A SPECIAL MINIMUM DROPOFF PSU FOR FEEDING THE BAROGRAPH OFF THE USB BUS WITH ZERO ERROR & MAXIMUM DISPLAY QUALITY
In this section we consider the second option of PART 6, namely the use of a special, extremely low dropout PSU, capable of delivering a very stable Vcc supply voltage to the PK235. With a PSU of adequate quality we can have a zero error Barograph, with performance even superior to that obtained with dynamic compensation, at the cost of a little more hardware.
The ideal situation is to have a self standing unit, equipped with the required Op Amp buffers to interface with the EXT. inputs of the DrDAQ and working off the + 5V USB Bus. If the PC is a portable unit, the internal battery will guarantee immunity from A.C. mains failure. Furthermore this PSU will eliminate the short term instability of the USB Bus voltage and the Barograph output will be completely clean, no more lumps.
DESCRIPTION of KP-235 PSU FROM USB BUS VOLTAGE
The KP-235 specification calls for a minimum supply Vcc of 4.5 V, while the USB bus spec. is 4.75 to 5.25 V. The simplest solution would seem to be a small Boost, self regulated inverter that would supply a stable 5V to the chip, regardless of USB Bus voltage variations. The writer tried this solution, using a miniature packaged Boost inverter, but it does not work, because the switching noise of the inverter severely disturbs the USB Bus and causes unreliable readings in Picolog. The problem persisted even with an ad hoc design of a switch mode booster and with the introduction of heavy filtering.
The winning solution has been that of creating a linear PSU capable of delivering a highly stable regulated 4.6 V to the Barometer chip with an USB Bus voltage as low as 4.75 V. When Bus voltage rises, the change in output voltage is virtually non-measurable. This supply loads the Bus with only 3 mA, which become a measly 13 mA when the Barometer chip is connected. Any linear mode PSU basically consists of a Reference Voltage Generator (RVG) and a Series Regulator (SR). In this design the SR element is a P Mosfet, whose ON resistance is in the milliohm range, but this is not enough. To ensure short term accuracy and stability the RVG output must be as low as possible, to keep it as far away as possible from Bus Voltage variations. In this PSU the reference voltage to the SR is only 1.5 V. To ensure long term accuracy and stability we use a virtual zener IC delivering 1.24 V.
With reference to the schematic diagram, output voltage from series pass P Mosfet Q1 (4.6 V nominal) drops down to 1.5V at the junction of R18-R19. IC1a compares this voltage to the reference coming from IC1b. IC1a then drives the Mosfet to keep the voltages at pins – and + equal and does this at a very high gain. The P Mosfet has been selected for a very low RdsOn resistance, therefore the SR is able to control the output at 4.6V even if input drops as low as 4.65V, provided that there is no change in the reference voltage.
The virtual zener diode D1 is really a complex IC in disguise and provides the required long term accuracy and stability, but it would not be able to provide the necessary short term stability with a Bus voltage change from 5.25 to 4.75 V, if driven by a simple series resistor. To obtain the required stability, D1 is placed inside the feedback loop of IC1b. Any variation in Bus voltage is automatically compensated by a variation in zener current, to keep voltages at pins - & + of IC1b equal. With this arrangement there is no measurable variation in RVG output voltage when USB Bus voltage goes from maximum to minimum. The multi-turn pot R13 sets the reference voltage at exactly 1.5V.
The function of capacitor C2 is very important: with no C2 present, turn-on output of IC1b is zero, therefore no voltages at the input pins and the system is locked in a zero condition. At turn-on C2 is a virtual short that briefly brings Pin 5 up to positive voltage; IC1b inputs are then out of balance, output rises and the circuit starts working.
The complete Barograph schematic, including the special PSU is shown below. In the next post the modified Picolog Baro set-up for a supply of 4.6V (instead of 5.0V) will be shown.