1.5MHz Monolithic Synchronous Step-Down Regulator Brings High Efficiency to WCDMA Cellular Telephone Applications

Feb 1 2004

Introduction

To extend talk time and save battery power, new 3rd generation WCDMA cellular telephones are expected to adjust their power levels to the requirements of each transmission. The highest power levels are reserved for data transmission far from the base station; the lowest for voice transmission near the station. Power amplifiers in these phones require a DC power supply which can slew quickly from one voltage to another. To perform this function, the LTC3403 and LTC3408 provide DC/DC conversion from single lithium-ion cell voltages down to a dynamically adjustable 0.3V–3.5V, which can be easily modulated with an external DAC.

These monolithic, synchronous step-down regulators are offered in a tiny, low profile 8-lead (3mm square, 0.8mm high) plastic DFN package. Their 1.5MHz switching frequency allows the use of tiny inductors and capacitors. With power switches located on-chip, the minimal number of required external components permits an entire regulator to occupy less than 8mm² of board space. The power switches’ low 0.4Ω of on-resistance increases efficiency when delivering as much as 600mA of current; efficiency as high as 96% can be achieved in buck mode. Further efficiency gains are possible in bypass mode where V_OUT connects directly to V_IN through the internal bypass P-Channel MOSFET, which has an on-state resistance of 0.08Ω for the LTC3408 and 0.20Ω for the LTC3403. The LTC3403 includes a gate driver for controlling an external bypass MOSFET, providing even higher efficiency in this mode.

The regulators employ a constant frequency, current mode architecture. In forced continuous mode, they switch at 1.5MHz, permitting the use of low inductor values. Because smaller case sizes are usually offered for lower inductor values, the overall solution size is reduced. Minimal output voltage ripple is generated, since output voltage ripple is inversely proportional to the high switching frequency. Below output voltages of 0.6V, the frequency decreases linearly to 700KHz, allowing the output to reach 0.3V. In practice, the output ripple voltage is just 10mV–15mV throughout the range of output voltages.

The LTC3403 can be configured for either Burst Mode^® operation or forced continuous mode. Burst Mode operation provides high efficiency and extends battery life by reducing gate charge loss at light loads. Forced continuous mode is not as efficient at light loads but it offers lower output voltage ripple in noise-sensitive applications. With no load, the LTC3403 consumes as little as 20µA in Burst Mode; both the LTC3403 and the LTC3408 draw less than 1µA in shutdown.

Because the LTC3408 uses only an internal bypass PFET and runs only in forced continuous mode, two extra terminals are available to halve the I²R losses associated with the V_IN and V_OUT bonding wires leading to this PFET. With all power switches contained within the device, current comparators are available to provide short-circuit protection to the main and bypass PFETs. In the presence of a short from V_OUT to ground, the bypass PFET will immediately turn off then iteratively attempt to turn on at about 2/3 of its normal current limit. The buck regulator will not change its current limit, but will lower its oscillator frequency to avoid inductor current runaway.

All Ceramic Capacitor, 2.5W, Dynamically Controlled Power Converter for WCDMA Power Amplifiers

Figure 1 shows a WCDMA transmitter power supply that is capable of providing output voltages from 0.3V to V_IN at up to 600mA of output current.

Figure 1. WCDMA transmitter power supply

Efficiency for the circuit is as high as 99% when operated in bypass mode, and above 90% for much of its useful range. The input and output capacitors are ceramic, which are desirable because of their small size, low cost, and low ESR. Many switching regulators are unstable using ceramic capacitors because they rely on the ESR of tantalum capacitors. But the LTC3403 and LTC3408 feature internal compensation specifically designed to work with ceramic capacitors, eliminating the cost, complexity, and circuit board space associated with external compensation networks. Even with small ceramic input and output capacitors, the LTC3403 and LTC3408 maintain stable output voltage with minimal voltage ripple.

Low Profile Alternative

Figure 2 shows the same circuit, next to a version modified to include components with a maximum height of 1.1 mm¹. The inductor is shielded, but occupies a larger footprint. Switching losses in the inductor cause nearly a 10% drop in efficiency at low currents (Figure 3). At higher currents, DC resistance predominates, allowing the low profile inductor to match the efficiency of the unshielded inductor.

Figure 2. The circuit in Figure 1 (left) and an equivalent low profile version (right), which uses components with a maximum height of 1.1mm

Figure 3. Efficiencies of the circuit shown in Figure 1 and the low profile version

Conclusion

The LTC3403 and the LTC3408 are high performance monolithic, synchronous step-down DC/DC converters well suited for applications requiring up to 600mA of output current while dynamically adjusting output voltage from 0.3V–3.5V to prolong battery life. Their internal low R_DS(ON) power switches and bypass switches, their high switching frequency, and the small number of ancillary components allow these regulators to offer compact, high efficiency power supply solutions for WCDMA power amplifiers.

Notes

¹ L = Sumida CDRH2D11 2.2µH
C_IN = 2× TDK C1608X5ROJ475M
C_OUT = TDK C1608X5ROJ475M