AD8315ARMZ datasheet(19/24 Pages) AD

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AD8315

Rev. C | Page 19 of 24

increases the phase margin, which helps to make the step response

of the circuit more stable when the PA output power is low and

the slope of the PA’s power control function is the steepest.

A smaller filter capacitor can be used by inserting a series

resistor between VAPC and the control input of the PA. A

series resistor works with the input impedance of the PA to

create a resistor divider and reduces the loop gain. The size of

the resistor divider ratio depends upon the available output

swing of VAPC and the required control voltage on the PA.

This technique can also be used to limit the control voltage in

situations where the PA cannot deliver the power level being

demanded by VAPC. Overdrive of the control input of some

PAs causes increased distortion. It should be noted, however,

that if the control loop opens (that is, VAPC goes to its maximum

value in an effort to balance the loop), the quiescent current of

the AD8315 increases somewhat, particularly at supply voltages

greater than 3 V.

Figure 39 shows the relationship between VSET and output

power (POUT) at 0.9 GHz . The overall gain control function is

linear in dB for a dynamic range of over 40 dB. Note that for

VSET voltages below 300 mV, the output power drops off steeply

as VAPC drops toward its minimum level of 250 mV.

–10

–20

–30

–40

–1

–2

–3

–4

1.6

+85°C

+25°C

–30°C

+85°C

+25°C

–30°C

VSET (V)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 39. POUT vs. VSET at 0.9 GHz for Dual-Mode Handset

Power Amplifier Application, −30°C, +25°C, and +85°C

ENABLE AND POWER-ON

The AD8315 can be disabled by pulling the ENBL pin to

ground. This reduces the supply current from its nominal level

of 7.4 mA to 4 μA. The logic threshold for turning on the device

is at 1.5 V with 2.7 V supply voltage. A plot of the enable glitch

is shown in Figure 22. Alternatively, the device can be completely

disabled by pulling the supply voltage to ground. To minimize

glitch in this mode, ENBL and VPOS should be tied together. If

VPOS is applied before the device is enabled, a narrow 750 mV

glitch results (see Figure 29).

In both situations, the voltage on VSET should be kept below

200 mV during power-on and power-off to prevent any

unwanted transients on VAPC.

INPUT COUPLING OPTIONS

The internal 5 pF coupling capacitor of the AD8315, along with

the low frequency input impedance of 2.8 kΩ, give a high-pass

input corner frequency of approximately 16 MHz. This sets the

minimum operating frequency. Figure 40, Figure 41, and Figure 42

show three options for input coupling. A broadband resistive

match can be implemented by connecting a shunt resistor to

ground at RFIN (see Figure 40). This 52.3 Ω resistor (other

values can also be used to select different overall input impedances)

combines with the input impedance of the AD8315 to give a

broadband input impedance of 50 Ω. While the input resistance

and capacitance (CIN and RIN) of the AD8315 varies from device

to device by approximately ±20%, and over frequency (see

Figure 11), the dominance of the external shunt resistor means

that the variation in the overall input impedance is close to the

tolerance of the external resistor. This method of matching is

most useful in wideband applications or in multiband systems

where there is more than one operating frequency.

A reactive match can also be implemented as shown in

Figure 41. This is not recommended at low frequencies as

device tolerances dramatically vary the quality of the match

because of the large input resistance. For low frequencies,

Figure 40 or Figure 42 is recommended.

In Figure 41, the matching components are drawn as generic

reactances. Depending on the frequency, the input impedance

and the availability of standard value components, either a

capacitor or an inductor is used. As in the previous case, the

input impedance at a particular frequency is plotted on a Smith

Chart and matching components are chosen (shunt or series L,

shunt or series C) to move the impedance to the center of the chart.

AD8315

RFIN

RSHUNT

52.3V

RIN

CIN

Figure 40. Broadband Resistive Input Coupling Option

AD8315

RFIN

RIN

CIN

Figure 41. Narrow-Band Reactive Input Coupling Option

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