LT1970A
11
1970afc
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applicaTions inForMaTion
and VC
SRC
/10 in magnitude, cause the current limit ampli-
fiers to decouple from the signal path. This is functionally
indicated by diodes D1 and D2 in the Block Diagram. When
the voltage V
SENSE
increases in magnitude sufficient to
equal or overcome one of the offset voltages VC
SNK
/10 or
VC
SRC
/10, the appropriate current limit amplifier becomes
active and because of 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 be-
tween 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
–
. Feedback 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 LT1970A, 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 ensures 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 VC
SRC
and VC
SNK
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.
ENABLE Control
The ENABLE input pin puts the LT1970A into a low sup-
ply current, high impedance output state. The ENABLE
pin responds to TTL threshold levels with respect to the
COMMON pin. Pulling the ENABLE pin low is the best
way to for
ce zero current at the output. Setting VC
SNK
=
VC
SRC
= 0V allows the output current to remain as high
as ±4mV/R
SENSE
.
