LTC5551
20
5551fa
For more information www.linear.com/LTC5551
The IFBIAS pin (Pin 16) is available for reducing the DC
current consumption of the IF amplifier, at the expense of
reduced performance. This pin should be left open-circuited
for optimum performance. The internal bias circuit pro
-
duces a 4
mA 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, V
CC
= 3.3V)
R3
(kΩ)
I
CC
(mA)
G
C
(dB)
IIP3
(dBm)
P1dB
(dBm)
NF
(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, V
CC
= 3.3V)
R3
(kΩ)
I
CCIF
(mA)
G
C
(dB)
IIP3
(dBm)
P1dB
(dBm)
NF
(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
I
CC
(mA)
G
C
(dB)
IIP3
(dBm)
P1dB
(dBm)
NF
(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 (V
CC
) by more than 0.3V. If this
should occur, the supply current could be sourced through
the ESD diode, potentially damaging the IC.
If the EN pin is left floating, its voltage will be pulled low by
the internal pull-down resistor and the chip will be disabled.
Figure 16. Enable Input Circuit
Figure 15. ISEL Interface Schematic
LTC5551
8
ISEL
V
CC
5551 F15
7
BIAS
LTC5551
5
EN
V
CC
5551 F16
6
BIAS
applicaTions inForMaTion
