LT1970
11
1970fe
For more information www.linear.com/LT1970
applicaTions inForMaTion
its very high transconductance, takes control from the input
stage, GM1. The output current is regulated to a value of
I
OUT
= V
SENSE
/R
SENSE
= (VC
SRC
or VC
SNK
)/(10 • R
SENSE
).
The time required for the current limit amplifiers to take
control of the output is typically 4µs.
Linear operation of the current limit sense amplifier occurs
with the inputs SENSE
+
and SENSE
–
ranging between V
CC
– 1.5V and V
EE
+ 1.5
V
. Most applications will connect pins
SENSE
+
and OUT together, with the load on the opposite
side of the external sense resistor and pin SENSE
–
. Feed-
back to the inverting input of GM1 should be connected
from SENSE
–
to –IN. Ground side sensing of load current
may be employed by connecting the load between pins
OUT and SENSE
+
. Pin SENSE
–
would be connected to
ground in this instance. Load current would be regulated
in exactly the same way as the conventional connection.
However, voltage mode accuracy would be degraded in
this case due to the voltage across R
SENSE
.
Creative applications are possible where pins SENSE
+
and
SENSE
–
monitor a parameter other than load current. The
operating principle that at most one of the current limit
stages may be active at one time, and that when active,
the current limit stages take control of the output from
GM1, can be used for many different signals.
Current Limit Threshold Control Buffers
Input pins VC
SNK
and VC
SRC
are used to set the response
thresholds of current limit amplifiers “I
SINK
” and “I
SRC
”.
Each of these inputs may be independently driven by a
voltage of 0V to 5V above the COMMON reference pin.
The 0V to 5V input voltage is attenuated by a factor of 10
and applied as an offset to the appropriate current limit
amplifier. AC signals may be applied to these pins. The AC
bandwidth from a V
C
pin to the output is typically 2MHz.
For proper operation of the LT1970, these control inputs
cannot be left floating.
For low V
CC
supply applications it is important to keep
the maximum input control voltages, VC
SRC
and VC
SNK
,
at least 2.5V below the V
CC
potential. This ensures linear
control of the current limit threshold. Reducing the current
limit sense resistor value allows high output current from
a smaller control voltage which may be necessary if the
V
CC
supply is only 5V.
The transfer function from V
C
to the associated V
OS
is
linear from about 0.1V to 5V in, or 10mV to 500mV at
the current limit amplifier inputs. An intentional nonlinear
-
ity is built into the transfer functions at low levels. This
nonlinearity insures that both the sink and source limit
amplifiers cannot become active simultaneously. Simul
-
taneous activation of the limit amplifiers could result in
uncontrolled outputs. As shown in the Typical Performance
Characteristics curves, the control inputs have a “hockey
stick” shape, to keep the minimum limit threshold at 4mV
for each limit amplifier
.
Figure 1 illustrates an interesting use of the current
sense input pins. Here the current limit control ampli-
fiers are used to produce a symmetrically limited output
voltage swing. Instead of monitoring the output current,
the output voltage is
divided down by a factor
of 20 and
applied to the SENSE
+
input, with the SENSE
–
input
grounded. When the threshold voltage between SENSE
+
and SENSE
–
(V
CLAMP
/10) is reached, the current limit
stage takes control of the output and clamps it a level of
±2 • V
CLAMP
. With control inputs V
CSRC
and V
CSNK
tied
together, a single polarity input voltage sets the same +
and – output limit voltage for symmetrical limiting. In this
circuit the output will current limit at the built-in fail-safe
level of typically 800mA.
Figure 1. Symmetrical Output Voltage Limiting
VC
SRC
COMMON
V
EE
VC
SNK
V
–
FILTER
V
+
12V
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
+IN
R3
3k
80mV
TO
10V
–80mV
TO
–10V
±CLAMP
REACHED
OUTPUT CLAMPS
AT 2× V
CLAMP
V
CLAMP
OV TO 5V
V
IN
LT1970
–12V
–IN
R1
21.5k R
L
1970 F01
R2
1.13k
R
F
R
G
