SoftRock RXTX Ensemble USB Power Supply USB_5 Vdc and 3.3 Vdc

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1 Reference 1 Purpose and Function The 5 VDC from the USB port is unregulated but may be regulated by the PC, and used by the ATTiny85 which implements the USB interface in firmware. The 3.3Vdc is regulated and used by the Si570 to generate the local Oscillator frequency. This stage installs the power supplies for the USB section of the board. This section is galvanically isolated from the rest of the board, with its own ground plane (the "USB ground". Voltages measured in this stage are measured with respect to (WRT) this ground and NOT the "regular ground" of the rest of the board. To paraphrase a famous city's motto, "What happens in USB stays in USB!" This stage installs the USB connection (with its 5 V bus) and the (very tiny) 3.3V regulator whch translates the USB 5 volts to 3.3V for the Si570 and the microcontroller of the next stage.. 1 (Local Oscillator Editor) Theory and Design C1 and C53 are Bulk Capacitors on USB_5V C2 and C56 are Bulk Capacitors on the 3.3Volt power supply C54 is called a bypass and is used to reduce output noise according the LP2992 Data Sheet 2 Note that the 3.3V regulator is turned off by pin 3 when the USB is not connected William R. Robinson Jr. AJ4MC p1of 6

2 Calculated The following parameters are for the regulated 3.3V supply only. No Load Output Voltage 1% of 3.3 VDC 2 o 5V * 0.05 = 0.25 Volts o Vout_noload = 3.27 to 3.33 Volts Max Load Output Voltage Max load is 250 ma 2 o The worst Rload = 3.3V/250ma = 13.2 Ohms Vout_maxload = 4.75 to 5.25 Volts Dropout Voltage Dropout voltage is the minimum Vin-Vout o Any Vin above this voltage should maintain the output voltage o Any Vin below this value will cause Vout to drop also Vdropout= Vout V 2 o Vdropout= 3.75 Volts Ripple Rejection -45 db at 1 Khz 2 Over Temp/Over Current protection The documentation claims the device has this protection 2 William R. Robinson Jr. AJ4MC p2of 6

3 Simulation NONE William R. Robinson Jr. AJ4MC p3of 6

4 Real Circuit No Load Output Voltage Vout_noload = 3.29 Volts Max Load Output Voltage With Rload = 13.2 Ohms for 250 ma Vout_maxload = 3.24 Volts o Note this is below the specification of 1% o Vin from my USB hub was only 4.44 volts but this is still above the specified dropout voltage 3.75 volts Dropout Voltage Output started to Drop from 3.24 Volts at full load, when the input is at 3.71 Volts Vdropout= 3.71 Volts Ripple Rejection With 300Mvp-p at 1 Khz injected through at pin1 of the lp2292 there was no noticeable increase in the normal noise of about 20 mvp-p (which seems to be the noise floor of my scope) Vload 20 log( ) dbv = 1.0Volt 3 20mV dbv = 20 log( ) 300mV Ripple Rejection > dbv Over Temp/Over Current protection Because I do not have a spare LP2992 I did not test this. William R. Robinson Jr. AJ4MC p4of 6

5 Comparison The table below shows good correlation with the documentation except for Max Load Output Voltage which is a little low o This should not affect this circuit as it does not pull the maximum current. o e graph shows the response of simulation and the measured circuit for transmit Real-Measured Simulation Calculated No Load Output Voltage V 3.29 N/A Max Load Output Voltage V 3.24 N/A Dropout Voltage V 3.71 N/A 3.75 Ripple Rejection db > N/A -45 Thermal Protection NOT TESTED N/A Works William R. Robinson Jr. AJ4MC p5of 6

6 References 1. Robson, Richard R. Sr., WB5RVZ, Ensemble RXTX Project, online, accessed National Semiconductor, Micropower 250 ma Low-Noise Ultra Low-Dropout Regulator, online, accessed UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 2010) p , (Eq 146) William R. Robinson Jr. AJ4MC p6of 6