is the LED forward voltage in volt (V)
VD is the forward voltage of freewheeling diode in volt (V)
VIN is the input voltage in volt(V)
L is the inductor value in Henry(H)
Since the compensation slope must be at least half, and preferably equal to the down slope of the inductor
current sense waveform seen at ISW pin, namely,
Hence, a maximum value is placed on the inductor current sense resistor RSW when operating in continuous
conduction mode at 50% duty cycle or greater.
As a conclusion, RSW should simultaneously meet the requirements of the following 2 equations for inductor


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