‘Industry’s Smallest’ Integrated ICs for Bias Control of Power Transistors in Cellular Base-Station Infrastructure

(Product News, 6 Apr 2009)

Maxim Integrated Products has introduced the MAX1385/MAX1386/MAX11008 dual RF LDMOS bias controllers. At 7mm x 7mm, these highly integrated devices are the industry’s smallest LDMOS bias controllers to integrate all of the analog-to-digital (A/D) and digital-to-analog (D/A) interfaces and logic functions required for cellular base-station control. The MAX1385/MAX1386/MAX11008 are uniquely suited to bias the power-amplifier power transistors employed in CDMA, GSM, EDGE, and UMTS cellular base-stations.

These bias controllers integrate all the functions needed in a PA signal chain to achieve 25µV (MAX1385/MAX1386) or 50microvolts (MAX11008) of resolution for self-calibration. The MAX1385/MAX1386 provide two gate-drive channels, each consisting of an 8-bit coarse DAC, a 10-bit fine DAC, and a gate-drive amplifier with a gain of 2 (MAX1385) or 4 (MAX1386). These channels generate a positive gate voltage to bias LDMOS devices, allowing up to 18 bits of resolution. Each of the MAX11008's two gate-drive channels integrates a 12-bit DAC to generate this positive gate voltage.

The MAX1385/MAX1386/MAX11008 offer a 7-channel, 12-bit ADC, which includes three channels for temperature measurement. In addition, each IC includes dual high-side current-sense amplifiers (32V, CMRR 90dB), which offer up to ±0.5 percent accuracy and 1 least-significant-bit (LSB) linearity at 12 bits. These high-side current-sense amplifiers feature programmable gains of 2, 10, and 25 to monitor the LDMOS drain current over the range of 20mA to 5A. The temperature sensor offers 0.25 degrees Celsius resolution with up to ±1 degree Celsius accuracy.

The MAX11008 contains 4Kb of on-chip, nonvolatile EEPROM, organized as 256 bits x 16 bits, to store lookup-table and register information. The MAX11008 monitors current, voltage, and temperature, logically deciding if the DAC output voltage should be adjusted to maintain the bias setting. Other existing solutions require an external EEPROM, necessitating frequent communication with an external processor.

All three bias controllers offer self-calibration modes to minimize error over time, temperature, and supply voltage. Additionally, an internal reference provides a well-regulated, low-noise, +2.5V reference for the ADC, DAC, and temperature sensors, while allowing the flexibility to operate from individual ADC and DAC external references. These ICs communicate over a 20MHz, SPI(TM)-compatible, 4-wire serial interface or a pin-selectable, 3.4MHz, I²C-compatible serial interface. They operate from a +4.75V to +5.25V analog supply and a +2.7V to +5.25V digital supply.

The MAX1385/MAX1386/MAX11008 are available in 7mm x 7mm, 48-pin TQFN packages and are specified for the -40 degrees Celsius to +85 degrees Celsius temperature range.