Buck Regulator Compensates for Output Capacitor ESR

Jan 1, 2006 12:00 PM
By Mark Valentine, Technical Editor, Power Electronics Technology

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The LM3100 is the first device in a new component family that extends National Semiconductor's Simple Switcher product line. It is designed to simplify the design process and reduce the overall cost of switching power supplies. This device is a buck regulator with a wide input voltage range of 4.5 V to 36 V, and an output that can supply loads up to 1.5 A. The main switch and synchronous rectifier — the high- and low-side MOSFETs — are integrated into the device. In addition to standard thermal and electrical protection features, the LM3100 also uses a freewheeling current injection technique to compensate for output capacitors with low ESR. This feature allows for the use of low-profile, multilayer ceramic capacitors on the output without the need for external ESR compensation, usually a series resistor. The other external capacitors required for this component can also be ceramic, further reducing cost and simplifying a design's bill of materials.

The regulator uses a constant on-time control scheme that is triggered when the voltage at the feedback pin falls below the 0.8-V threshold, which is generated by an on-chip voltage reference. There are two basic modes of operation: continuous and discontinuous inductor current mode. Discontinuous operation allows the inductor current to reach zero before the main switch supplies it with more current. This increases efficiency at light loads by reducing switching losses. However, the part normally operates in continuous mode, which ensures the inductor current never falls to zero, even in the presence of ripple current. One part of the design process for LM3100 applications is to select filter inductance that satisfies this condition.

As with all synchronous buck regulators, part or all of the energy stored in the inductor during the on time is transferred to the load through the synchronous rectifier. In the LM3100, current mirrors are connected to the low-side MOSFET. They supply signals that sense an overcurrent condition in the inductor and provide ESR compensation for the filter capacitor. For overcurrent monitoring, the voltage across an internal sense resistor in series with one current mirror that scales the low-side MOSFET current is compared to an internal reference. When the low-side MOSFET current exceeds 1.9 A, a delay is inserted before the switching action that normally occurs immediately after the output feedback voltage falls below 0.8 V. This process continues until the inductor current falls below the 1.9-A overcurrent threshold.

The scaled voltage in the other current mirror also monitors the inductor current through the low-side MOSFET. However, this signal is used in a proprietary analog technique that compensates for a low-ESR filter capacitor.

Another feature of the LM3100 includes a nearly constant switching frequency over line and load variations. The switch's on time is a function of the input voltage and the value of the programming resistor R_ON. There is also a programmable soft-start feature to minimize in-rush current with a delay adjusted by the capacitance between the appropriate terminal and ground. Another external bootstrap capacitor stores energy for the high-side drive, ensuring full enhancement of the floating MOSFET. The programmable output voltage is routed through an external voltage divider to the feedback terminal, which is linked to the inverting input of the regulating comparator.

The device is housed in National's 20-lead eTSSOP plastic package, which measures 6.5 mm × 4.4 mm × 1.1 mm. The package is thermally enhanced with an exposed pad on the underside that is thermally connected to the IC substrate, complimenting the thermal shutdown feature of the part. Available now, the LM3100 is priced at $2.50 in 1000-unit quantities.

To learn more about these and other aspects of the LM3100, as well as design considerations for specific applications, visit www.national.com.