Adaptive On-Time DC-DC Converters Combine Speed and Efficiency

Aug 1, 2010 12:00 PM
Sam Davis, Editor in Chief

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Semtech Corp.’s newest EcoSpeed™ dc-dc converters are the SC173 and SC174 synchronous buck regulators. Their dc-dc converter platform incorporates a patented adaptive on-time (AOT) topology intended primarily for point-of-load (POL) applications. The SC173 and SC174 POL regulators satisfy today’s “green” power requirements by efficiently managing both standby and full current demands. Typical applications include office automation, networking and communications equipment; set-top boxes; portable products; power supply modules; and other embedded products.

Figure 1 is a typical configuration of the SC173 regulators. The SC174 configuration is similar, the only difference is their output, the SC173 can supply 3A and the SC174 can supply 4A. Their programmable switching frequency range of 200KHz to 1MHz enables circuit optimization for minimum board space and optimum efficiency.

The SC173 and SC174 converters exhibit advantages over conventional constant on-time control topologies, as shown in Table 1 that compares their characteristics. The adaptive on-time topology in common with hysteretic topologies, enables fast transient response while eliminating the need for external compensation components. This results in simple, space- and cost-efficient power management solutions. However, compared with constant on-time and hysteretic topologies, the adaptive on-time technique provides for more predictable switching frequencies.

Figure 2 shows the simplified circuit for pseudo-fixed frequency, adaptive on-time control. When VFB is less than the internal 750mV reference, the FB Comparator output goes high, turning on the high-side MOSFET via the flip-flop and gate drive. The resulting output ripple triggers the one-shot multivibrator, which sends a single pulse to the high-side MOSFET by way of the flip-flop (F/F) and gate drive. One FB comparator input accepts a percentage of VOUT determined by the voltage divider consisting of R1 and R2, its other input accepts the reference voltage (VREF).

You can configure the SC174 with various capacitor types, such as electrolytic, tantalum, POS and ceramic. For some applications that use low-ESR ceramic capacitors, a signal injection circuit may be implemented to provide 10 mVp-p, as shown in Fig. 3. The on-time control regulates the valley of the output ripple voltage. This ripple voltage is the sum of the two voltages: one produced by the ESR of the output capacitor, and the other due to capacitive charging and discharging of the output capacitor during the switching cycle. For most applications, the output capacitor dominates total output ripple voltage.

In the one-shot multivibrator used in the SC173 and SC174, a current source determined by VIN and RTON charges an internal 2.5pF capacitor. The high-side MOSFET turns off when the capacitor voltage hits VOUT, which is derived internally by the heavily filtered LX voltage. Therefore, at the selected frequency, the IC automatically anticipates the on-time needed to regulate VOUT based on the existing VIN. This method automatically produces an on-time proportional to VOUT and inversely proportional to VIN. Under steady-state operation, you can program the switching frequency from 200kHz to 1MHz, according to:

The SC173 and SC174 use external resistor RTON to set the on-time, which indirectly sets the frequency. You can determine RTON from the desired switching frequency:

The switcher output voltage (VOUT) is regulated by comparing the internal 750mV reference voltage with the percentage of VOUT determined by the R1 and R2 voltage divider at FB. Adjusting the values of the voltage divider controls VOUT.

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