AMC7823 Datasheet, PDF (32/48 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL CIRCUIT

AMC7823
SLAS453A – APRIL 2005 – REVISED OCTOBER 2005
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PRECISION CURRENT SOURCE
The AMC7823 provides the user with a precision current source for driving an external component such as a
thermistor. Output current from pin 5 is set by the value of the resistor (R_SET) connected from pin 4 to the analog
supply voltage. This resistor should be close to the AMC7823 to minimize any voltage drop from the analog
supply to the resistor. Internal closed-loop circuitry impresses a fixed voltage across the set resistor by means of
an operational amplifier and a PMOS-follower output driver. Current through the set resistor supplies the follower.
The follower driver supplies this current to the external load connected from PRECISION_I_OUTPUT (pin 5) to
ground. This circuit architecture maintains high output impedance and provides wide output voltage compliance.
See Figure 49.
+5 V
AVDD
Reference
Int. / Ext.
V_SET
R_SET
ISET_RESISTOR
GREF
SREF
PREFB
PTS
AMC7823
Buffer and
Control Circuit
V_SET
AGND
PRECISION_I_OUTPUT
I_O
Figure 49. Diagram of Current Source
In addition to the external setting resistor R_SET, four logic bits—PTS and PREFB in the Power-Down Register and
SREF and GREF in the AMC Status/Configuration Register—are used to configure the current source as well.
Table 7 describes how to configure the current source.
The bit PTS is the current source power-down bit. When PTS is cleared to '0', the current source is in
power-down mode, and the output current is zero. When PTS is set to '1' and PREFB is set to '1', the current
source is in normal operation.
Bit SREF is the reference source selection bit. When SREF is cleared, the internal reference is selected. When
SREF is set to '1', the internal reference is de-selected. Set SREF = 1 when an external reference is applied.
GREF is the internal reference gain bit. PREFB is the Reference Buffer Amplifier Power-Down bit. Both bits
provide information to the current source circuit, and affect the current source configuration.
If the internal reference is used, and no external reference is applied to pin 21, the output current is 0.5 V/R_SET
(V_SET = 0.5 V) or zero, depending on bits PREFB and GREF, as shown in Table 7.
For a precise calculation, measure the voltage across the set resistor (V_SET) and divide it by the precise value of
R_SET.
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