INTRODUCTION
This kit converts any DC input voltage from 6 to 12V up to an output of 16.5V DC. Nominal output current is 100mA. More current is available but with a corresponding increase in output voltage ripple. The main use for this kit is to provide the necessary input voltage for our range of microcontroller programmers, such as QK123. Most 12V DC unregulated plug packs have an open circuit output of 16 to 18 volts. This is sufficient to drive the programmers. However, in some countries, only regulated plug packs are available. These give an exact 12V DC output. This kit aims to overcome that problem. The following table gives performance specifications for this kit.
Test
Conditions
Results
Line Regulation
VIN = 6V - 12V IO = 100 mA
40 mV
Load Regulation
VIN = 12V IO = 50 300 mA
30 mV
Output Ripple
VIN = 12V IO = 100 mA
150 mV
Line Regulation the change in the output voltage in response to a change in the input voltage at a given current.
Load Regulation the change in the output voltage in response to a change in the average load current at a given input voltage.
Output Ripple the AC component of a steady DC output.
ﾠ
ASSEMBLY INSTRUCTIONS
Use the component overlay on the PCB to place the components, in the following order: ﾷ Resistors and diode ﾷ Ceramic capacitor and IC socket ﾷ DC power jacks ﾷ Electrolytic capacitors. Make sure they are the right way around ﾷ Inductor
CIRCUIT DESCRIPTION
The circuit is based on a Motorola MC34063A DC-DC converter IC. Data on this chip is available from the Motorola web site at http://mot-sps.comThe input voltage is applied to pin 6 of the IC and also to the 170uH inductor via resistors R1-4. These resistors provide current monitoring and the IC shuts down if the voltage across them exceeds 0.3V. An internal oscillator, its operating frequency set by capacitor C3, drives a high current switch whose output appears at pin 1. When this output goes low it switches current through the 170uH inductor. Each time the switch turns off, the collapsing magnetic field associated with the inductor will try to maintain the current through it. With the switch off the only available current path is via the 1N5819 diode, D1, to the 470uF capacitor C3. This capacitor charges to a higher voltage than the input voltage. Pin 5 is the negative input of an internal voltage comparator, while an internal 1.25V reference is connected to the positive input. Negative feedback, via the resistor network R6 and R7, is used to set the output voltage. When the voltage at pin 5 is just above 1.25V the output (pin 1) is off and when it is below 1.25V the output is switching on and off at a high frequency. This will regulate the output voltage to within ﾱ 50mV. The formula to set the output voltage is:ﾠ
VOUT =1.25(1+ R7/R6)
Earning 1point for this item
Validate your login
Sign In
Create New Account