Power Electronics

Self-Synchronization Empowers Buck Regulator ICs

A line of synchronous buck regulators from National Semiconductor offers a combination of high power density and self-synchronization that increases the value of the sync feature in point-of-load power designs. While synchronization is typically used to avoid interference where multiple switching regulators are generating different voltage rails, National's LM20154, LM20134 and LM20133 synchronous buck regulators can use their sync-out and sync-in pins for current sharing to double the output current. Alternatively, they can use synchronization to reduce the required input capacitance, rms current and EMI.

The three models mentioned above are among 10 buck regulators (LM201xx and LM202xx) National has added to its PowerWise energy-efficient product family. These regulators offer output current ratings of 2 A for the LM20242, 3 A for the LM20123/20133/20143, 4 A for the LM20124/20134/20144/20154 and 5 A for the LM20125/20145. These devices all feature an input-voltage range of 2.95 V to 5.5 V, except for the LM20242, which has an input range of 4.5 V to 36 V. In addition, these regulators generate adjustable output voltages as low as 0.8 V.

The 4-A rated LM20154 provides the sync-out pin that enables self-synchronization with either the 4-A LM20134 or the 3-A LM20133. Sync out provides a clock output that is 180 degrees out of phase with the LM20154's internal switching frequency. When this signal is fed to either the LM20134 or the LM20133, it causes the two regulators to operate out of phase while sharing a common input bus. As a result, input ripple and the required input capacitance can be cut by 50%. This benefit can even be achieved while generating two different output voltages.

Offered in 16- or 20-pin thermally enhanced TSSOPs, the LM201xx and LM202xx also feature power sequencing, soft-start, monotonic startup into prebiased loads, and options of fixed- or adjustable-frequency operation. Fixed-frequency options include 1.5 MHz at 3 A, 1 MHz at 4 A or 500 kHz at 5 A. For those with adjustable switching frequency, the range is 250 kHz to 1.5 MHz on the low input-voltage devices and 300 kHz to 750 kHz on the high input-voltage devices.

Each regulator integrates two switching MOSFETs and uses an externally compensated peak current-mode control loop, providing fast load-transient response at low duty cycles. The external compensation allows the performance to be optimized for the output capacitor and inductor.

Each chip also provides an internal voltage reference that is accurate to within ±1.5% from -40°C to 125°C. In addition, these devices include a precision enable pin that allows accurate setting of the turn-on threshold and sequencing of the turn-on of multiple devices, eliminating the need for an external voltage supervisor chip.

For enhanced system reliability, the regulators employ an accurate current limit (±10%) to provide robust protection against long-term hard shorts while minimizing the inductor size. Overvoltage and undervoltage events are monitored at the output voltage.

To simplify the application of these regulators, the company offers a downloadable power-supply design tool in a spreadsheet format. This tool calculates external component values based on user-specified design parameters and allows the designer to quickly analyze control-loop response, thermal performance and efficiency.

The regulators range in price from $2.30 to $3 in 1000-unit quantities. For more, see http://power.national.com.

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