Principle of Operation
837
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Reference Module (REF_A)
21.2 Principle of Operation
The REF_A module provides all of the necessary voltage references to be used by various analog
modules on the device.
The REF_A module contains a high-performance bandgap. This bandgap has good accuracy (factory
calibrated), low temperature coefficient, and high PSRR even while operating at low power. The bandgap
voltage is used to generate 1.2 V, 1.45 V, and 2.5 V reference voltages through a noninverting amplifier
stage. One voltage can be selected at a time. A second output of the REF_A module provides a buffered
bandgap reference line. The REF_A module also supports voltage references that are required for the
DAC12 module, when it is available. The REF_A module also includes the temperature sensor circuitry
that operates from the bandgap. The temperature sensor is used by an ADC to measure a voltage
proportional to temperature.
When the REFON bit is set to 1 in the REFCTL0 register, the bandgap, the bandgap bias, the
noninverting buffer stage, and the unity gain buffer are enabled.
21.2.1 Low-Power Operation
The REF_A module can support low-power applications such as LCD generation. Many of these
applications do not require a very accurate reference, compared to data conversion, but low-power
consumption is very important. To support these kinds of applications, the bandgap can be used in a
sampled mode. In sampled mode, the bandgap circuitry is clocked by the VLO at an appropriate duty
cycle. This reduces the average power of the bandgap circuitry significantly, but at the cost of accuracy.
When not in sampled mode, the bandgap is in static mode. Its power is at its highest, but so is its
accuracy.
Analog modules can automatically request static mode or sampled mode through their own individual
request lines. In this way, each module determines which mode is appropriate for its proper operation and
performance. Any one active analog module that requests static mode causes all other analog modules to
use static mode, even if another analog module is requesting sampled mode. In other words, static mode
request always has higher priority than sampled mode request. When the REFON bit is set, the bandgap
and the bandgap bias operate in static mode.
21.2.2 Reference System Requests
Three basic reference requests are used by the REF_A module. Each analog module can use these
requests to obtain the proper response from the REF_A module. The three basic requests are
REFGENREQ, REFBGREQ, and REFMODEREQ. No interaction is required by the user code. The analog
modules automatically select the proper request.
A reference request signal, REFGENREQ, is available as an input into the REFGEN subsystem. This
signal represents a logical OR of individual requests coming from the various analog modules in the
device that require a voltage reference to be available on the variable reference line. When a module
requires a voltage reference, it asserts its corresponding REFGENREQ signal. When the REFGENREQ is
asserted, the REFGEN subsystem is enabled. After the specified settling time, the variable reference line
voltage is stable and ready for use. The REFVSEL settings determine which voltage is generated on the
variable reference line.
After the specified settling time of the REFGEN subsystem, the REF_A module sets the REFGENRDY
signal. This signal is used by each analog module, for example, to wait before an ADC conversion is
started after a REFGENREQ was set. The generation of the reference voltage can be triggered by a timer
or by software to make sure the reference voltage is ready when an analog module requires it.
In addition to the REFGENREQ, a second reference request signal, REFBGREQ is available. The
REFBGREQ signal represents a logical OR of requests coming from the various analog modules that
require the buffered bandgap reference line. When the REFBGREQ is asserted, the bandgap with its bias
circuitry and local buffer are enabled, if it is not already enabled by a prior request.
After the specified settling time of the REFBGREQ subsystem, the REF_A module sets the REFBGRDY
signal. This signal is used by each analog module to hold operation while the buffered bandgap reference
voltage is settling. The generation of the buffered bandgap voltage can be triggered by a timer or by
software to make sure the reference voltage is ready when an analog module requires it.