Discussion on power semiconductor technologies with TI's Ram Ananth

By Srinivas N.

Analog IC market is a kind of evergreen (in sense of opportunities) market. Each and every digital system need analog devices as human interface to digital ICs and also required to power the digital ICs from unregulated power sources. The new market term "green semiconductor" is music to the ears of semiconductor professionals. To analog market this 'green semiconductor' market opportunities tend to make it even more evergreen market.
Texas Instruments is leading the rest of analog IC vendors in some of the areas of low power device technology. The quiescent current of few of their latest devices are really outstanding. To discuss about their power semiconductor technologies, I sat with Ram Ananth, Director, Power Management Products, Texas Instruments India. He is placed in Bangalore, India.

Here are the excerpts.

We see now an industry average of 95% power efficient dc/dc converters at particular load. Can we achieve 98% efficient devices in coming years?

Ram Ananth: This optimal efficiency of 95% or 96% depending on the topology is at particular load current. It's not possible to have >95% efficiencies through out the load current.

So have we reached a kind of wall at 96%!

Ram Ananth: Always, right! Up to 90% it's easily doable. To cross this industry norm, you need innovation. The kind of innovation is happening in MOSFETs and the switching topologies. Resonant converters are one such example, where they recycle quite a bit of energy. But for any converters achieving constant and higher load efficiencies above 95% is challenging.

In MOSFET and in other power ICs, what are the technologies used by TI to reduce leakage and dynamic currents?

Ram Ananth: If you look at the MOSFET, it's a switch. Actually TI is not doing MOSFETs until recently it has acquired Ciclon Semiconductor. What we do at TI in our regulators with built-in MOSFETs are, to reduce on-resistance of MOSFET as low as possible and at same time reduce the gate charge. It's also depends on how the IC is designed. Self-consumption of the IC has to be reduced; now it's in the range of micro amps. Operating voltage in portables devices is coming down and the MOSFET and other device geometries in the IC are optimized for such low voltages.

The lowest operating voltage has now gone below 1 Volt level. Is this going further low?

Ram Ananth: I have seen the today's processing technologies going down to 0.6 Volts in sleep mode and also seen threshold voltages down to 0.4V in some of our devices. These are the voltages mainly to keep processor barely alive for doing some housekeeping functions during standby mode.

What are the new trends in battery monitoring ICs in predicting exact amount of energy left in the battery? Can you elaborate on this technology?

Ram Ananth: The older versions of battery monitoring IC in some of the old mobile phones and other portable electronic devices measure the battery voltage and digitally indicate it in the form of bar graph. That's not a good indication of how much of battery energy available in the battery indication. To indicate correctly, the battery-monitoring device has to measure the charge going into the battery and charge taken out of the battery. We need to count the coulombs. Coulombs can be measured by measuring the current flowing into the battery. Using a shunt resistor connected in series to a battery, the charging current is converted to voltage. This voltage is fed to a voltage to frequency converter (VFC). The VFC along with an embedded microcontroller tracks the energy flow while the battery is charging by incrementing a counter in proportional to charging current. While discharging, the same concept is used in reverse direction by decrementing the counter.

The stored energy is displayed by using the counter's data along with considering battery's different parameters, which is already stored in the device. So the number of coulombs getting into the battery and getting out of the battery is measured. With this measured data, battery can be exactly characterized. This coulomb-count data along with the power consumption parameters of the portable equipment available, the user of portable equipment can be provided with the exact time the battery will last for each activity of the user. To give an example in cell phone, exact hours of standby time and talk time can be displayed.

Also we have a latest feature called impedance track. When the battery is charged and discharged its impedance changes. One-year-old battery will have different impedance than brand new battery due to multiple numbers of charges/discharges by that time. This battery-monitoring device with impedance tracking feature can manage to predict the energy available in the aging batteries.

Is this already available in the portable electronics equipments sold in the market?

Ram Ananth: If not in all cell-phones, it's available in nearly every laptops and high-end cell phones.

When an old PC gets scrapped, the working SMPS is also trashed. If we have power supply unit external to the PC, Industry and user should have saved lot more money by reusing it. What's stopping analog companies from doing this?

Ram Ananth: It's all driven by consumerism. Few years ago, PCs were not commodity and now along with PC even mobile phone is a commodity. The buyers don't really understand the differences of power supply being external and internal.

Switch Mode Power Supplies (SMPS) in PCs lose their reliability after 3-5 years due to the use of aluminum electrolytic capacitors. Let's say, we replace aluminum electrolytic capacitors with some better alternative to extend the reliable life of SMPS to 10 years. But I don't think PC last for so many years and motherboards inside those PCs will be obsolete by that time. Also customers don't want SMPS to be fitted outside their PC. Another point is the thermal issues of SMPS will not allow us to reduce the physical size to that of a notebook's external power supply unit.

Is there not a new technology coming up to take care of thermals and other disadvantages?

Ram Ananth: Whenever Intel changes its processor on motherboard, the AT power requirement changes. What used to be the 150-Watt AT power supply, the same size box now generated 340 Watts by using devices called active clamp forward converters, what designers mostly use inside latest SMPS to reduce the size and increase the efficiency. Resonant converters can improve the efficiency by 4 times. As the efficiencies go up the heat generated will come down and the unit can manage the heat without a fan.

So the cooling fan will go away!

Ram Ananth: The fan has not gone away as far as PC is concerned but surely not used in notebook adapters. As I said, there are devices and topologies like active clamp forward converters and resonant converters for better efficiencies but the fan is still used in PCs.

Let me ask me ask this question on linear regulators. These old 78xx, LM317 and other linear regulators are still used; do you see some worthy stuff in these devices?

Ram Ananth: Cell phones still use lot of Low Dropout Regulators (LDOs). The reason is Power Supply Rejection Ratio (PSRR) offered by LDOs can't be matched by switching regulators. For powering noise sensitive functions like Phase Locked Loops (PLL), Voltage Controller Oscillators (VCO), and many analog applications like camera engine actually need LDOs. So, there are advantages and disadvantage of both linear and switching regulator ICs.

What are your 3 favorite power supply ICs, which you like to appreciate them for their performance?

Ram Ananth: My favorite ICs are,
UCC3895 - BiCMOS advanced phase shift controller
UCC3818 - BiCMOS powerfactor pre-regulator
UCC92XX - Point of load type digital PWM power controller

These are all good ICs.

Being an analog engineer do you see lot of opportunities in green semiconductor and other analog semiconductor area in Indian domestic market?

Ram Ananth: Absolutely. Good example is streetlights. In some parts of Bangalore, bright mercury lamps are installed. They consume 275 Watts of power. We can replace them with LED lighting fixtures with probably 40 times more longevity and reliability. LED lights for same brightness consume only 50 watts of power. This is classical example.

TI has big initiative in this direction to produce green semiconductor devices and technologies and is fully focusing its R&D efforts on Indian as well as global market for a greener world.

Where TI leads ahead of your competitors in analog IC technology?

Ram Ananth: We have one of the latest fab called LBC7 to make analog ICs of low quiescent current supporting voltages up to 30 volts covering voltage sources ranging from single cell lithium ion battery to 19.5 volts laptop adapter. To give an example, our power management IC TPS720 family works at very low quiescent current, which is half of our competitor's similar device.