7
FN3180.6
October 2, 2015
High output currents can be obtained only as far as package
dissipation allows. It is strongly recommended that output
current-limit sensing be used in such cases.
Current-Limit Sensing - The on-chip comparator (C in the
Functional Diagram) permits shutdown of the regulator
output in the event of excessive current drain. As Figure 8
shows, a current-limiting resistor, R
CL
, is placed in series
with V
OUT2
and the SENSE terminal is connected to the
load side of R
CL
. When the current through R
CL
is high
enough to produce a voltage drop equal to V
CL
(0.5V) the
voltage feedback is by-passed and the regulator output will
be limited to this current. Therefore, when the maximum load
current (I
LOAD
) is determined, simply divide V
CL
by I
LOAD
to
obtain the value for R
CL
.
Logic-Controllable Shutdown - When equipment is not
needed continuously (e.g., in remote data-acquisition
systems), it is desirable to eliminate its drain on the system
until it is required. This usually means switches, with their
unreliable contacts. Instead, the ICL7663S can be shut
down by a logic signal, leaving only I
Q
(under 4A) as a
drain on the power source. Since this pin must not be left
open, it should be tied to ground if not needed. A voltage of
less than 0.3V for the ICL7663S will keep the regulator ON,
and a voltage level of more than 1.4V but less than V+
IN
will
turn the outputs OFF. If there is a possibility that the control
signal could exceed the regulator input (V+
IN
) the current
from this signal should be limited to 100A maximum by a
high value (1M) series resistor. This situation may occur
when the logic signal originates from a system powered
separately from that of the regulator.
Additional Circuit Precautions - This regulator has poor
rejection of voltage fluctuations from AC sources above
10Hz or so. To prevent the output from responding (where
this might be a problem), a reservoir capacitor across the
load is advised. The value of this capacitor is chosen so that
the regulated output voltage reaches 90% of its final value in
20ms. From:
In addition, where such a capacitor is used, a current-limiting
resistor is also suggested (see “Current-Limit Sensing”).
Producing Output Voltages with Negative Temperature
Coefficients -The ICL7663S has an additional output which
is 0.9V relative to GND and has a tempco of +2.5mV/°C. By
applying this voltage to the inverting input of amplifier A (i.e.,
the V
SET
pin), output voltages having negative TC may be
produced. The TC of the output voltage is controlled by the
R
2
/R
3
ratio (see Figure 9 and its design equations).
Applications
Boosting Output Current with External Transistor
The maximum available output current from the ICL7663S is
40mA. To obtain output currents greater than 40mA, an
external NPN transistor is used connected as shown in
Figure 10.
SHDN
V
OUT2
V
OUT1
V
TC
V
SET
SENSE
GND
R
2
C
L
V
IN
0.047F
V+
IN
604k
R
CL
20
210k
R
1
10F
V
OUT
+5V
FIGURE 8. POSITIVE REGULATOR WITH CURRENT LIMIT
V
OUT
=
R
2
+ R
1
R
1
V
SET
= 5V
I
CL
=
V
CL
R
CL
= 25mA
I =
V
C ,C
t
=
I
OUT
(20 x 10
-3
)
0.9V
OUT
= 0.022
I
OUT
V
OUT
V
REF
V
SET
-
+
V
TC
-
+
R
1
R
2
V
OUT
R
3
-
+
Where:V
SET
= 1.3V
V
TC
= 0.9V
TCV
TC
= +2.5mV/°C
FIGURE 9. GENERATING NEGATIVE TEMPERATURE
COEFFICIENTS
EQ. 1: V
OUT
= V
SET
( R
2
1 +
R
1
) R
2
+
R
3
(V
SET
- V
TC
)
EQ. 2: TC V
OUT
=
R
2
-
R
3
(TC V
TC
) in mV/°C
SHDN
V
OUT2
V
OUT1
V
SET
SENSE
GND
V
IN
10F
V+
IN
604k
100
210k
V
OUT
+5V
0.47
EXTERNAL PIN
POWER
TRANSISTOR
FIGURE 10. BOOSTING OUTPUT CURRENT WITH EXTERNAL
TRANSISTOR
ICL7663S
