Overview:
Introduction In modern applications, traditional low-dropout regulators (LDOs) are being replaced by switching power supplies (SMPS). Although LDO is a low cost and rugged power solution, it consumes a lot of power. More and more portable device manufacturers, such as digital cameras, mobile phones, and PDA manufacturers, are studying the feasibility of replacing LDOs with more efficient solutions. The size of the switching solution, the physical size of the power supply, is often an obstacle that these vendors cannot overcome. The STw4141 is an innovative switching power supply that uses two external coils to generate two independent output voltages. Because of its inherent switching characteristics, this chip is very efficient and requires very few external components. The product is comparable in efficiency to two independent switching power supplies and is sized to two independent LDO power supplies. Therefore, it is possible to replace the linear power supply in a portable device or to reduce the physical size and cost of the switching regulator.
working principle
To get a glimpse of the traditional step-down DC-DC converter, the STw4141's innovative dual-output topology is derived from this design. Figure 1 is a circuit diagram of a simple buck converter, and Figure 2 is the waveform of its coil current. The buck converter topology component includes a power stage composed of PMOS and NMOS, a coil L, an output capacitor C, and a feedback control loop. The PMOS and NMOS are inverted at a frequency of 1/T, and the duty ratio is D1. When the PMOS transistor is turned on, the coil current begins to rise and the slope is:
(1) When the NMOS transistor is turned on, the coil current begins to drop with a slope of:
Figure 1 Buck topology
(2) The following conditions must be valid during steady state:
(3) Equation 3 means that the coil current IL at the beginning of each clock cycle must equal the coil current IL at the end of each clock cycle (otherwise the system is not steady). From this condition, we can derive the duty cycle formula of the buck converter.
(4) Equation 4 means that the total amount of electricity generated by the coil must equal the total amount of power consumed by the load, assuming all switches and RDSon losses are ignored.
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