Download How to Set Control Voltage In an Application Circuit

RF Applications Engineering
Technical Note
µPG2250T5N
How to Set Control Voltage
In an Application Circuit
uPG2250T5N Pin Connections
As shown in the Pin Connections above, uPG2250T5N has three VDD pins for connection to
external supply voltage. In most applications these pins are connected to the main power
line/plane of the system. In addition, uPG2250T5N also has a control pin, Vcont, for the control
of the PA. As indicated on the data sheet, Vcont should be set around 1.8V while VDD can be
selected as high as 3.5V. Often the control voltage from a controller is the same or close to the
main supply voltage, as a result when the system supply voltage is higher than 1.8V a question
arises as to how control voltage can be lowered to the required level for Vcont of uPG2250T5N.
This note provides a brief discussion on this issue.
Generally there is a variety of methods to convert a DC voltage. In this particular case, one of the
following two simple methods should be sufficient. The schematic for the two methods are
shown at the end of this document.
i. Serial Voltage-dropping Resistor
This is the simplest way to lower the control voltage. In this method the voltage drop is simply
proportional to the current drawn by the device, as a result the usual design consideration is
whether it provides a stable enough bias point against current variation to ensure consistent PA
performance. This is particularly important in a design for mass production. In general there are
several factors that determine whether the voltage dropping resistor is an appropriate method for
a real application. In the following a quantitative analysis is provided for the case of
uPG2250T5N based on its performance characteristics
As shown in the plots under the section of TYPICAL CHARACTERISTICS on the data sheet,
the control current increases as a function of control voltage with a rate of 0.8mA/V in the range
around Vcont=1.8V. This feature provides a negative feedback against the current variation.
Also shown in the plot is that in the range of Vcont=1 to 2V, both Pout and IDD are relatively
insensitive to Vcont. Both are favorable features for using voltage dropping resistor.
Quantitatively, the plot shows the typical value of control current is about 1mA at Vcont=1.8V.
For a system supply voltage of 3V, a 1.2kΩ resistor would drop the voltage from 3V to 1.8V. As
2
for the effect of current variation, the data sheet states the maximum control current at
Vcont=1.8V is 3mA. Assuming in this case the Icont vs. Vcont curve has the same shape but
with larger slope (Icont should reach 3mA instead of 1mA at Vcont=1.8V), a simple analysis
shows the 1.2kΩ resistor would drop the voltage from 3V to about 1.4V in this case. The same
plot shows that at Vcont=1.4V, Pout and IDD are only slightly lowered.
In conclusion, a single serial resistor of 1.2 to 1.5kΩ can lower a supply voltage of 3 to 3.3V to
the appropriate range for Vcont. The effect of the control current variation on the PA
performance should be acceptable in most applications.
ii. Voltage Divider
For those applications where it is desirable to have minimum variation in bias condition, two
resistors can be used to form a voltage divider. This method provides a more stable bias point
against current variation. Its disadvantage is the extra component and power consumption. In this
bias scheme the voltage drop is determined by the ratio of the two resistors’ values and the
voltage stability is determined by their absolute values. A voltage divider with smaller resistor
values provides a more stable voltage point, but consumes more power.
uPG2250T5N
R
To microcontroller
3
Vcont
Serial Resistor Method
uPG2250T5N
To microcontroller
R1
3
Vcont
R2
Voltage Divider Method
3