LT1970A
17
1970afc
For more information www.linear.com/LT1970A
applicaTions inForMaTion
Figure 6 shows the LT1970A driving an inductive load
with a controlled amount of current. This load is shown
as a generic magnetic transducer, which could be used to
create and modulate a magnetic field. Driving the current
limit control inputs directly forces a current through the
load that could range up to 2MHz in modulation. Clamp
diodes are added to protect the LT1970A output from large
inductive flyback potentials caused by rapid di/dt changes.
Supply Bypassing
The LT1970A can supply large currents from the power
supplies to a load at frequencies up to 4MHz. Power
supply impedance must be kept low enough to deliver
these currents without causing supply rails to droop. Low
ESR capacitors, such as 0.1µF or 1µF ceramics, located
close to the pins are essential in all applications. When
large, high speed transient currents are present additional
capacitance may be needed near the chip. Check supply
rails with a scope and if signal related ripple is seen on the
supply rail, increase the decoupling capacitor as needed.
To ensure proper start-up biasing of the LT1970A, it is
recommended that the rate of change of the supply volt
-
ages at turn-on be limited to be no faster than 6V/µs.
Application Circuit Ideas
The digitally controlled analog pin driver is shown in
Figure 7. All of the control signals are provided by an
LTC
®
1664 quad, 10-bit DAC by way of a 3-wire serial
interface. The LT1970A is configured as a simple differ-
ence amplifier with a gain of 3. This gain is required to
produce
±15V
from the 0V to 5V outputs from DACs C
and D. To provide voltage headroom, the supplies for the
LT1970A are set to the maximum value of ±18V. As ±18V
is the absolute maximum rating of supply voltage for the
LT1970A, care must be taken to not allow the supply voltage
to increase. DACs A and B separately control the sinking
and sourcing current limit to the load over the range of
±4mA to ±500mA. An optional on/off control for the pin
driver using the ENABLE input is shown. If always enabled
the ENABLE pin should be tied to V
CC
.
In some applications it may be necessary to know what
the current into the load is at any time. Figure 8 shows an
LT1787 high side current sense amplifier monitoring the
current through sense resistor R
S
. The LT1787 is biased
from the V
EE
supply to accommodate the common mode
input range of ±10V. The sense resistor is scaled down
to provide a 100mV maximum differential signal to the
current sense amplifier to preserve linearity. The LT1880
amplifier provides gain and level shifting to produce a 0V
to 5V output signal (2.5V DC ±5mV/mA) with up to 1kHz
full-scale bandwidth. An A/D converter could then digi
-
tize this instantaneous current reading to provide digital
feedback from the circuit.
The L
T1970A is just as easy to use as a standard opera
-
tional amplifier. Basic amplification of a precision reference
voltage creates a very simple bench DC power supply as
shown in Figure 9. The built-in power stage produces an
adjustable 0V to 25V at 4mA to 100mA of output current.
Voltage and current adjustments are derived from the
LT1634-5 5V reference. The output current capability is
500mA, but this supply is restricted to 100mA for power
dissipation reasons. The worst-case output voltage for
maximum power dissipated in the LT1970A output stage
occurs if the output is shorted to ground or set to a voltage
near zero. Limiting the output current to 100mA sets the
maximum power dissipation to 3W. To allow the output to
range all the way to 0V, an LTC1046 charge pump inverter
is used to develop a –5V supply. This produces a negative
rail for the LT1970A which has to sink only the quiescent
current of the amplifier, typically 7mA.
Using a second LT1970A, a 0V to ±12V dual tracking power
supply is shown in Figure 10. The midpoint of two 10k
resistors connected between the + and – outputs is held
at 0V by the LT1881 dual op amp servo feedback loop. To
maintain 0V, both outputs must be equal and opposite in
polarity, thus they track each other. If one output reaches
current limit and drops in voltage, the other output fol
-
lows to maintain a symmetrical + and – voltage across
a common load. Again, the output current limit is less
than the full capability of the L
T1970A due to thermal
reasons. Separate current limit indicators are used on
each L
T1970A because one output only sources current
and the other only sinks current. Both devices can share
the same thermal shutdown indicator, as the output flags
can be ORed together.
