Recently, the performance of white LED has been greatly improved. It can compress the extremely high luminous density in an ultra compact package. Compared with more traditional lighting devices, the latest White LED has many advantages, including improved spectral characteristics, shorter response time, higher brightness, stronger durability, longer life and smaller size.
For example, Luxeon V of Lumileds has 50 times the flow rate and at least 20 times the luminous density of standard through-hole LEDs. When used with an efficient and simple switching regulator, white LED has the best versatility and performance in many applications, including automatic switching lights, dome lights, instrument panel lights and emergency lights; Portable flash lamp; Lighting of building details; Edge light marking; Airport emergency lights and taxi guide lights; Medical lighting and decorative lighting.
When using white LEDs in many applications, one of the challenges is to power them through the wide input voltage range provided by the battery. The DC-DC converter from the battery to the LED must not only gradually increase the power supply voltage to the forward voltage of the standard 6.8vled, but also gradually reduce the power supply voltage to the forward voltage (lxhl-lw6c of Lumileds), and keep the current of the LED unchanged (for constant brightness).
Shows an 800KHz single ended primary inductive converter (SEPIC) that provides 700mA drive current for Luxeon V series LEDs over a wide input voltage range (3.6 to 7V). The output voltage of the converter is 6.8v, that is, the forward voltage of lxhl-lw6c. U1 is a heat dissipation enhanced lt34efe with high efficiency, 3A current on-board power switch, which can simplify the regulator design and layout. It can also provide enough current to the LED driver without overheating. U2 (lt1738is5 operational amplifier encapsulated in sot-2) converts the lt3436 from a current mode, constant output voltage switching regulator to a current mode, constant current source converter with the least inductive resistance power loss.
Press and hold the switch (on / off) button to turn on the LED and connect the off pin to the VIN. Release the switch (on / off) button to turn off the IC and limit the battery power consumed by the IC. The advantage of SEPIC configuration on a more common boost application is that all outputs are disconnected during IC shutdown.
If the output LED is physically disconnected from the circuit, R7, R6 and Q1 will limit the output voltage to avoid any damage to the IC. If you do not want low-voltage operation (input voltage lower than 3.6V), you can reduce the size of L2 by connecting cdrh4d23 10uh inductor with cdrh6d28 22uh inductor. When the overall input current is higher, a larger inductance and a smaller ripple current are required to limit the peak switching current below the maximum rated current 3a of the IC.
Source; International LED network