Power Electronics

6-A, 17-V Buck Converter Fits In 3.5-mm2 QFN

A step-down dc-dc converter IC shrinks in size using a unique semiconductor process to produce monolithic, low on-resistance, power MOSFETs.

The TPS54620 from Texas Instruments improves on older IC technology that could not reduce the low-side on-resistance (RDS(on)) for monolithic on-chip power MOSFETs to less than 100 mΩ from a 12-V input. In contrast, the 6-A TPS54620 has an on-chip synchronous rectifier with an RDS(on) of 26 mΩ for the high-side MOSFET and 19 mΩ for the low-side MOSFET.

Low on-resistance and synchronous rectification also provide higher efficiency for the TPS54620. By achieving at least 25% lower on-resistance than previous 6-A ICs, the TPS54620 exhibits 95% power-conversion efficiency (Fig. 1). Therefore, the converter easily powers deep sub-micron DSPs and other embedded processors, such as FPGAs and ASICs.

High efficiency obtained by comparatively low on-resistance allows the IC to occupy a 3.5- × 3.5-mm QFN package with a footprint of 195 mm2, one-fourth the size of a postage stamp. The company says the TPS54620 is the industry's smallest single-chip, 6-A (8-A peak) dc-dc converter.

Another characteristic of this IC is improved performance, such as a voltage reference with ±1% accuracy over temperature. Plus, its total operating current is approximately 600 µA when not switching and under no load.

When disabled, the supply current is typically less than 2 µA. The TPS54620 supports input voltages from 4.5 to 17 V, allowing it to manage space-constrained 5-V and 12-V point-of-load designs such as wireless base stations or high-density servers.

Switching frequency is adjustable from 200 kHz to 1.6 MHz. The IC's high switching frequency minimizes the size of its associated external capacitors and inductor. An internal phase lock loop (PLL) allows synchronization of the switching cycle to the falling edge of an external system clock, eliminating potential beat noise in sensitive data-acquisition circuits.

Other features include a constant-frequency, peak-current-mode control that also simplifies external frequency compensation and improves performance during line and load transients. And, as a synchronous buck converter, it normally works in continuous conduction mode (CCM) under all load conditions.

THE TPS54620 AS A STEP-DOWN CONVERTER

Fig. 2 shows a typical circuit when the IC is used as a step-down converter. A resistor divider from VOUT to the VSENSE pins sets the output voltage. Using divider resistors with a tolerance of 1% or better will minimize drift. To improve efficiency at light loads consider using larger-value resistors. If resistor values are too high the converter will be more susceptible to noise. Output voltage can be stepped down to as low as its internal 0.8-V voltage reference.

An integrated boot regulator requires an external capacitor (CBOOT) between the BOOT and PH pins to provide the gate-drive voltage for the high-side MOSFET. The value of CBOOT should be 0.1 µF. Ceramic capacitors with an X7R- or X5R-grade dielectric and a voltage rating of 10 V or higher are recommended because of their stability over temperature and voltage.

The TPS54620 achieves design flexibility by using the VIN and PVIN pins together or separately. The VIN pin voltage supplies the internal control circuits of the device. The PVIN pin voltage provides the input voltage to the power converter system.

If tied together, the input voltage for VIN and PVIN can range from 4.5 V to 17 V. When using the VIN separately from PVIN, the VIN pin must be between 4.5 V and 17 V, and the PVIN pin can range from as low as 1.6 V to 17 V. A voltage divider connected to the EN pin can adjust the input voltage UVLO (under voltage lockout). Adjusting the input voltage UVLO on the PVIN pin helps provide consistent power-up behavior.

The IC has a power-good comparator with hysteresis that monitors the output voltage via the VSENSE pin. The PWRGD (power good) pin is an open-drain MOSFET that goes low when the VSENSE pin voltage is less than 91% or greater than 109% of the reference voltage. It goes high when the VSENSE pin voltage is 94% to 106% of the internal reference voltage.

The SS/TR (soft-start/tracking) pin minimizes inrush currents during initial power up and power-supply sequencing. A low-value capacitor or resistor divider on the SS/TR pin controls the soft-start period. Power sequencing involves the use of the EN and PWRGD pins.

An output overvoltage protection (OVP) circuit monitors output voltage overshoot. If an overvoltage occurs, the OVP feature minimizes overshoot by comparing the VSENSE pin voltage to the OVP threshold.

If the VSENSE pin voltage is greater than the OVP threshold the high-side MOSFET turns off, preventing current from flowing to the output and minimizing output overshoot. When the VSENSE voltage drops lower than the OVP threshold, the high-side MOSFET is allowed to turn on at the next clock cycle.

Protection against overcurrent conditions is handled with cycle-by-cycle current limiting on both the high-side and low-side MOSFETs. The IC implements current-mode control that uses the COMP pin voltage to control the turn-off of the high-side MOSFET and the turn-on of the low-side MOSFET on a cycle-by-cycle basis.

Internal thermal shutdown circuits force the IC to stop switching if its junction temperature exceeds 175°C (typ.). The IC reinitiates the power up sequence when the junction temperature drops below 165°C (typ).

SEMICONDUCTOR PROCESSING

Over the last decade monolithic dc-dc converters with on-chip MOSFETs were relatively inefficient because manufacturers were limited by MOSFET on-resistance in the hundreds of milliohms. The semiconductor manufacturing process used then was not able to produce low-resistance MOSFETs and transistors on the same monolithic structure. Therefore, power supply manufacturers used external discrete MOSFETs with an RDS(on) under 10 mΩ to maximize converter efficiency.

This is changing as a result of Texas Instruments' LBC7 processing technology, used for the TPS54620. The LBC7 Power BiCMOS process is built with 0.25-µm CMOS technology and can integrate power transistors, CMOS logic, bipolar transistors and passives to enable the full range of power, control and protective circuitry used in power management devices. LDMOS transistors support voltages to 30 V with low on-resistance. Thick copper metal significantly reduces resistance for high current-carrying capability, and bonding over active circuitry moves high-currents off-chip quickly. Special LDMOS device construction ensures a large, safe operating area which protects the device and improves reliability while supporting high-power operation. The LBC7 low-voltage dense isolated power technology also makes possible power management chips that enable safer, smaller battery chargers with faster charge time.

A companion process, the LBC5, features the same components as LBC7 but is based on 0.35-µm CMOS technology and can support voltages up to 80 V, suiting it for automotive, industrial and telecommunications applications.

DESIGN SUPPORT

The TPS54620 is compatible with the SwitcherPro online or desktop design software that allows the design of power supplies with Texas Instruments SWIFT (TPS54xxx) point-of-load, step-down dc-dc products. This tool creates, manages and shares custom designs. It provides evaluation model designs that you can use for reference or starting points for custom designs. In addition, passive components such as inductors and MOSFETs are now included in both the online and desktop applications. The desktop application now allows more flexibility in creating designs since there is no need to be connected to the internet. Design and go at your own pace.

The SwitcherPro software enables designers to:

  • Create designs online or from the desktop the Power Design Wizard
  • Calculate efficiency and loop responses
  • View stress information on all key parts in the design
  • Customize designs by changing part labels, parts and outputs
  • Change multiple parameters to model various what/if cases
  • Get a simple schematic for the design
  • Send designs, with comments attached, to other system users with the online application

More Featured Products

Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish