Datasheet

15
LTC1744
1744f
APPLICATIO S I FOR ATIO
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An external bypass capacitor of 4.7µF or larger is required
for the 2.5V reference output, V
CM
. This provides a high
frequency low impedance path to ground for internal and
external circuitry. This is also the compensation capacitor
for the reference. It will not be stable without this capacitor.
The difference amplifier generates the high and low refer-
ence for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins: REFHA and REFHB
for the high reference and REFLA and REFLB for the low
reference. The doubled output pins are needed to reduce
package inductance. Bypass capacitors must be con-
nected as shown in Figure 5.
V
CM
REFHA
REFLB
SENSE
TIE TO V
DD
FOR 3.2V RANGE;
TIE TO GND FOR 2V RANGE;
RANGE = 2 • V
SENSE
FOR
1V < V
SENSE
< 1.6V
2.5V
REFLA
REFHB
4.7µF
4.7µF
INTERNAL ADC
HIGH REFERENCE
BUFFER
0.1µF
1744 F05
LTC1744
4Ω
DIFF AMP
1µF
1µF
0.1µF
INTERNAL ADC
LOW REFERENCE
2.5V BANDGAP
REFERENCE
1.6V
1V
RANGE
DETECT
AND
CONTROL
Figure 5. Equivalent Reference Circuit
V
CM
SENSE
2.5V
1.1V
4.7µF
14k
1µF
11k
1744 F06a
LTC1744
V
CM
SENSE
2.5V
5V
1.25V64
1, 2
4.7µF
1µF0.1µF
1744 F06b
LTC1744
LT1790-1.25
Figure 6a. 2.2V Range ADC
Figure 6b. 2.5V Range ADC with an External Reference
Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 6a. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device since the logic threshold is close to ground and
V
DD
. The SENSE pin should be tied high or low as close to
the converter as possible. If the SENSE pin is driven
externally, it should be bypassed to ground as close to the
device as possible with a 1µF ceramic capacitor.