Document No. DS13649 Rev. C, ECN 11508
Revision Date: 4/3/2012
MPD2T28125-700
PIN Diode Driver
magnitude of the forward voltage across the PIN diode
and the resistance of R4. This current is nominally 25
mA.
The series PIN diode which is connected between the J0
and J2 ports must be reverse biased during State 1. The
reverse bias voltage must be sufficiently large to maintain
the diode in its non-conducting, high impedance state
when a large RF signal voltage may be present in the J0-
to-J1 path. The reverse voltage across this diode is the
arithmetic difference of the bias voltage applied to the J2
bias port and the DC forward voltage of the forward-
biased J0-to-J1 series PIN diode.
The minimum voltage required to maintain the series
diode between J0 and J2 out of conduction is a function
of the magnitude of the RF voltage present, the standing
wave present at the series diode’s anode, the frequency
of the RF signal and the characteristics of the series
diode, among other factors. Minimum control voltages
for several signal frequencies are shown in the table
“Minimum Reverse Bias Voltage”, assuming the input
power to the J0 or J1 port to be 100 W CW and the VSWR
on the J0-J1 path to be 1.5:1. For other conditions,
please contact the Aeroflex Metelics applications engi-
neering team for assistance in determining the required
bias voltage.
State 2
In State 2, the series PIN diode between the J0 and J2
ports is forward biased by applying 0 V to the J2 bias
input port (P1-J2). The magnitude of the resultant bias
current through the diode is primarily determined by the
voltage applied to the J0 bias port (P1-J0), the magnitude
of the forward voltage across the PIN diode and the
resistance of R1. This current is nominally 100 mA. At
the same time, the PIN diode connected between J2 and
B2 ports is reverse biased by applying a high bias voltage,
nominally 28 V, to the B2 bias port (P1-B2). A high volt-
age, nominally 28 V, is also applied to the J1 bias port
(P1-J1). Under this condition, the PIN diode connected
between the J0 and J1 ports is reverse biased thus
isolating the J1 RF port from the RF signal path between
J0 and J2. The reverse voltage across this diode is the
arithmetic difference of the bias voltage applied to the J1
bias port and the DC forward voltage of the
forward-biased J0-to-J2 series PIN diode. As described
above, the minimum voltage required to maintain the
series diode on the J0-to-J1 side of the switch out of
conduction is a function of the magnitude of the RF
voltage present, the standing wave present at the diode’s
anode, the frequency of the RF signal and the character-
istics of the series diode, among other factors.
Calculation of Resistor Values
The magnitude of the forward bias current applied to the
series diode is set by the magnitude of the supply voltage
+VCC1, which is nominally 5 V, the value of resistor R1
and the forward voltage of the series diode, VDIODE,
among other factors. Given the desired current value, the
resistance is given by the formula:
The magnitude of the current through the shunt diode is
set by the magnitude of the supply voltage +VCC2, the
value of resistor in series with the shunt diode (R2 or R4)
and the forward voltage of the shunt diode, VDIODE,
among other factors. Given the desired current value,
this resistance is given by the formula:
Single Control Operation
The logic level available at output CTL0 is the comple-
ment of the control voltage applied to input CTL1. For
single control operation via input CTL1, connect CTL0
directly to control input CTL2.
Control of Asymmetrical SP2T Switch
The MPD2T28125-700 can control an SP2T T/R switch
comprising a series diode on the Transmit (Tx) side of the
switch and a series shunt topology on the Receive (Rx)
side of the switch. Each driver section is connected to
bias a series diode on one side of the switch. The output
of Driver Section A, which controls the series diode on
the Tx side of the switch, also controls the shunt diode on
the Rx side of the switch. Driver Section B controls the
series diode on the Rx side of the switch only.
A typical asymmetric switch/driver application circuit is
shown below. In this circuit, the MPD2T28125-700
driver is used to control the Aeroflex / Metelics
MSW2000-200 asymmetrical SP2T switch. The switch
may be controlled to one of two operational states, which
are called Transmit State and Receive State. In the
descriptions of these states, it is assumed that +VCC1 =
5 V and +VCC2 = 28 V.