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2271f

LTC2271

APPLICATIONS INFORMATION

Reference

The LTC2271 has an internal 1.25V voltage reference. For

a 2.1V input range using the internal reference, connect

SENSE to VDD. For a 1.05V input range using the internal

reference, connect SENSE to ground. For a 2.1V input

range with an external reference, apply a 1.25V reference

voltage to SENSE (Figure 9).

The input range can be adjusted by applying a voltage to

SENSE that is between 0.625V and 1.30V. The input range

will then be 1.68 • VSENSE.

The VREF, REFH and REFL pins should be bypassed as

shown in Figure 8. A low inductance 2.2μF interdigitated

capacitor is recommended for the bypass between REFH

and REFL. This type of capacitor is available at a low cost

from multiple suppliers.

Alternatively, C1 can be replaced by a standard 2.2μF

capacitor between REFH and REFL. The capacitors should

be as close to the pins as possible (not on the back side

of the circuit board).

Figure 8c and 8d show the recommended circuit board

layout for the REFH/REFL bypass capacitors. Note that in

Figure 8c, every pin of the interdigitated capacitor (C1)

is connected since the pins are not internally connected

in some vendors’ capacitors. In Figure 8d the REFH and

REFL pins are connected by short jumpers in an internal

layer. To minimize the inductance of these jumpers they

can be placed in a small hole in the GND plane on the

second board layer.

Figure 8a. Reference Circuit

SENSE

1.25V

EXTERNAL

REFERENCE

2.2μF

1μF

VREF

2271 F09

LTC2271

Figure 9. Using an External 1.25V Reference

VREF

REFH

SENSE

TIE TO VDD FOR 2.1V RANGE;

TIE TO GND FOR 1.05V RANGE;

SENSE FOR

0.625V < VSENSE < 1.300V

1.25V

REFL

INTERNAL ADC

HIGH REFERENCE

BUFFER

2271 F08a

LTC2271

5Ω

0.84x

DIFF AMP

INTERNAL ADC

LOW REFERENCE

C1: 2.2μF LOW INDUCTANCE

INTERDIGITATED CAPACITOR

TDK CLLE1AX7S0G225M

MURATA LLA219C70G225M

AVX W2L14Z225M

OR EQUIVALENT

1.25V BANDGAP

REFERENCE

0.625V

RANGE

DETECT

AND

CONTROL

2.2μF

1μF

–

REFH

REFL

2271 F08b

LTC2271

CAPACITORS ARE 0402 PACKAGE SIZE

1μF

2.2μF

1μF

Figure 8b. Alternative REFH/REFL Bypass Circuit

Figure 8c. Recommended Layout for the REFH/REFL Bypass

Circuit in Figure 8a

Figure 8d. Recommended Layout for the REFH/REFL Bypass

Circuit in Figure 8b

2271 F08c

2271 F08d

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