January 15, 1998

DC/DC converter operates from phone line

Gary Shockey, Linear Technology Corp, Milpitas, CA

DC/DC converters for use inside the telephone handset require operation from the high-source-impedance phone line. Additionally, the CCITT specifications call for maximum on-hook power consumption of 25 mA. The dc/dc converter in Figure 1 is 70%-efficient at an input power of 25 mA, providing 5V at 3.4 mA. Controlled, low-peak switch current ensures that the ­48V input line experiences no excessive voltage drops during switching.

The circuit operates as a flyback regulator with an auxiliary winding to provide power for the LT1316 switching-regulator IC. When you first apply power, the LBI pin is low, causing the SHDN pin to connect to ground through LBO. Grounding the SHDN pin places the part in shutdown mode, and only the low-battery comparator remains active. During this state, V_IN rises at a rate determined by R₁ and C₁. The IC draws only 6 mA in shutdown mode. R₁ needs to supply only this shutdown current, the current through R₂ and R₃, and C₁'s charging current.

When LBI reaches 1.17V (which corresponds to a V_IN of approximately 3.7V), the LBO pin lets go of SHDN and the IC enters the active mode; switching action begins, and the output voltage begins to increase. As the device switches, the V_IN pin draws current out of C₁. V_IN then decreases sufficiently to trip the low-battery detector, stopping the switching. Start-up proceeds in this irregular fashion until, eventually, the voltage at V_A increases to 5V. (V_A is the same as V_OUT because L₂ and L₃ have the same number of turns.) After start-up, current flows to the IC from V_A rather than from the ­48V rail, increasing efficiency. The circuit will not start if V_OUT is loaded before it reaches 5V.

During each switch cycle, current in the transformer primary ramps up until reaching the current limit (Figure 2a). The value of R₄ sets the peak switch current. The circuit uses a 69.8-kilo-ohm resistor to provide a peak switch current of 50 mA; increasing R₄ decreases the current limit. Secondary peak current is approximately equal to the primary peak current multiplied by the transformer's turns ratio (Figure 2b). The FB pin has a sense voltage of 1.23V, and you can set V_OUT by the following formula:

For the load currents of 4 to 80 mA, the circuit achieves a minimum of 70% efficiency. Less than 80 mA quiescent current flows when the converter supplies 0.5 µA over 36 to 72V. (DI #2148)  

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