| The LM2754, featuring low input noise, adjustable output power control and automatic LED shutdown, delivers up to an 800mA load in handheld applications. Many of today's cellular phones feature built-in digital cameras capable of both high-resolution still and video images.
Gains in camera technologies have created the need for a high-power white-light source for camera use indoors or in dim ambient light.
White LEDs, already popular for backlighting colour displays, have emerged as the flash light source in camera phones. National's LM2754 charge-pump-based, high current, flash LED driver boasts a unique circuit design that enables several exclusive system-level features, such as reduced noise spikes on the input and adjustable output power control to prevent battery drain. The LM2754 also features automatic LED shutdown to avoid excessive heat from the LED if it stays on longer than it should. In addition, National's LM2754 can effectively drive one, two, three or four LEDs at the same time. This makes the LM2754 well-suited for both multiple- and single-die flash LED technologies in cellphone cameras. National's LM2754 drives up to an 800mA load with an input voltage as low as 3.0V. A unique circuit design enables several exclusive technical features. National's LM2754 reduces noise spikes on the input battery, a common problem when pulling high flash currents from the battery. To drive a high output current in a cellphone design for example, switched capacitor boosts must pull an even higher current from a portable device's battery. This causes large current spikes at the battery terminals that interfere with sensitive RF and analogue circuitry also supplied by the battery. In typical high-power boost convertors, the input current and associated noise levels can cause the mobile device to be knocked off of the network, resulting in a dropped call. National's LM2754 features unique circuitry that draws current from the battery using a graduated and controlled regime, resulting in significantly lower peak current levels on the input line. The unique circuit design also enables National's LM2754 to 'share' the battery with the RF power amplifier (PA). Rather than pulling high current from the battery in parallel during transmission power bursts from the power amplifier, the LM2754's TX pin allows the device to reduce the output power and therefore current pull from the battery. This unique feature reduces dropped calls due to two high-power loads operating at the same time. A third unique feature protects the handheld mobile device from overheating due to the flash LED staying on too long. If the flash LED driver stays on after the picture is taken, the LED power consumption causes excess heat. National's LM2754 prevents excessive heating by automatically shutting down the LED after a time-out period. Available in a very small, low profile 24-pin LLP package, the LM2754 provides a total solution size of less than 28mm2 and can also drive flash LEDs made up of one, two three, or four smaller white LEDs. Competing devices drive just one large flash LED, which provides less flexibility to the mobile device designer in component selection. | 
12:27 [25/11/2006] Post by