ISL78171 Datasheet, PDF (12/29 Pages) Intersil Corporation – 6-Channel, 50mA Automotive LED Driver with Ultra-high Dimming Ratio and Phase Shift Control

ISL78171
Theory of Operation
PWM Boost Converter
The PWM boost converter operating in current mode produces
the minimum voltage needed to enable up to six parallel LED
strings having a combination of high and low forward voltage
drop to run at the programmed current. The ISL78171 employs
current mode control boost architecture that has a fast current
sense loop and a slow voltage feedback loop. Such architecture
achieves a fast transient response that is essential for notebook
backlight applications in which drained batteries can be instantly
changed to an AC/DC adapter without noticeable visual
brightness change. The number of LEDs that can be driven by
ISL78171 depends on the type of LED chosen in the application.
The ISL78171 is capable of boosting up to 40V max and typically
driving 12 LEDs in series for each of the 6 channels, enabling a
total of 72 pieces of the 3.2V/20mA type of LEDs. At start-up, the
LX pin injects a fixed current into the output capacitor. The device
does not start unless the voltage at LX exceeds 1.2V. Also note
that the VIN has to be 12V or greater to support an LED string
current of 50mA per channel.
Enable
The EN pin is used to enable or disable the ISL78171 operation. It
is a high voltage pin that can be tied directly to V_IN up to 26.5V. If
EN is pulled low for longer than 30ms, the device will shut down.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 24.
The LED peak current is set by translating the R_SET current to the
output with a scaling factor of 410.5/R_SET. The sink terminals of
the current source MOSFETs are designed to run at 500mV to
optimize the power loss versus accuracy requirements. The
sources of errors of the channel-to-channel current matching
come from the op amps offset, internal layout, reference and
current source resistors. These parameters are optimized for
current matching and absolute current accuracy. On the other
hand, the absolute accuracy is additionally determined by the
external R_SET, and therefore, additional tolerance will be
contributed by the current setting resistor. A 1% tolerance
resistor is therefore recommended.
+
+
-
REF
-
RSET
+
-
PWM DIMMING
DC DIMMING
FIGURE 24. SIMPLIFIED CURRENT SOURCE CIRCUIT
Dynamic Headroom Control
The ISL78171 features a proprietary Dynamic Headroom Control
circuit that detects the highest forward voltage string or
effectively the lowest voltage from any of the CH0-CH5 pins to
GND. When this lowest channel voltage is lower than the short
circuit threshold, V_SC, such voltage will be used as the feedback
signal for the boost regulator. The boost makes the output to the
correct level such that the lowest channel pin is at the target
headroom voltage. Since all LED stacks are connected to the
same output voltage, the other channel pins will have a higher
voltage, but the regulated current source circuit on each channel
will ensure that each channel has the same programmed
current. The output voltage will regulate cycle-by-cycle and is
always referenced to the highest forward voltage string in the
architecture.If VSC function is enabled then the forward voltages
across the LED string matching must be less than 7.5V.
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an appropriate
value for R_SET. This should be chosen to fix the maximum
possible LED current:
I_LEDmax = -_R4---¹-_S--⁰--_E--^.--⁵_T-
(EQ. 1)
Where I_LEDmax is in mA and R_SET in k. Once R_SET is fixed, the
LED DC current can be adjusted by writing a value to the DC
Brightness register 0x07 (BRTDC) as follows:
I_LED = 1.61xBRTDC  R_SET
(EQ. 2)
BRTDC can be programmed from 0 to 255 in decimal and
defaults to 255 (0xFF). If left at the default value, LED current will
be fixed at I_LEDmax. BRTDC can be adjusted dynamically on the fly
during operation. BRTDC = 0 disconnects all channels.
For example, if the maximum required LED current (I_LED(max)) is
20mA, rearranging Equation 1 yields Equation 3:
R_SET = 410.5  0.02 = 20.52k
(EQ. 3)
If BRTDC is set to 200 then:
I_LED = 1.61  200  20.52 = 15.69mA
(EQ. 4)
PWM DIMMING CONTROL
The ISL78171 provides multiple PWM dimming methods, as
described in the following. Table 1 summarizes the dimming
mode selection. Each of these methods results in PWM chopping
of the current in the LEDs for all 6 channels to provide a lower
average LED current. During the On periods, the LED current will
be defined by the value of R_SET and BRTDC, as described in
Equations 1 and 2. The source of the PWM signal can be
described as follows:
1. Internally Dimming Duty Cycle control: The dimming duty
cycle is controlled by a 256 step PWM brightness control
register (BRT) which is programmed through the SMBus/I²C.
The dimming frequency is set by the resistor connected to the
FPWM pin.
2. External Dimming Duty Cycle control: The signal applied to
the PWM input provides the dimming duty cycle information
for the LED display. The dimming frequency is set by the
resistor connected to the FPWM pin.
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FN8602.0
June 15, 2015