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

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

Advantages of adaptive on-time control are:
• Predictable operating frequency compared to other variable frequency methods.
• Reduced component count by eliminating the error amplifier and external compensation components.
• Reduced component count by removing the need to sense and control inductor current.
• Fast transient response, controlled by a fast comparator rather than a typically slow error amplifier used in traditional regulator circuits. Figures 4a and 4b illustrate the transient response of an SC174 during a rising and falling load current.
• Operates with ceramic input and output capacitors.
• Reduced output capacitance due to fast transient response

Low Standby Power

Very low power during standby mode is increasingly important to comply with emerging “green” initiatives. However, most electronic products also require fast “wake-up” capability, which demands fast recovery from low-power standby to full-power steady state mode. The SC173 and SC174 converters solve this with an ultrasonic power-save (UPSAVE) mode. UPSAVE reduces the switching frequency to meet low-power standby requirements, which cuts power MOSFET switching losses and results in up to 95% peak standby efficiency. Then, when the load changes from standby to steady state, the EcoSpeed technology cycle-by-cycle response quickly exits UPSAVE mode. To keep the switching frequency from exceeding 25 kHz (high end of the audible range) on-times are prevented from occurring at intervals greater than 40μs.

You can see the effect of power-save in Figures 5 and 6 that show the SC173 and SC174 efficiency vs. output current, respectively. The red curve is with power-save and the blue curve is without power-save.

Circuit Protection

Active loads may leak current from a higher voltage into the switcher output. Under light load conditions with power-save enabled, this can force VOUT to slowly rise and reach the over-voltage threshold, resulting in a hard shutdown. Smart Power Save prevents this condition. When the FB voltage exceeds 10% above nominal (825mV), the ICs immediately disable power-save, and turn on the low-side MOSFET. This draws current from VOUT through the inductor and causes VOUT to fall. When VFB drops back to the 750mV trip point, a normal TON switching cycle begins. This method prevents a hard over-voltage protection (OVP) shutdown and also cycles energy from VOUT back to VIN.

These ICs feature fixed current limiting that employs the RDS(ON) of the lower MOSFET for current sensing. While the low-side MOSFET is on, the inductor current flows through it and creates a voltage across its RDS(ON). During this time, the voltage across the MOSFET is negative with respect to ground. If this MOSFET voltage drop exceeds the internal reference voltage, it activates the current limit. The current limit then keeps the low-side MOSFET on and will not allow another high side on-time, until the current in the low-side MOSFET reduces enough to drop below the internal reference voltage once more.

Additional Protection Features

The SC173 and SC174 synchronous buck regulators incorporate a full range of protection features, including:
• Over-Voltage Protection (OVP)
• Over-Temperature Protection (OTP)
• Output short circuit protection
• Under-voltage lock-out (UVLO)
• Soft-Start

To indicate that the SC173 and SC174 are operating properly, the regulators provide a power good (PGOOD) output, with open-drain that requires a pull-up resistor. When the output voltage is 10% below the nominal voltage, PGOOD goes low and stays low until the output voltage returns to the nominal voltage. PGOOD is held low during soft start and activated approximately 1ms after VOUT reaches regulation. The total PGOOD delay is typically 2ms.

PGOOD will transition low if VFB exceeds +20% of nominal, which is also the over-voltage shutdown threshold (900mV). PGOOD also pulls low if the EN/PSV pin is low when VDD is present.

Summary

Designers wishing to implement point-of-load power circuits now have an energy-saving alternative in the SC173 and SC174 synchronous buck regulators. The parts efficiently manage current in standby and full current modes, using an advanced adaptive on-time topology to provide fast transient response, reduced output capacitance, and predictable switching frequencies.

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