LTC1745 datasheet(14/20 Pages) LINER

14 / 20 page

LTC1745

1745f

APPLICATIO S I FOR ATIO

Reference Operation

Figure 5 shows the LTC1745 reference circuitry consisting

of a 2.35V bandgap reference, a difference amplifier and

switching and control circuit. The internal voltage refer-

ence can be configured for two pin selectable input ranges

of 2V(

±1V differential) or 3.2V(±1.6V differential). Tying

the SENSE pin to ground selects the 2V range; tying the

SENSE pin to VDD selects the 3.2V range.

The 2.35V bandgap reference serves two functions: its

output provides a DC bias point for setting the common

mode voltage of any external input circuitry; additionally,

the reference is used with a difference amplifier to gener-

ate the differential reference levels needed by the internal

ADC circuitry.

An external bypass capacitor of 4.7

µF or larger is required

for the 2.35V reference output, VCM. 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.

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

VDD. 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 1

µF ceramic capacitor.

VCM

REFHA

REFLB

SENSE

TIE TO VDD FOR 3.2V RANGE;

TIE TO GND FOR 2V RANGE;

RANGE = 2 • VSENSE FOR

1V < VSENSE < 1.6V

2.35V

REFLA

REFHB

4.7

µF

4.7

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

1745 F05

LTC1745

Ω

DIFF AMP

µF

0.1

µF

INTERNAL ADC

LOW REFERENCE

2.35V BANDGAP

REFERENCE

1.6V

RANGE

DETECT

AND

CONTROL

Figure 5. Equivalent Reference Circuit

VCM

SENSE

2.35V

1.1V

4.7

µF

12.5k

µF

11k

1745 F06a

LTC1745

VCM

SENSE

2.35V

1.25V

1, 2

4.7

µF

0.1

µF

1745 F06b

LTC1745

LT1790-1.25

Figure 6a. 2.2V Range ADC

Figure 6b. 2.5V Range ADC with an External Reference

LTC1745 Datasheet(PDF) 14 Page - Linear Technology

LTC1745 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL