IS32LT3181
Integrated Silicon Solution, Inc. – www.issi.com 12
Rev. B, 04/20/2017
APPLICATION INFORMATION
The IS32LT3181 is a 6-channel linear current driver
optimized to drive Rear Combination Lamp for
automotive applications. A single input is used to
select between two fully programmable intensity levels,
one for the ‘STOP’ condition, and the other for the
‘TAIL’ condition. The full intensity ‘STOP’ condition is
easily set using an external resistor, R
STOP
. The lower
intensity ‘TAIL’ condition is realized via an integrated
PWM circuit, the duty cycle of which is easily
programmed using an external resistor, R
TAIL
.
IS32LT3181 also includes an integrated drive circuit
for an external PMOS FET linear regulator for the case
where the voltage across the LED loads must be
accurately maintained to control power dissipation.
The ERC pin current (I
ERC
) flows through 1kΩ resistor
(R
GS
) and generates a voltage across gate and drain of
PMOS FET. IS32LT3181 regulates this ERC current
by sensing feedback reference voltage (V
FB
) to control
the R
DS_ON
of PMOS FET and get the expected V
STRING
,
which is set by resistor divider R
FB1
and R
FB2
.
The integrated feedback reference is trimmed to be
9% accuracy, while the ERC pin current (I
ERC
) for the
external regulator control can drive up to 6mA.
PROGRAMMING THE OUTPUT CURRENT
A single programming resistor (R
STOP
) controls the
maximum sink current for each LED channel. The
STOP pin provides a reference voltage of 1.08V (Typ.).
The programming resistor may be computed using the
following Equation (1):
STOP
OUT
R
V
I
08.1
100
(1)
The current which is drawn from the STOP pin is
internally mirrored to each of the 6 outputs with a
multiplication factor of 100A/A. Thus, an output current
of 50mA would require a current to be drawn from
STOP of 500µA, corresponding to an external
programming resistance of 2.16kΩ.
OVER CURRENT PROTECTION
A 1mA current limiting on STOP pin limits the current
which can be referenced from the STOP pin.
Exceeding the STOP pin current limit will shut down
the output currents to prevent unexpected excessive
power dissipation (Figure 10). When the STOP pin
current reaches 1mA, the current outputs are
shutdown, the fault register is triggered and FAULTB
pin will be asserted low after the FAULTB delay time.
PROGRAMMING THE PWM DUTY CYCLE
A single external resistor on the TAIL pin sets the
PWM duty cycle which determines the lower intensity
TAIL condition. The TAIL pin supplies a constant
current of 100µA. The PWM duty cycle (D
cycle
) is set by
the following Equation (2):
%1001.0
4
100
V
RA
D
TAIL
cycle
(2)
Internally, a sawtooth waveform with a peak value of
4.4V and a minimum value of 0.4V is compared to the
voltage of the TAIL pin (100uA x R
TAIL
). The fixed
frequency of the sawtooth waveform is 1kHz resulting
in a PWM signal of 1kHz at the programmed duty cycle.
Thus, for example, a 50% duty cycle would require the
reference voltage at TAIL to be 2.4V, corresponding to
an external resistance value of 24kΩ.
The internal PWM generator can also be driven by an
external DC voltage supply. Providing a DC input
voltage to the TAIL pin in the range from 0.4V to 4.4V
programs the output duty cycle linearly from 0% to
100% duty cycle.
THERMAL SHUTDOWN
If the die temperature exceeds the thermal shutdown
temperature of 160°C (Typ.) then the device will
shutdown, and the sink currents are shut off for all
channels. After a thermal shutdown event, the
IS32LT3181 will not try to restart until its temperature
has reduced to less than 145°C (Typ.). The fault
register is triggered and FAULTB pin will be asserted
low after delay time.
OUTPUT CURRENT SLEW RATE CONTROL
To minimize the effects of EMI, the output current rise
and fall time is controlled. The slew rate control
circuitry is designed to control the rise time, 10% to
90% and fall time 90% to 10% at 5mA/µs (Typ.).
OPEN LED DETECTION
Each of the outputs of the IS32LT3181 is monitored for
an output voltage of less than 450mV (Typ.). If any of
the output voltages drops below the threshold voltage,
the fault register is triggered and the FAULTB pin is
asserted low after delay time.
During normal operation, it is possible that current may
still be flowing in the output LED string even if the
output voltage of the IC falls below the OC detect
threshold – for example, if the LED string remains intact,
but the supply voltage dips momentarily (the duration is
longer than t
D
which is configured by the DELAY pin)
and UVLO pin remain higher than threshold. In this
case, the FAULTB pin would assert then de-assert
when the output voltage returns to the nominal value.
OUTPUT DISABLE DETECTION
As IS32LT3181 powers up, the device will check OUTx
pin of each channel to see if it is connected to GND. If
any channel is connected to GND (disable typical
threshold is 100mV), the fault diagnostic function will
