The rapid proliferation of 3-V logic systems is generating requirements for circuits that operate from supply voltages as low as 2.7 V. The device shown here (Fig. 1) underscores that point, as it was devised in response to a customer request for a 1.25-V "bus-termination" voltage in a system that was powered from a 3-V main power bus. This step-down (buck) switching regulator will provide a regulated 1.25 V at 5 A from a 2.7-to-3.3-V input.
When trying to come up with a solution to the customer's request, it became clear that the minimum input supply voltage of 2.7 V was too limiting of a constraint for power FETs: There are no power FETs that can be fully turned on with only 2.7 V. This meant that FETs couldn't be used without some kind of internal converter to generate a higher voltage to drive the gates (this was eventually ruled out because of additional circuitry and cost).
As an alternative, the device in Figure 1 was designed. It uses a single fast-switching pnp transistor (type D45H5) driven by an LM3578 controller (selected because it will operate on voltages as low as 2 V and it provides up to 750 mA of drive for the pnp). A 16-A Schottky diode (TO-220 package) was used for the output diode to minimize power loss (an 8-A diode can be used with an efficiency loss of about 3% at a 5-A load). No heat sink is required.w
Looking more closely at the circuit design, the LM3578 is connected in the basic step-down configuration, with component values selected for a 1.25-V output and 40-kHz switching frequency. The 8-A D45H5 has good saturation current at 5 A and low switching losses at the 40-kHz operating frequency. Only a small heat sink is required, as the power dissipation was less than 3 W (worst case).
The toroidal-type inductor is recommended for the output filter because it produces less EMI noise. The input and output capacitors must be aluminum electrolytics with very low equivalent series resistance (ESR). In the breadboard unit, Panasonic HFQ-type capacitors were employed.
Generally, good power-conversion efficiency in a switching converter requires that the voltage drops (and power dissipation) associated with the switching devices be very small when compared to the output voltage (and power delivered to the load). However, for this buck regulator, the 1.25-V output voltage means that the voltage drops across the pnp switching transistor and the output diode aren't negligible. In fact, they're quite significant.
To minimize power dissipated in the switching transistor, a single pnp was used (as opposed to a pnp/npn Darlington combination). If sufficient base drive is provided to the pnp, it can be pulled hard enough into saturation to force its collector-emitter voltage well below 1 V. This keeps the "on time" power losses as low as possible.
The following results were obtained with the device: load regulation = 0.45% (Vin = 3 V, IL = 1.2 A - 5 A); line regulation = 0.32% (IL = 5 A, Vin = 2.7 V - 3.3 V); output noise (rms) = 1.28 mV (Vin = 3 V, IL = 5 A, Fluke 8050A).
The circuit efficiency (defined as the ratio Pload/Pin) was about 65% at a 5-A load, and is essentially independent of input voltage at that current level (Fig. 2).