Datasheet

LT1970
16
1970fc
APPLICATIONS INFORMATION
Figure 6 shows the LT1970 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 fi eld. Driving the current
limit control inputs directly forces a current through the
load that could range up to 2MHz in modulation. Biasing
the input stage to the midpoint of the modulation signal
allows symmetrical bidirectional current fl ow through
the load. Clamp diodes are added to protect the LT1970
output from large inductive fl yback potentials caused by
rapid di/dt changes.
Abrupt Load Short Protection
An abrupt short-circuit connection, often referred to as
screwdriver or crowbar short, to ground or other supply
potentials is the worst-case load condition for the LT1970.
The current limit sense amplifi er normally operates with
an input voltage differential equal to the voltage across the
sense resistor, which is only 500mV maximum in a typical
application. During an abrupt load short to ground, the
load end of the sense resistor is immediately connected to
ground while the amplifi er output remains at the normal
output voltage. This can impose a large differential voltage
to the sense amplifi er inputs for a brief period. If this delta
V can be greater than ±2V, it is benefi cial to add clamps
between the current limit sense amplifi er inputs. These
clamps ensure a smooth transition from the main amplifi er
control to the current limit amplifi er control under all load
short conditions that may arise.
Figure 7 shows the connection of these back-to-back
clamp diodes between the FILTER pin and the SENSE
+
pin.
With this connection, the internal 1k resistor between the
SENSE
–
pin and the FILTER pin limits the diode current. The
BAV99 diodes are small SOT-23 packaged general purpose
silicon diodes. The maximum current limit sense voltage
is now the diode voltage drop, determined by the voltage
across the sense resistor and the 1k internal resistor. As
the diode begins to conduct current, with a voltage drop
of around 300mV, an error in the expected current limit
level at the high end of the control becomes apparent, as
Figure 6. Current Modulation of a Magnetic Transducer
Figure 7. Adding Protection for Abrupt Load Shorts
4
5
1k
100Ω*
SENSE
+
OUT
FILTER
SENSE
–
*OPTIONAL—SEE TEXT
SENSE
+
FILTER
12
3
R
SENSE
BAV99
LOAD
2N3904
1970 F07
HIGHER CLAMP VOLTAGE
ALTERNATIVE
2N3904
1nF TO 10nF
–
+
6
3
MAIN
AMPLIFIER
CURRENT
LIMIT SENSE
AMPLIFIER
VC
SRC
COMMON
V
EE
VC
SNK
V
–
FILTER
V
+
12V
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
D2
1N4001
C1
500pF
R1
95.3K
LT1634-2.5
+IN
5V
V
IN
0V
LT1970
–12V
–IN
12V
2.5V
R
S
1
±500mA
MAGNETIC
TRANSDUCER
1970 F06
D1
1N4001