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CN5815 Datasheet(PDF) 6 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) 6 Page - Shanghai Consonance Electronics Incorporated

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CONSONANCE
www.consonance-elec.com
6
REV 1.0
LED Dimming
The CN5815 provides pulsed or chopped current dimming. Generally, high-brightness LEDs are binned to match
at their full-rated current. However, LEDs from the same bin exhibit poor matching at currents other than
full-rated current. To achieve uniformity, high brightness LED manufacturers recommend PWM pulsing of the
LED current at their full-rated value. This can be achieved by pulsing the DIM pin while having a constant
voltage at VIN. The frequency of PWM signal applied to DIM pin should be between 30KHz and 100KHz.
When DIM pin is pulled low, internal PWM converter is disabled, DRV pin outputs low, no energy is transferred
to LED string; When DIM pin is high, the PWM converter operates normally.
N-Channel MOSFET Gate Driver (DRV Pin)
The CN5815 offers a built-in gate driver for driving an external N-channel MOSFET. The DRV pin can
source/sink currents in excess of 800mA/1500mA. The gate driver is powered by on-chip 5V regulator, so the
voltage at DRV pin is 5V while output high.
Duty Cycle Estimation
For a boost LED driver operating in continuous conduction mode (CCM), the duty cycle is:
Where, VIN is the input voltage, VLED is LED voltage, VD is the forward voltage of freewheeling diode.
So the maximum duty cycle occurs when VIN is minimum, namely:
The minimum duty cycle occurs at VIN is maximum, namely:
Maximum Inductor Current (Input Current)
CN5815 measures the inductor current (Input current) by sensing the voltage across the inductor current sense
resistor (RSW in Figure 1) between the source of external N-channel MOSFET and GND. So the LED current
needs to be reflected back to the input in order to guarantee the correct current regulation.
Based on the fact
that, ideally, the output power is equal to the input power, the maximum average inductor current is:
The internal current mode control loop will not allow the inductor peak to exceed 0.18/RSW. In practice, one
should allow some margin for variations in the CN5815 and external component values, and a good guide for
selecting the peak inductor current (Input current) is:
Inductor Selection
An inductor should be chosen that can carry the maximum input DC current which occurs at the
minimum input voltage. The peak-to-peak ripple current is set by the inductance and a good
starting point is to choose a ripple current of 30% of its maximum value:


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