AD8315
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.
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.
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.
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 20ꢁ, 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.
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.
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.
40
4
+85°C
30
3
+25°C
20
2
+85°C
+25°C
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.
10
1
–30°C
0
0
–10
–20
–1
–2
–30°C
–3
–4
–30
–40
AD8315
0
0.2
0.4
0.6
0.8
(V)
1.0
1.2
1.4
1.6
V
SET
C
C
RFIN
Figure 39. POUT vs. VSET at 0.9 GHz for Dual-Mode Handset
Power Amplifier Application, −30°C, +25°C, and +85°C
R
52.3V
SHUNT
C
R
IN
IN
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).
Figure 40. Broadband Resistive Input Coupling Option
AD8315
C
C
X1
RFIN
C
R
IN
X2
IN
Figure 41. Narrow-Band Reactive Input Coupling Option
Rev. C | Page 19 of 24
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