MAX16826_15 Datasheet, PDF (16/26 Pages) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

MAX16826
Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
is 2μs, so the ADC requires at least 5 of these minimum
pulses within the 190ms measurement window to com-
plete a conversion. During PWM dimming, LED current
pulse widths of less than 2μs are possible, but the ADC
may not have enough sampling time to complete a con-
version in this scenario and the corresponding data may
be incomplete or inaccurate. Therefore, adaptive volt-
age optimization may not be possible when the LED
current-pulse duration is less than 2μs. The LED current
pulse duration is shorter than the pulse applied at the
DIM_ inputs because of the LED turn-on delay.
Faults and Fault Detection
The MAX16826 features circuitry that automatically
detects faults such as overvoltage or shorted LED string.
An internal fault register at the address OAh is used to
record these faults. For example, if a shorted LED string
is detected, the corresponding fault register bit is set and
the faulty output is shut down.
Shorted LED strings are detected with fast comparators
connected to DR1–DR4. The trip threshold of these
comparators is 1.52V (typ). When this threshold is
exceeded, the shorted string is latched off and the cor-
responding bit of register OAh is set.
After the internal ADC completes a conversion, the
result is stored in the corresponding register and can
be read out by the external μC. The μC then compares
the conversion data with the preset limit to determine if
there is a fault.
When an LED string opens, the voltage at the corre-
sponding current-sink FET drain node goes to 0V.
However, the ADC can only complete a conversion if
the LED current comes into regulation. If an LED string
opens before the LED current can come into regulation,
the ADC cannot complete a conversion and the MSB
(eighth bit) is set to indicate an incomplete conversion
or timeout condition. Thus, an examination of the MSB
provides an indication that the LED string is open. If the
LED string opens after the LED current is in regulation,
the ADC can make conversions and reports that the
drain voltage is 0V. Therefore, to detect an open condi-
tion, monitor the MSB and the ADC measurement. If the
MSB is set and the CS_ on-time is greater than 2μs, or
if the ADC measures 0 at the drain, then there is an
open circuit.
ADC
DAC
EXTERNAL
EVENTS
SYSTEM
CLOCK
I²C
REGISTER
FILE UNIT
OVP
POWER
MANAGEMENT
Figure 3. Digital Block Diagram
Table 1. ADC Response
CONDITION
Shorted string fault
Shorted string fault while
converting
ADC register read when it is
being updated
UVLO
STBY
SHDN
ADC RESPONSE
Load full-scale code into register, no conversions on affected channel until power or enable is
cycled.
Immediately load full-scale code into register and cease conversion effort on this channel until
power or enable is cycled.
Previous sample is shifted out through the I²C interface and then the register is updated with the
new measurement.
Immediately terminate conversions, do not update current register.
Immediately terminate conversions, do not update current register.
Immediately terminate conversions, do not update current register.
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