IS32LT3170/71
Integrated Silicon Solution, Inc. – www.issi.com 14
Rev. B, 12/21/2016
APPLICATIONS INFORMATION
IS32LT3170/71 provides an easy constant current
source solution for LED lighting applications. It uses
an external resistor to adjust the LED current from
10mA to 150mA. The LED current can be
determined by the external resistor R
EXT
as Equation
(1):
mAI
mA
R
SET
EXT
10
10610
(1)
Where I
SET
is in mA.
Paralleling a low tolerance resistor R
EXT
with the
internal resistor R
INT
will improve the overall
accuracy of the current sense resistance. The
resulting output current will vary slightly lower due to
the negative temperature coefficient (NTC) resulting
from the self heating of the IS32LT3170/71.
HIGH INPUT VOLTAGE APPLICATION
When driving a long string of LEDs whose total
forward voltage drop exceeds the IS32LT3170
V
BD_OUT
limit of 42V, it is possible to stack several
LEDs(such as 2 LEDs) between the EN pin and the
OUT pins 2,3, and 5 so the voltage on the EN pin is
higher than 5V. The remaining string of LEDs can
then be placed between power supply +V
S
and EN
pin, (Figure 34). The number of LEDs required to
stack at EN pin will depend on the LED’s forward
voltage drop (VF) and the +V
S
value.
IS32LT3170
GND
OUT
+V
S
> 42V
EN
1
4
2,3,5
6
REXT
R
EXT
Figure 34 High Input Voltage Application Circuit
Note: when operating the IS32LT3170 at voltages
exceeding the device operating limits, care needs to
be taken to keep the EN pin and OUT pin voltage
below 42V.
THERMAL PROTECTION AND DISSIPATION
The IS32LT3170/71 implements thermal foldback
protection to reduce the LED current when the
package’s thermal dissipation is exceeded and
prevent “thermal runaway”. The thermal foldback
implements a negative temperature coefficient
(NTC) of -0.26%/K.
When operating the chip at high ambient
temperatures, or when driving maximum load
current, care must be taken to avoid exceeding the
package power dissipation limits. Exceeding the
package dissipation will cause the device to enter
thermal protection mode. The maximum package
power dissipation can be calculated using the
following Equation (2):
JA
AMAXJ
MAXD
TT
P
)(
)(
(2)
Where T
J(MAX)
is the maximum junction temperature,
T
A
is the ambient temperature, and θ
JA
is the junction
to ambient thermal resistance; a metric for the
relative thermal performance of a package.
The recommended maximum operating junction
temperature, T
J(MAX)
, is 125°C and so the maximum
ambient temperature is determined by the package
parameter; θ
JA
. The θ
JA
for the IS32LT3170/71
SOT23-6 package is 130°C/W.
Therefore the maximum power dissipation at T
A
=
25°C is:
W
WC
CC
P
MAXD
77.0
/130
25125
)(
The actual power dissipation P
D
is:
ENENOUTOUTD
IVIVP
(3)
To ensure the performance, the die temperature (T
J
)
of the IS32LT3170/71 should not exceed 125°C. The
graph below gives details for the package power
derating.
0
0.2
0.4
0.6
0.8
1
Temperature (°C)
Power Dissipation (W)
-40 -25 -10 5 20 35 50 65 80 95 110 125
SOT23-6
Figure 35 P
D
vs. T
A
(SOT23-6)
