Power Electronics

Data Points

Secondary-Side Synchronous PWM Controller

Secondary-side synchronous PWM controllers offer an alternative to mag amps when building multi-output low-voltage supplies. They make it possible to add one or more stages of highly efficient post regulation to a supply's main output without adding windings to the transformer secondary.

The UCC2540 secondary-side-synchronous buck PWM controller from Texas Instruments expands these benefits, offering an efficiency-boosting gate-drive technique, high drive-current capability, a wide bias-voltage range and a supply tracking function.

A synchronous buck controller employing voltage-mode control, the UCC2540 incorporates TI's predictive gate-drive technology, which reduces body-diode conduction to achieve higher efficiency at low output voltages or high switching frequencies. The gate-drive technique is suitable for outputs at or below 3.3 V, and switching frequencies at or above 300 kHz.

The UCC2540 drives two external N-channel MOSFETs synchronously at switching frequencies up to 1 MHz. Synchronization input can be taken directly from a pulsed-rectified voltage from the secondary winding device or from a pulse transmission from the primary-side PWM controller.

The controller also exploits TI's TrueDrive technology, which uses bipolar gate drivers. The chip's dual ±3 A TrueDrive outputs allow the design of multiple power stages capable of up to 50-A outputs at voltages as low as 0.7 V. The controller is distinguished by its ability to operate from 2.7-V to 35-V bias supplies, enabling operation from a 3.3-V supply rail versus 5-V rails for some competing parts. On-chip tracking functions let multiple UCC2540 controllers be stacked to generate multiple outputs with each output sequenced in simultaneous, ratio-metric or sequential modes. The UCC2540 is offered in a 20-pin, thermally enhanced HTSSOP PowerPAD package and is priced at $1.85 each in quantities of 1,000.

For more information, visit power.ti.com/sc04124.

Charge Pump Drives Ultrabright Flash LEDs

Austriamicrosystems (Raleigh, N.C.) has launched its first power and lighting management standard product, the AS3681. Constructed to address all lighting functions in mobile applications, this highly integrated high-power charge pump delivers up to 600 mA to drive ultrabright flash LEDs. Featuring a power conversion efficiency of up to 95%, it supports fractional conversion with 1X, 1.5X and 2X modes. The flash, which can be synchronized with an external strobe signal by using a GPIO input, supports a preview mode. A total of four GPIO pins provide access to an internal 10-bit ADC (2 inputs) to take ambient light conditions into consideration or to apply external PWM signals to control the current sinks determining LED brightness. The AS3681 integrates 11 4-bit programmable current sinks with currents starting at 2.5 mA and reaching up to 37.5 mA, 150 mA or 300 mA.

A second charge pump allows for generation of high positive and negative supply voltages for different displays. For instance, for TFTs it supplies 15 V, 5 mA or organic LED, -6 V, 10 mA. The on-chip programmable dc-dc step-up converter provides up to 40 V, 20 mA to drive up to eight white LEDs with a constant current. The same dc-dc converter can be used to efficiently bias charge coupled devices (CCD) applications.

All functions are real-time programmable via serial interface, and software lets design engineers optimally configure the AS3681 to suit different system architectures.

Because each functional block is individually accessed, the chip is optimized for power consumption, leading to less than 1 µA in shutdown mode. Available in a QFN32, 5-mm × 5-mm package, the device consumes approximately 40% less pc-board space and costs up to 30% less than comparable discrete solutions. The AS3681 costs approximately $1.50 each in quantities of 250,000 pieces.

For more information, visit www.austriamicrosystems.com.

POLs Ensure Reliable SMT Assembly

Datel's LSM-10A series of nonisolated point-of-load dc-dc converters (POLs) are surface-mount devices offered with a heat shield option. The heat shields prevent the POLs' internal components from reflowing when the POLs are assembled to the customer's board using lead-free assembly procedures. Datel adds no lead in manufacturing these POLs.

Surface-mount versions of the LSN Series POLs, the LSM-10A measure 1.3-in. × 0.53-in. × 0.34-in. They offer 3.3-V, 5-V or 12-V inputs and resistor-programmed 0.8-V to 5-V outputs at currents up to 10 A. With efficiencies up to 96%, they can operate without heatsinks. Unit pricing is $9.95 each in lots of 10,000.

For more information, visit www.datel.com.

LDOs Enhance Performance at High Currents

Micrel Semiconductor's MIC5190/91 LDO controllers feature low dropout voltage at high output-current levels along with small package size, low-output capacitance requirements and the ability to post-regulate higher-frequency switchers. Designed for up to 10 A of output with an external NMOS pass transistor, they limit dropout voltage to less than 100 mV.

A 1-MHz gain bandwidth product enables very fast transient response, while enabling the ICs to post-regulate the outputs of switch-mode power supplies operating at higher frequencies. PSRR is greater than 40 dB at 500 kHz.

Package options include the 3-mm × 3-mm MLF-10 and the MSOP-10. In lots of 1000, unit pricing is $2.69 for the MLF and $2.56 for the MSOP.

Seiko Instruments' S-1170 Series LDO voltage regulator delivers 800 mA of output, a high current capability for a CMOS-based LDO. The high output rating comes with a quiescent current of 80 µA during operation, which reduces to 0.1 µA during shutdown. Offered in the SOT-89-5 and 6-pin HSON(A), it features 70 dB of ripple rejection ratio without an external bypass capacitor.

To learn more, visit www.micrel.com or www.sii-ic.com.

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