With the continually decreasing costs of LEDs, white light LEDs are now witnessing a great expansion in many product applications. In view of these developments and taking a firm and specific aim at the backlight application area, Holtek has released a series of new LED driver devices, the HT7936, HT7937, HT7938 and HT7939.
The HT7936 is a non inductive charge pump DC/DC converter that can generate a stable and constant voltage output of 4.5V and 5.0V with an output current of 150mA and 110mA respectively. The HT7936 uses few external components, comprising of only three capacitors, making the device extremely easy to use by product designers. With its soft start function, over-current and short-circuit protection circuits, these devices will find suitable use in small backlight display LED drivers whose power is supplied by batteries.
The HT7937 provides a higher drive voltage which is able to drive up to 6 LEDs in series. Moving up the series we see the HT7938 which uses a 40V high voltage process and which is able to drive up to 9 LEDs in series. Using LEDs in series means that the individual LED currents are identical resulting in the same generated illumination level from each LED. Finally we see the HT7939 which includes an internal high current 950mA MOSFET which can drive up to 39 LEDs. The backlight LED arrangement can be of various series and parallel connection configurations according to specific user demands. For example, an LED array of 3 in series and 13 in parallel could be a suitable arrangement. In addition to the over-voltage protection of the HT7937 device, both the HT7938 and HT7939 also include over-temperature and over-current protection circuits as well low voltage lock functions.
The HT7937, HT7938 and HT7939 devices include a voltage reference circuit, which together with an external feedback resistor connected to ground, can be used to setup different values of LED currents. One major and important feature is the device’s ability to achieve an excellent conversion efficiency of up to 90%. Also by using an operating frequency of 1.2MHz, designers are now free to use smaller value inductors and capacitors with the resulting advantage of smaller scale component footprints which of course then result in small PCB areas.
Holtek’s WLED driver devices are all supplied in the very small SOT23-6 type package, which are more suitable for WLED backlight drive display applications of 8 inch or less. With flexibility in terms of backlight area and LED series and parallel connections, this new series provides users with a range of devices for a wide range of backlight applications. The new range of devices should find excellent use in a wide array of products such as mobile phones, digital cameras, audio/multi-media products etc.
