Power Electronics
PWM Controller Runs at Low Duty Cycles and High Frequencies

PWM Controller Runs at Low Duty Cycles and High Frequencies

The LM3495 PWM buck controller from National Semiconductor employs the company’s emulated-current-mode (ECM) architecture to generate output voltages with very low duty cycles at switching frequencies up to 1.5 MHz. Using this controller, high-side MOSFET on-times as short as 50 ns are possible. In addition, the controller features wide (2.9 V to 18 V) input range, low output-voltage capability (down to 0.6 V), and a highly accurate voltage reference. Another benefit is the chip’s ability to perform low-side sensing and programmable current limit without an external sense resistor.

The ECM architecture overcomes one of the limitations typically associated with current-mode control. Although power supply circuits employing current-mode control are easy to compensate, the noise sensitivity of these circuits makes them difficult to use at low duty cycles. But with ECM, the control uses a measurement of inductor voltage to generate a noise-free signal representing the inductor current ramp. This technique allows the LM3495 to achieve high-side MOSFET on- times down to 50 ns. Contrast that with a conventional current-mode controller, which may be limited to a minimum on-time of about 150 ns.

The LM3495 was developed to power digital ASICs, FPGAs, DSPs and other embedded processors, which may require tight voltage regulation. The controller achieves accuracy of ±1% over a wide temperature range, while integrating system features optimized for those applications. The IC also addresses the requirements of point-of-load module (POL) manufacturers by offering fault-protection features.

The LM3495’s 2.9-V to 18-V input range enables the part’s use in 3.3-V, 5.0-V, 12-V and intermediate-bus systems. Meanwhile, the part's adjustable 0.6-V to 5.5-V output accommodates existing and future core and I/O voltages. The IC operates from a single input rail, eliminating the need for an external bias supply (see the figure).

The controller’s ability to perform low-side sensing and implement a programmable current limit without a sense resistor allows for reduced power dissipation, component count and system cost. Another notable function is an adaptive duty cycle limit, which reduces the designer’s need to oversize the inductor in order to prevent saturation. Other functions include input undervoltage lockout, switch-node short protection, and hiccup-mode current limit protection. The chip also has an internal soft start with tracking capability.

Switching frequency is in the range of 200 kHz to 1.5 MHz, which can be synchronized to reduce radiated system noise. The LM3495’s on-chip gate drivers are capable of driving FETs suitable for output currents in excess of 20 A. Additionally, the LM3495 has soft shutdown and glitch-free start-up into pre-biased loads.

National first introduced its ECM architecture with the LM5005, a high-voltage 2.5-A buck regulator, and the company plans to use ECM in products that will be introduced throughout the rest of the year. These will include a dual-output, second-generation, ECM controller designed to optimize performance in the most demanding digital cores.

Available now in a 16-pin TSSOP, the LM3495 is priced starting at $3.08 in 1000-unit quantities. For more information about the LM3495, including how to order samples and evaluation boards, visit www.national.com/pf/LM/LM3495.html or for a datasheet, see http://cache.national.com/ds/LM/LM3495.pdf.

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