Datasheet
LT1970
13
1970fe
For more information www.linear.com/LT1970
applicaTions inForMaTion
take protective measures when a current limit condition
is detected automatically.
The current limit status flag can also be used to produce
a dramatic change in the current limit value of the ampli
-
fier. Figure 4 illustrates a “snap-back” current limiting
characteristic. In this circuit, a simple resistor network
initially sets a high value of current limit (500mA). The
cir
cuit operates normally until the signal is large enough
to enter current limit. When either current limit flag goes
low, the current limit control voltage is reduced by a fac
-
tor of 10. This then forces a low level of output current
(50mA) until the signal is reduced in magnitude. When
the load current drops below the lower level, the current
limit is then restored to the higher value. This action is
similar to a self resettable fuse that trips at dangerously
high current levels and resets only when conditions are
safe to do so.
THERMAL MANAGEMENT
Minimizing Power Dissipation
The LT1970 can operate with up to 36V total supply volt
-
age with output currents up to ±500mA. The amount of
power dissipated in the chip could approach 18W under
worst-case conditions. This amount of power will cause
die temperature to rise until the circuit enters thermal
shutdown. While the thermal shutdown feature prevents
damage to the cir
cuit, normal operation is impaired.
Thermal design of the LT1970 operating environment is
essential to getting maximum utility from the circuit.
The first concern for thermal management is minimizing
the heat which must be dissipated. The separate power
pins V
+
and V
–
can be a great aid in minimizing on-chip
power. The output pin can swing to within 1.0V of V
+
or
V
–
even under maximum output current conditions. Using
separate power supplies, or voltage regulators, to set V
+
Figure 3. Adding Positive Feedback to Sharpen the Transition
Edges of the Current Limit Status Flags
Figure 4. “Snap-Back” Current Limiting
VC
SRC
COMMON
V
EE
VC
SNK
V
–
FILTER
V
+
12V
I
SOURCE
FLAG
WHEN CURRENT LIMIT
IS FLAGGED, I
LIMIT
TRESHOLD IS REDUCED
BY 0.5%
I
SINK
FLAG
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
+IN
V
IN
LT1970
–12V
–IN
R
S
1Ω
R
L
1970 F03
R
G
R
F
R2
100Ω
R1
100Ω
CURRENT
LIMIT
CONTROL
VOLTAGE
(0.1V TO 5V)
R4
20k
R3
20k
VC
SRC
COMMON
V
EE
VC
SNK
V
–
FILTER
V
+
12V
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
+IN
V
IN
LT1970
–12V
–IN
R
S
1Ω
R
L
1970 F04
R
G
10k
R
F
10k
R3
2.55k
R2
39.2k
R1
54.9k
I
MAX
500mA
–500mA
50mA
0I
OUT
I
LOW
V
CC
• R2
(R1 + R2) • 10 • R
S
I
MAX
≈
V
CC
• (R2||R3)
[R1 + (R2||R3)] • 10 • R
S
I
LOW
≈
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