IS32LT3175N Datasheet, PDF (14/19 Pages) Integrated Silicon Solution, Inc – SINGLE CHANNEL LINEAR LED DRIVER WITH FADE IN/OUT AND PWM DIMMING

IS32LT3175N/P
THERMAL CONSIDERATIONS
The package thermal resistance, θJA, determines the
amount of heat that can pass from the silicon die to the
surrounding ambient environment. The θJA is a
measure of the temperature rise created by power
dissipation and is usually measured in degree Celsius
per watt (°C/W). The junction temperature, TJ, can be
calculated by the rise of the silicon temperature, ∆T,
the power dissipation, PD, and the package thermal
resistance, θJA, as in Equation (3):
P_D  V_CC  I_CC  (V_CC  V_LED )  I_OUT
(3)
and,
T_J  T_A  T  T_A  P_D  _JA
(4)
Where ICC is the IC quiescent current, VCC is the supply
voltage, VLED is the voltage across VCC to OUT and TA
is the ambient temperature.
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. The maximum power dissipation can
be calculated using the following Equation (5):
P_{D(MAX )}

125C  25C
 _JA
(5)
So,
P_{D(MAX )}

125C  25C
50.98C /W
 1.96W
Figure 28, shows the power derating of the
IS32LT3175 on a JEDEC board (in accordance with
JESD 51-5 and JESD 51-7) standing in still air.
2.5
SOP-8-EP
2
1.5
1
0.5
0
-40 -25 -10 5 20 35 50 65 80
Temperature (°C)
Figure 28 Dissipation Curve
95 110 125
The thermal resistance is achieved by mounting the
IS32LT3175N/P on a standard FR4 double-sided
printed circuit board (PCB) with a copper area of a few
square inches on each side of the board under the
IS32LT3175N/P. Multiple thermal vias, as shown in
Figure 29, help to conduct the heat from the exposed
pad of the IS32LT3175N/P to the copper on each side
of the board. The thermal resistance can be reduced
by using a metal substrate or by adding a heatsink or
thicker copper plane.
Figure 29 Board Via Layout For Thermal Dissipation
EMI AT THE CABLE AND INTERCONNECT LEVEL
Vehicle electronics can be affected by electromagnetic
interference (EMI) caused by ‘stray’’ magnetic and
electric fields from automotive inductive load switching.
Running throughout the vehicle are wiring harnesses
which behave as ‘‘hidden antennas’’ and pickup these
harmonic frequencies.
Because the IS32LT3175 is usually connected with a
long wire to the vehicle’s central computer, it could be
susceptible to EMI transients. For example, a coupled
EMI transient on the wiring harness connected to the
IS32LT3175’s PWM pin 8 can be passed through and
cause a slight LED flicker.
To avoid this, an RC low-pass filter can be
implemented to attenuate high frequency signals at the
PWM pin. The low-pass filter will allow only low
frequency signals from 0Hz to its cut-off frequency (ƒc)
to pass while attenuating frequencies above this cut-off
frequency.
The formula to calculate the cut-off frequency of an RC
filter is:
1
f
C

2

R
C
(6)
PWM
PWM
As shown in Figure 30, typical values for RPWM=10kΩ
and CPWM=3.3nF. For the IS32LT3175 the value of
RPWM is fixed at 10kΩ (must always be installed) while
CPWM is optional and its value can vary depending on
the vehicle’s EMI environment.
Integrated Silicon Solution, Inc. – www.issi.com
14
Rev.A, 07/19/2016