User`s guide

Chapter 4 Theory of Operation

A-to-D Converter

The analog-to-digital converter (ADC) is used to change dc voltages into digital

information. The circuitry consists of an integrator amplifier (U25 and U28), current

steering switch U32, resistors (R72, R73, and R98), voltage reference U31, ADC

controller U20, and residue ADC in U19.

The ADC method used by the power supply is called multislope III. Multislope III is

a charge balancing continuously integrating analog-to-digital converter. The input

voltage continuously forces charge onto the integrator capacitors C50 and C52

through R73.

Switch U32 steers fixed positive or negative reference currents onto the integrator

capacitors to cancel, or balance the accumulated input charge. The level shifted (R99

and R100) output of the integrator is checked every 2.66 msec by the U20 COMP

input. Logic state machines in U20 control the U32 current steering to continuously

seek an approximate 2.5 V level on the integrator amplifier output, FLASH. If the

ADC input voltage is between ±15 V, the integrator output (FLASH) will remain

within the 0 V to 5 V range of the U19 on-chip ADC. The U19 ADC input (FLASH)

is clamped to 0 V or 5 V by R47 and CR14 to protect U19.

The integrator amplifier is formed by U25 and U28. Resistors R60 and R61 affect the

amplifier stability. Amplifier oscillation may occur if their values are incorrect.

Amplifier U28 improves the offset voltage characteristics of integrator amplifier U25.

Each analog-to-digital conversion occurs continuously. The ADC starts by clearing

the integrator slope count in U20. At the end of the integration period, the slope count

is latched. The slope count provides the most significant bits of the input voltage

conversion. The least significant bits are converted by the on-chip ADC of U19.

U40 provides a stable +5 V reference voltage for ADC. U31A amplifies the voltage

reference to +10 V while amplifier U31B inverts the +10 V reference to -10 V. The

reference voltage forces precision slope currents for the integrating ADC through R72

and R98.