Datasheet

ADA4433-1 Data Sheet
Rev. 0 | Page 10 of 16
THEORY OF OPERATION
The ADA4433-1 with short-to-battery and short-to-ground
protection is designed as a fifth-order, low-pass filter with a
fixed gain of 2 that is capable of driving 2 V p-p video signals
into doubly terminated video transmission lines on a single
supply as low as 2.7 V. The filter has a 1 dB flatness of 9 MHz
and provides a typical out-of-band rejection of 45 dB at 27 MHz.
The ADA4433-1 is a fully differential filter/driver that is also
designed for compliance with both ac- and dc-coupled inputs
and outputs. The ADA4433-1 can be driven by a differential or
single-ended source and provides a fully differential output signal
that is biased at a voltage equal to half the supply voltage (+V
S
/2).
When the devices is used with a single-ended input source, bias
the inverting input, −IN, at the middle of the input voltage range
applied to the noninverting input, +IN, allowing each output
signal to swing equally around the midsupply point (see the
Configuring the ADA4433-1 for Single-Ended Input Signals
section). This is particularly important to maximize output
voltage headroom in low supply voltage applications.
SHORT CIRCUIT (SHORT-TO-GROUND) PROTECTION
The ADA4433-1 includes an internal protection circuit that limits
the output sink or source current to 60 mA. This short-circuit
protection prevents damage to the ADA4433-1 when the output(s)
are shorted to ground, to a low impedance source, or together for
an extended time. In addition, the total sink or source current for
both outputs is limited to 50 mA, which helps protect the device in
the event of both outputs being shorted to a low impedance.
However, the short circuit protection does not affect the normal
operation of the device because one output sources current
while the other output sinks current when driving a differential
output signal.
OVERVOLTAGE (SHORT-TO-BATTERY) PROTECTION
The ADA4433-1 includes an internal protection circuit to ensure
that internal circuitry is not subjected to extreme voltages or
currents during an overvoltage event applied to its outputs. A
short-to-battery condition usually consists of a voltage on the
outputs that is significantly higher than the power supply
voltage of the amplifier. Duration can vary from a short
transient to a continuous fault.
The ADA4433-1 can withstand voltages of up to 18 V on its
outputs. Critical internal nodes are protected from exposure to
high voltages by circuitry that isolates the output devices from
the high voltage and limits internal currents. This protection is
available whether the device is enabled or disabled, even when
the supply voltage is removed.
The output devices are disconnected when the voltage at the
output pins exceeds the supply voltage. After the overvoltage
condition is removed, internal circuitry pulls the output voltage
back within normal operating levels. The output devices are
reconnected when the voltage at the output pins falls below the
supply voltage by about 300 mV. When the ADA4433-1
is used
with a doubly terminated cable, the voltage sensed at the output
pins is lower than the voltage applied to the cable by the voltage
drop across the back termination resistor. The maximum voltage
drop across the back termination resistor is limited by the short-
circuit current protection; therefore, the threshold at which the
overvoltage protection responds to a voltage applied to the cable is
V
THRESH (CABLE)
= +V
S
+ I
LIMIT
R
T
where:
V
THRESH (CABLE)
is the voltage applied to the cable that activates the
internal isolation circuitry.
+V
S
is the positive supply voltage.
I
LIMIT
is the internal short-circuit current limit, typically 50 mA.
R
T
the back termination resistance.
If the voltage applied to the cable is lower than V
THRESH (CABLE)
, the
voltage seen at the output pins is lower than the supply voltage,
so no overvoltage condition is detected. However, the internal
circuitry is protected by the short-circuit current limit; therefore,
the ADA4433-1 can withstand an indefinite duration short to any
positive voltage up to 18 V without damage.
SHORT-TO-BATTERY OUTPUT FLAG
In addition to the internal protection circuitry, the short-to-battery
output flag (STB pin) indicates an overvoltage condition on either
or both output pins. The flag is present whenever the internal
overvoltage protection is active; therefore, it is available when
the device is enabled or disabled. It is not available, however, when
the supply voltage is removed, although the internal protection
is still active. The threshold at which the short-to-battery flag is
activated and deactivated is the same as the threshold for the
protection circuitry.
Table 5. STB Pin Logic
STB Pin Output Device State
High (Logic 1) Overvoltage fault condition
Low (Logic 0) Normal operation