Buck converter works efficiently from phone line

Wayne Rewinkel, National Semiconductor, Santa Clara, CA -- EDN, 2/15/2001

A switching converter provides an inexpensive way to generate 5V, 18 mA (48V, 5 mA maximum) directly from a standard phone line (Figure 1). The high input voltage and low available current require a unique design approach to achieve high efficiency. The circuit uses the LM2597HVM, which has a 60V rating and power-saving features. Its VB_IAS pin permits bootstrapping bias power whenever the output is higher than 4.4V. This feature reduces the bias current by at least a factor of four to a typical current of 1 mA. Additionally, the IC has a shutdown/soft-start pin that, when pulled low, shuts the regulator off (with 10-µA maximum quiescent current). When you release this pin, the IC starts switching with an increasing duty cycle. These features combine with an external comparator and voltage reference to provide the basis for a low-power switching regulator.

The circuit uses the LM4041 adjustable-voltage reference with Q₁ to form a small, low-power comparator. Its 1.23V threshold controls the regulator's output voltage according to the following expression: V_OUT=1.23V·(1+R₃/R₂). If the divided output voltage is below the threshold, IC₁ turns on, causing V_OUT to ramp up until the voltage crosses the threshold. Then Q₁ pulls Pin 5 low, forcing IC₁ into shutdown. IC₁ stays in this state until the output voltage again decays to the comparator's threshold and the cycle repeats. In this always-stable, discontinuous-switching mode, the inductor current is many times the load current, so its stored energy at turn-off always forces a slight overshoot that has an effect similar to hysteresis. This overshoot is the ripple voltage (ignoring capacitor ESR); you can estimate it from the energy transferred to C₂+C₅ during a switching cycle. The following equation gives ripple:

R₁ and C₅ remove the large ESR-induced spike from C₂ caused by the 1A peak charging current. This lowpass filter is small and inexpensive, and it allows the use of capacitors with almost any ESR ratings. C₃ and C₄ speed the comparator action and thus reduce output ripple. R₅ ensures that the 4041 receives sufficient bias current, and R₄sets the maximum bias current. L₁ should have a value lower than 100 µH and must have a saturation rating exceeding 1A. Coilcraft's DO3316683 inductor fills the bill nicely. Smaller inductors degrade efficiency, because the 2597 goes into its pulse-current-limited protection mode. Larger values also degrade efficiency by using additional switching cycles. The circuit in Figure 1 provides satisfactory results with input voltages of 10 to 60V. For an input of 48V, 5 mA, the available output current measures 34 mA. The output regulation is less than 2-mV output variation for inputs of 10 to 60V and load currents of 0 to 100 mA.