IC specification
MLX10803
Automotive high power LED driver
3901010803
Page 5/25
Rev 018
28/SEP/05
Author: ALX, VAR
revised: TFR/VAR
1.1.3. Principle soft start up LED driver diagram
ROSC
IREF2
VREF
GND
RSENSE
DRVGATE
VS/PWM
VBAT
GND
100nF
Cap for EMC directly
on the connector
100nF...1uF
IREF1
Figure 3:
Application with gradual increase of light intensity after power up (soft start)
1.1.4. LED driver application notes
The MLX10803 is optimised for the use of low cost coils and n-channel MOSFETs. For a standard application with
1 LED and an average current of 350mA, a coil of about 100µH…220µH and ≤ 1
Ω DC resistance should be
chosen. The sense resistor should have a value between 0.27
Ω…0.47Ω / 250mW.
As a general rule: the higher the load current, the lower the inductance of the coil as higher currents lengthen the
charging time of the coil. Thus, switching frequencies may become lower than 20kHz which is often not desired.
It is possible to set the peak current and the average current of the LED by variation of the RSENSE resistor, the
coil value and the internal oscillator frequency (ROSC resistor).
The flyback diode that carries the load current during the passive state (driver OFF) should be a fast switching and
low intrinsic capacitance diode like ES1D or BYG80 in order to avoid parasitic spikes on RSENSE. The diode must
be able to carry the LED current flowing during the OFF time of the driver.
The n-channel MOSFET should have low intrinsic capacitances, a drain-source voltage suitable for the application
and must be able to carry the current flowing through the LED(s) during the ON time. To decrease the time of
transistor switching and to improve the thermal behaviour of the module, the lines between transistor and IC should
me minimized.
For applications that use an NTC resistor for temperature sensing, the NTC value has to be selected according to
the application requirements. For most applications, a NTC value up to 470k
Ω will be suitable.
In case of longer lines between the IC and the coil (which should be avoided because of EMI), a capacitor might be
placed in parallel to RSENSE to avoid crosstalk and parasitic switching. Well chosen parameters for external
components can help to avoid such conditions. The goal should be to unload the coil as much as possible during
the selected off time (see also chapter 7).
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