The MAX796 evaluation kit (EV kit) is a preassembled and tested demonstration board that generates 5V and 15V. It features a flyback transformer rather than a simple inductor, providing the extra 15V output with very little added cost. This loosely regulated 15V output is normally post-regulated to 12V with a linear regulator such as the MAX667, in order to generate VPP programming voltages for flash memory and PCMCIA sockets. The board comes configured to accept battery voltages between 6.5V and 28V, but can be reconfigured for voltages between 5.7V and 30V by reducing the expectation on secondary load-current capability at low voltages (60mA) and substituting MOSFETs with higher breakdown voltage ratings.
The standard board is guaranteed to deliver at least 3A of load current on the main output and a 120mA minimum on the secondary output (VSEC > 13V). To modify the load-current capability, change the sense-resistor (R1) value and re-size the external components according to the Design Procedure in the MAX796/MAX797/MAX799 data sheet.
The main output voltage comes preset to 5.08V (nominal). To select 3.3V operation, move jumper J2 to position 2-3. For operation in adjustable mode, install resistors R4 & R5 and remove the jumper. There is a small PC trace jumper that shunts J2 on the board. This default jumper must be cut apart for either adjustable-mode or fixed 3.3V operation. Don't operate the circuit if a jumper or resistor divider has not been installed, as this will damage the IC due to output overvoltage. Be sure to change the transformer turns ratio if the secondary feedback resistor divider is changed.
In addition to the standard components, the EV kit has some extra pull-up and pull-down resistors (R2-R8) to set default logic input levels. These resistors can usually be omitted in the final design. There is also an optional HF noise filter on the current-sense leads (R6 and C9) that may be needed with some transformer types. If the main output becomes noisy when the secondary output is heavily loaded, the noise filter should be left installed.
The MAX796 EV kit can be used to evaluate the MAX799 IC by replacing the IC and re-wiring the transformer secondary. Changes needed include connecting the SECFB resistor divider to REF instead of GND, changing the transformer, and reversing the secondary rectifier (D3) and filter capacitor (C7) polarities.
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