LTC5544
14
5544f
applicaTions inForMaTion
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 4mA reference for the IF amplifier, which causes
the amplifier to draw approximately 98mA. If resistor R1
is connected to Pin 16 as shown in Figure 6, a portion of
the reference current can be shunted to ground, resulting
in reduced IF amplifier current. For example, R1 = 1kΩ
will shunt away 1.5mA from Pin 16 and the IF amplifier
current will be reduced by 40% to approximately 59mA.
The nominal, open-circuit DC voltage at Pin 16 is 2.1V.
Table 5 lists RF performance at 5250MHz versus IF ampli-
fier current.
Table 5. Mixer Performance with Reduced IF Amplifier Current
(RF = 5250MHz, Low Side LO, IF = 240MHz, V
CC
= V
CCIF
= 3.3V)
R1
(kΩ)
I
CCIF
(mA)
G
C
(dB)
IIP3
(dBm)
P1dB
(dBm)
NF
(dB)
OPEN 98 7.4 25.9 11.4 11.3
4.7 89 7.2 25.7 11.5 11.4
2.2 77 6.9 25.2 11.6 11.5
1.0 59 6.3 23.8 11.3 11.6
(RF = 5250MHz, High Side LO, IF = 240MHz, V
CC
= V
CCIF
= 3.3V)
R1
(kΩ)
I
CCIF
(mA)
G
C
(dB)
IIP3
(dBm)
P1dB
(dBm)
NF
(dB)
OPEN 98 7.3 24.0 11.4 11.7
4.7 89 7.0 23.8 11.4 11.9
2.2 77 6.6 23.5 11.4 12.2
1.0 59 5.8 22.6 11.3 12.4
Shutdown Interface
Figure 12 shows a simplified schematic of the SHDN pin
interface. To disable the chip, the SHDN voltage must be
higher than 3.0V. If the shutdown function is not required,
the SHDN pin should be connected directly to GND. The
voltage at the SHDN 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.
The SHDN pin must be pulled high or low. If left floating,
then the on/off state of the IC will be indeterminate. If a
three-state condition can exist at the SHDN
pin, then a
pull-up or pull-down resistor must be used.
Figure 12. Shutdown Input Circuit
Figure 13. TEMP Diode Voltage vs Junction Temperature (T
J
)
LTC5544
4
SHDN
500Ω
V
CC1
5544 F12
5
Temperature Diode
The LTC5544 provides an on-chip diode at Pin 12 (TEMP)
for chip temperature measurement. Pin 12 is connected to
the anode of an internal ESD diode with its cathode con-
nected to internal ground. The chip temperature can be
measured by injecting a constant DC current into Pin 12
and measuring its DC voltage. The voltage vs temperature
coefficient of the diode is about –1.73mV/°C with 10µA
current injected into the TEMP pin. Figure 13 shows a
typical temperature-voltage behavior when 10µA and 80µA
currents are injected into Pin 12.
Supply Voltage Ramping
Fast ramping of the supply voltage can cause a current
glitch in the internal ESD protection circuits. Depending on
the supply inductance, this could result in a supply volt-
age transient that exceeds the maximum rating. A supply
voltage ramp time of greater than 1ms is recommended.
JUNCTION TEMPERATURE (°C)
–40
TEMP DIODE VOLTAGE (mV)
600
850
900
0
40 80
500
750
550
800
450
400
700
650
–20
20
60
100
5544 F13
80µA
10µA
