LTC5582IDD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5582IDD' PDF : 16 Pages View PDF
LTC5582
Applications Information
Since the output buffer amplifier of the LTC5582 is capable
of driving an arbitrary capacitive load, the residual ripple
can be further filtered at the output with a series resistor
RSS and a large shunt capacitor CLOAD. See Figure 9. This
lowpass filter also reduces the output noise by limiting
the output noise bandwidth. When this RC network is
designed properly, a fast output transient response can
be maintained with a reduced residual ripple. For example,
we can estimate CLOAD with an output voltage swing of
1.7V at 2140MHz. In order not to allow the maximum
5mA souring current to limit the fall time (about 5μs), the
maximum value of CLOAD can be chosen as follows:
CLOAD
≤
5mA
•
allowable
additional
1.7V
time
=
5mA
•
0.25µs
1.7V
=
735pF
Once CLOAD is determined, RSS can be chosen properly
to form a RC low-pass filter with a corner frequency of
1/[2π(RSS + 100) • CLOAD].
In general, the rise time of the LTC5582 is much shorter
than the fall time. However, when the output RC filter is
used, the rise time can be dominated by the time constant
of this filter. Accordingly, the rise time becomes very similar
to the fall time. Although the maximum sinking capability
of the LTC5582 is 5mA, it is recommended that the output
load resistance should be greater than 1.2k in order to
achieve the full output voltage swing.
Temperature Compensation of Logarithmic Intercept
The simplified interface schematic of the intercept tempera-
ture compensation is shown in Figure 10. The adjustment
of the output voltage can be described by the following
equation with respect to the ambient temperature:
ΔVOUT = –TC1 • (TA – TNOM) – TC2 • (TA – TNOM)2–
detV1 – detV2
where TC1 and TC2 are the 1st-order and 2nd-order
temperature compensation coefficients, respectively; TA
is the actual ambient temperature; and TNOM is the refer-
ence room temperature; detV1 and detV2 are the output
voltage variations when RT1 and RT2 are not set to zero at
room temperature. The temperature coefficients TC1 and
TC2 are shown as functions of the tuning resistors RT1
and RT2 in Figures 11 and 12, respectively.
VCC
LTC5582
250Ω
RT1 OR RT2
RT1 OR RT2
5582 F10
Figure 10. Simplified Interface Circuit Schematic of the
Control Pins RT1 and RT2
When Pins RT1 and RT2 are shorted to ground, the tem-
perature compensation circuit is disabled automatically.
Table 2 lists the suggested RT1 and RT2 values at various
RF frequencies for the best output performance over
temperature.
1.2
120
1.0
100
TC1
0.8
80
0.6
60
0.4
40
0.2
20
detV1
0
0
5 10 15 20 25 30 35 40
RT1 (kΩ)
5582 F11
Figure 11. 1st-Order Temperature Compensation Coefficient
TC1 vs RT1 Value
Table 2. Suggested RT1 and RT2 Values for Optimal Temperature
Performance vs RF Frequency
FREQUENCY (MHz)
450
RT1 (kΩ)
12
RT2 (kΩ)
2
880
12
2
2140
0
2
2700
0
1.6
3600
0
1.6
5800
0
3
5582f
13
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