MAX16826 Datasheet, PDF (21/26 Pages) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

Programmable, Four-String HB LED Driver with
Output-Voltage Optimization and Fault Detection
register. First, activate standby mode and then deacti-
vate this mode using the I²C interface. Next, perform a
read operation on the fault status register. The old fault
information is reported in this first read operation. The
conclusion of the read operation clears the data con-
tained in the register. Subsequent read operations con-
firm that the fault status register has been cleared.
The description of this register is as follows:
Bit 0: Overvoltage sense flag. This flag is set if the volt-
age at OVP exceeds 1.25V; switching stops until power
or the enable or standby is cycled.
• Bit 1: Not used.
• Bit 2: LED string 1 shorted flag. A diode short in LED
string 1 has been detected if this bit is set.
• Bit 3: LED string 2 shorted flag. A diode short in LED
string 2 has been detected if this bit is set.
• Bit 4: LED string 3 shorted flag. A diode short in LED
string 3 has been detected if this bit is set.
• Bit 5: LED string 4 shorted flag. A diode short in LED
string 4 has been detected if this bit is set.
Register 0Bh Bit 0: Device Standby Command
When register 0Bh bit 0 is set to 1, the IC enters a low-
current standby mode. In this mode, the system clock is
off and no operation is allowed. Set this bit to 0 to leave
standby mode and back to normal operation mode.
Register 0Ch Bit 2-0: Device Revision Code
These 3 bits are a hardwired value that identifies the
IC’s revision.
Applications Information
Programming LED Currents
The MAX16826 uses sense resistors (R28, R29, R30,
R31 in the Typical Application Circuit) to set the output
current for each LED string. To set the LED current for a
particular string, connect a sense resistor across the
corresponding current-sense input (CS1–CS4) and
GND. For optimal accuracy, connect the low-side of the
current-sense resistors to GND with short traces. The
value needed for the sense resistor for a given current
is calculated with the equation below:
R31 = V_CS1/I_OUT1
where V_CS1 can be set from 97mV to 316mV by the
internal registers through the I²C interface and I_OUT1 is
the desired LED string 1 current.
Calculating the Value of Peak
Current-Limit Resistor
The value of R12 sets the peak switching current that
flows in the switching FET (Q1). Set the value of resistor
R12 using the equation below:
R12 = 0.19/(1.2 x I_PK)
where I_PK is the peak inductor current at minimum input
voltage and maximum load.
Boost Inductor Value
The value of the boost inductor is calculated using the
following equation:
( ) L1 = VINMIN × VOUT − VINMIN
V_OUT × f_SW × ∆I_L
where V_INMIN is the minimum input voltage, V_OUT is the
desired output voltage, and f_SW is the switching fre-
quency, and ∆I_L is the peak-to-peak ripple in the boost
inductor. Higher inductor values lead to lower ripple but
at a higher cost and size. Choose an inductor value
that gives peak-to-peak ripple current in the order of
30% to 40% of the average current in the inductor at
low-line and full-rated load. This choice of inductor is a
compromise between cost, size, and performance for
the boost converter.
Setting Output Voltage
Set the switch regulator output voltage by connecting
FB to the center of a resistive voltage-divider between
the switching regulator output and GND. V_FB is regulat-
ed to a voltage from 0.88V to 1.25V (typ) set by an
internal register through the I²C interface. Choose R13
and R14 in the Typical Application Circuit for a reason-
able bias current in the resistive divider and use the fol-
lowing formula to set the output voltage:
V_OUT = (1 + R13/R14) x V_FB
where V_FB is the regulated voltage set by the internal
register.
Adaptive Voltage Optimization
The availability of the digitized switching regulator output
voltage and current sink drain voltages and the ability to
change the switching regulator output voltage provide
the ability to do adaptive voltage optimization. A slow
digital control loop is established with an external µC
closing the loop. Firmware residing in the external µC is
tasked to read each one of the current sink FET drain
voltages and select the minimum value of the four LED
strings. The minimum value is subtracted from the scaled
output voltage reading, and then the switching regulator
output is forced to maintain the difference required to
provide current regulation in the current sink FETs.
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