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LTC5551

5551f

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The IFBIAS pin (Pin 16) is available for reducing the DC

current consumption of the IF amplifier, at the expense of

reducedperformance.Thispinshouldbeleftopen-circuited

for optimum performance. The internal bias circuit pro-

duces a 4mA reference for the IF amplifier, which causes

the amplifier to draw approximately 134mA. If resistor R3

is connected to Pin 16 as shown in Figure 7, a portion of

the reference current can be shunted to ground, resulting

in reduced IF amplifier current. For example, R3 = 1kΩ will

shunt away 1.5mA from Pin 16 and the IF amplifier cur-

rent will be reduced to approximately 90mA. The nominal,

open-circuit DC voltage at Pin 16 is 2.1V. Table 5 lists RF

performance at 1950MHz vs IF amplifier current.

Table 5. Mixer Performance with Reduced IF Amplifier Current

(RF = 1950MHz, Low Side LO, IF = 153MHz, VCC = 3.3V)

(kΩ)

ICC

(mA)

(dB)

IIP3

(dBm)

P1dB

(dBm)

(dB)

OPEN

204

2.4

35.5

18.0

9.7

4.7

194

2.4

35.0

17.9

9.4

2.2

186

2.4

34.2

17.8

9.2

1.0

164

2.4

31.9

17.3

8.7

(RF = 1950MHz, High Side LO, IF = 153MHz, VCC = 3.3V)

(kΩ)

ICCIF

(mA)

(dB)

IIP3

(dBm)

P1dB

(dBm)

(dB)

OPEN

204

2.4

33.0

17.9

10.5

4.7

194

2.3

32.6

17.8

10.2

2.2

186

2.3

32.1

17.6

9.9

1.0

164

2.3

30.5

17.0

9.4

Low Power Mode

The LTC5551 can be set to low power mode using a digital

voltage applied to the ISEL pin (Pin 8). This allows the

flexibility to reduce current when lower RF performance is

acceptable. Figure 15 shows a simplified schematic of the

ISEL pin interface. When ISEL is set low (<0.3V), the mixer

operates at maximum DC current. When ISEL is set high

(>1.2V), the DC current is reduced, thus reducing power

consumption. When floating, the ISEL is pulled low by

an internal pull-down resistor, and operates at maximum

supply current. The performance in low power mode and

nominal power mode are compared in Table 6.

Table 6. Performance Comparison – Low Power vs High Power

Mode RF = 1950MHz, Low Side LO, IF = 153MHz, EN = High

ISEL

ICC

(mA)

(dB)

IIP3

(dBm)

P1dB

(dBm)

(dB)

Low

204

2.4

35.5

18.0

9.7

High

139

2.4

29.3

16.7

8.3

Enable Interface

Figure 16 shows a simplified schematic of the EN pin in-

terface. To enable the chip, the EN voltage must be higher

than 1.2V. The EN voltage at the pin should never exceed

the power supply voltage (VCC) by more than 0.3V. If this

shouldoccur,thesupplycurrentcouldbesourcedthrough

the ESD diode, potentially damaging the IC.

If the EN pin is left floating, its voltage will be pulled low by

theinternalpull-downresistorandthechipwillbedisabled.

Figure 16. Enable Input Circuit

Figure 15. ISEL Interface Schematic

LTC5551

ISEL

VCC

5551 F15

BIAS

LTC5551

VCC

5551 F16

BIAS

applicaTions inForMaTion

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