AN752| Application Note
Maxim > App Notes > POWER-SUPPLY CIRCUITS Keywords: load step, load transients, power supply, DC-DC converters, pulse test, power supplies
Jun 05, 2001
APPLICATION NOTE 752
Creating a Fast Load Transient
Abstract: A simple circuit generates fast load transients for thorough testing of power supplies. Many of Maxim's devices show a typical operating characteristic (TOC) graph with a very fast load transient. Our customers sometimes ask us how we create such fast changes in load. Although commercially available dynamic load boxes are capable of supplying transients, they are not sharp enough for an attractive-looking TOC. To obtain the best-looking load transients, we use a combination of a power MOSFET and a couple of resistors right on the PC board of the device under test (DUT). Choosing the components is relatively easy. If the output voltage is positive, an n-channel MOSFET works great. The MOSFET is connected with a resistor from the source to ground (see the figure below). The MOSFET's gate is terminated with a 51!& resistor to impedance-match the signal generator's output.
Component connections Determining the value of R1 and selecting M1 depend on the output voltage and current required. Ideally, the components should be chosen using the following equation: R1 = 70% x (VOUT / IOUT) And for the MOSFET: RDS(ON) = 20% x (VOUT / IOUT) If the DUT requires relatively low output currents, it may be difficult to find a MOSFET that satisfies the above
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