Spread Spectrum, Step-Down DC-DC Converter

Nov 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

Today's portable battery-powered equipment calls for compact and highly efficient power conversion solutions. The LTC3251 from Linear Technology is an inductorless, spread spectrum step-down dc-dc converter that delivers up to 500mA of continuous output current from a tiny MSOP package (Fig. 1). It operates within a V_IN range of 2.7V to 5.5V, making it ideal for single-cell Li-Ion or multicell NiMH/alkaline applications. An external resistor programs the output voltage from 0.9V to 1.6V, enabling it to drive low-voltage DSPs and microcontrollers.

Switched capacitor fractional conversion enables the LTC3251 to attain efficiencies as high as 88% — only slightly lower than that of an equivalent buck switching regulator. The combination of ultralow noise, high efficiency, and a very compact/low-profile footprint provide a step-down solution for handheld wireless applications requiring low noise and maximum battery life — without using an inductor.

The converter regulates by sensing the output voltage through an external resistor divider and modulating the charge pump output current based on the error signal. A 2-phase nonoverlapping clock activates its two charge pumps that work in parallel, but out of phase from each other. On the first clock phase, the IC transfers current from V_IN, through the external Flying Capacitor 1, to V_OUT via Charge Pump 1 switches. During this phase, it delivers current to V_OUT and charges the flying capacitor. On the second clock phase, it connects Flying Capacitor 1 from V_OUT to ground, transferring the charge stored during the first phase of the clock to V_OUT via the switches of Charge Pump 1. Charge Pump 2 operates in the same manner, but with the clock phases reversed.

This dual-phase architecture achieves extremely low output and input noise by providing constant charge transfer from V_IN to V_OUT. Using this method of switching, only half the output current is delivered from V_IN, achieving a 50% increase in efficiency over a conventional LDO.

The IC uses a 2-phase spread spectrum switching architecture to provide a very low noise regulated output as well as low noise at the input. The switching frequency randomly shifts within the 1-MHz to 1.6-MHz range, to minimize harmonic switching spikes that can introduce noise into nearby RF circuitry. The spread spectrum oscillator sets the rate of charging and discharging of the flying capacitors.

The four operating modes are: Continuous Spread Spectrum, Spread Spectrum with Burst Mode^® operation, Super Burst^™ mode and shutdown. Low operating current and low external parts count make the LTC3251 ideally suited for space-constrained battery-powered applications.

A logic low applied to mode pins MD0 and MD1 forces the LTC3251 into the shutdown where all circuitry turns off. The LTC3251 draws only leakage current from the V_IN supply and V_OUT disconnects from V_IN. The mode pins are CMOS inputs with threshold voltages of approximately 0.8V to allow regulator control with low-voltage logic levels.

The LTC3251 is available from stock in a thermally enhanced 10-lead MSOP package. Pricing starts at $1.95 each in 1,000-piece quantities.

Charge Pump DC-DC Converter

Fairchild Semiconductor's FAN5660 is an inductorless, 100mA dc-dc charge pump converter with over 90% efficiency at full load, making it suitable for portable systems. It achieves this efficiency with a typical operating current of just 120µA. Using only two low-cost capacitors, the charge pump's 100mA output replaces switching regulators — eliminating inductors and their associated cost, size, and EMI. It is housed in a space-saving 8-lead SOIC package.

For highest performance, capacitors with low effective series resistance (ESR) should be used. When using the inverting mode with a supply voltage less than 2V, LV may be connected to V_SS. This bypasses the internal regulator circuitry and provides best performance in low-voltage applications. When using the inverter mode with a supply voltage above 2V, LV must be left open.

The FAN5660 inverts a +1.5V to +5.5V input to a corresponding -1.5V to -5.5V output. It can split or double a full load, +5V input voltage, of a power supply or battery, providing +2.25V in splitter and +9.5V in doubler mode. Fig. 2a is the inverter circuit. Fig. 2b shows the doubler, and Fig. 2c is the splitter circuit.

Its most common application is as a charge pump voltage inverter, which uses only two identical external capacitors, C1 and C2. A typical output source resistance of 5Ω means that with an input of +5V, the output voltage is -5V under light load, and decreases to 4.5V with a load of 100mA.

Equipped with an onboard oscillator, its charge pump allows flexibility in selecting appropriate operating frequencies. The oscillator can operate at low or high frequencies. When selecting the low, 5-kHz operating frequency, a very low quiescent current will result. A smaller capacitor can be used at the higher 50-kHz operating frequency. Because the FAN5660 doesn't require an external diode in voltage doubler mode, it occupies less p. c. board area.

LDO Regulators

MAX6469-MAX6484 from Maxim are low-dropout (LDO) linear regulators with a fully integrated microprocessor reset circuit. Each is available with preset output voltages from +1.5V to +3.3V in 100-mV increments and delivers up to 300mA of load current. They consume only 82µA of supply current. The low supply current, low dropout voltage, and integrated reset functionality make them ideal for battery-powered portable equipment.

Input voltage range for the regulator core is +2.5V to +5.5V. The MAX6469-MAX6472/MAX6477-MAX6480 offer an adjustable output voltage implemented with an external resistor divider network between OUT, SET, and GND (Fig. 3). SET must be connected to either GND for fixed V_OUT or to an external divider for adjustable V_OUT. They automatically determine the feedback path depending on the connection of SET. OUT is an internally regulated LDO regulator that powers the microprocessor.

Their reset output indicates when the regulator output drops below standard microprocessor supply tolerances (-7.5% or -12.5% of nominal output voltage). This eliminates the need for an external microprocessor supervisor, while ensuring that supply voltages and clock oscillators have stabilized before processor activity is enabled. Push-pull and open-drain active-low reset outputs are available, with reset timeout periods of 2.5 ms, 20 ms, 150 ms, or 1200 ms (min).

A microprocessor's reset input starts the µP in a known state. The MAX6469-MAX6484 supervisory circuits assert RESET during power-up, power-down, and brownout conditions. RESET asserts when the input voltage is below the undervoltage lockout threshold. RESET asserts when V_OUT is below the reset threshold and remains asserted for at least the minimum selected reset timeout period after V_IN rises above the undervoltage lockout threshold and V_OUT rises above the reset threshold. RESET asserts when MR is pulled low (MAX6471-MAX6474/MAX6479-MAX6482). RESET asserts when SHDN is pulled low (MAX6469/MAX6470/MAX6473-MAX6478/MAX6481-MAX6484).

MAX6469-MAX6484 also have a shutdown feature that reduces the supply current to 1µA (max). The MAX6471-MAX6474/MAX6479-MAX6482 offer a manual reset input to assert a microprocessor reset while the regulator output is within specification. They feature a remote feedback sense pin for use with an external NPN transistor for higher current applications. MAX6469-MAX6476 are available in 6-pin SOT23 and 8-pin thin QFN packages. MAX6477-MAX6484 are available in a 3 × 3 chip-scale package (UCSP^™). All devices are specified for operation from -40°C to +85°C, 300mA LDO regulators with internal microprocessor reset circuit.

Many of the microprocessor-based products require manual reset capability, allowing the operator, a test technician, or external logic circuitry to initiate a reset. A logic low on MR asserts reset while the regulator output voltage is still within tolerance. Reset remains asserted while MR is low and for the reset timeout period after MR returns high. The MR input has an internal pull-up of 40kΩ (typ) to OUT. MR can be driven with TTL/CMOS logic levels or with open-drain/collector outputs.

Linear Technology, Milpitas, Calif.
Circle 347 on Reader Service Card

Fairchild Semiconductor, Irving, Texas
Circle 348 on Reader Service Card

Maxim, Sunnyvale, Calif.
Circle 349 on Reader Service Card