LTC5582

14

Rev. D

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APPLICATIONS INFORMATION

larger filtering capacitor C3 at the expense of a slower

transient response.

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 time

1.7V

=

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

ofthisfilter.Accordingly,therisetimebecomesverysimilar

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

Thesimplifiedinterfaceschematicoftheintercepttempera-

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

Figure 10. Simplified Interface Circuit Schematic of the

Control Pins RT1 and RT2

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.

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.

Table 2. Suggested RT1 and RT2 Values for Optimal Temperature

Performance vs RF Frequency

FREQUENCY (MHz)

RT1 (kΩ)

RT2 (kΩ)

450

12

2

880

12

2

2140

0

2

2700

0

1.6

250

RT1 OR RT2

RT1 OR RT2

VCC

5582 F10

LTC5582

RT1 (k)

5

0.8

0.2

15

25

35

40

30

20

10

0

1.2

0.4

0.6

1.0

100

40

80

60

20

0

120

5582 F11

TC1

detV1

Figure 11. 1st-Order Temperature Compensation Coefficient

TC1 vs RT1 Value