Design Note DN1040: Introduction

The LTC3649 is a monolithic step-down regulator capable of operating from an input voltage range of 3.1V to 60V, and efficiently producing a (single) resistor-programmable output voltage at up to 4A of output current. These features alone make it a compelling industrial or automotive supply for output voltages from (V_IN – 0.5V) to ground. One unique feature of the LTC3649 is that it is capable of providing power to critical systems when there is a power outage, without any extra components.

Hold-up circuits supply power to critical systems when the main power rail fails, allowing them to perform important housekeeping tasks, such as data retention, for a short period before all available energy is lost. Typical hold-up solutions employ dedicated controllers and large storage capacitors [1, 2] thus additional cost and complexity is warranted if the critical circuits require significant power and hold-up time. But if the required hold-up energy is relatively low, the LTC3649 can easily perform this task with no additional circuitry.

The dual output converter described herein works as a conventional step-down power supply under normal operating conditions. But during a power interruption, the converter itself becomes the energy source, maintaining the programmed output voltage to critical circuits. To perform this task, U1 becomes a step-up converter when input voltage is disconnected, discharging its output capacitor to provide hold-up energy.

Dual Output Converter and Hold-Up Circuit

Figure 1 shows a hold-up design using the LTC3649. Under normal conditions, the unregulated rail, V_IN (V_INS via a blocking diode) supplies a converter based on U1 (Converter A). This converter works in buck mode, generating a stable 5V on V_OUT1. V_INS is connected to a U2-based second converter (Converter B), which supplies 3.3V on V_OUT2 to a critical load. When V_IN fails, Converter A enters boost mode and maintains its programmed output voltage (V_INS) by discharging its output filter capacitors C_O1 and C_O2. Resistors R_IT and R_IB program this voltage level. The PGOOD (PG) signal produced by U1 can be used to communicate the power failure to systems that can disconnect noncritical circuitry to preserve energy. The MODE/SYNC pin is left floating to allow the LTC3649 to enter boost mode.

 

 

Figure 2 shows what happens to the LTC3649 in a boost mode. For the first 7ms of the capture, all voltages are stable. At 7ms, the power is turned off; both V_IN and V_INS begin to decline. When V_INS reaches 8V, it stabilizes and the PG signal changes state, signaling the beginning of the V_OUT1 collapsing. V_INS remains at 8V as long as C_O1 and C_O2 have charge. V_OUT2 holds constant during the entire process, supplying steady power to the critical load long after the power is interrupted.

 

 

Conclusion

LTC3649 is a monolithic step-down regulator with integrated power MOSFETs. It is highly efficient, with low quiescent current, important in many battery operated systems. It is also highly versatile, with programmable frequency, a wide V_IN range up to 60V and an output voltage range down to ground. It simplifies the design of automotive and industrial supplies, especially when its inherent ability as a hold-up circuit is taken into account.