MAX16826_15 Datasheet, PDF(21/26 Page) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

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MAX16826_15 Datasheet, PDF (21/26 Pages) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

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MAX16826

Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

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 o perations 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.

â¢ Bit 1: Not used.

â¢ Bit 2: LED string 1 shorted flag. A diode short in LED

string 1 has been detected if this bit is set.

â¢ Bit 3: LED string 2 shorted flag. A diode short in LED

string 2 has been detected if this bit is set.

â¢ Bit 4: LED string 3 shorted flag. A diode short in LED

string 3 has been detected if this bit is set.

â¢ Bit 5: LED string 4 shorted flag. A diode short in LED

string 4 has been detected if this bit is set.

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.

These 3 bits are a hardwired value that identifies the

ICâs 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 (CS1âCS4) 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 = VCS1/IOUT1

where VCS1 can be set from 97mV to 316mV by the

internal registers through the I2C interface and IOUT1 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 IPK)

where IPK 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 Ã VOUT â VINMIN

VOUT Ã fSW Ã ÎIL

where VINMIN is the minimum input voltage, VOUT is the

desired output voltage, and fSW is the switching fre-

quency, and ÎIL 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 compro-

mise 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. VFB 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:

VOUT = (1 + R13/R14) x VFB

where VFB is the regulated voltage set by the internal

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 dig-

ital control loop is established with an external Î¼C closing

the loop. Firmware residing in the external Î¼C 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 volt-

age reading, and then the switching regulator output is

forced to maintain the difference required to provide cur-

rent regulation in the current sink FETs.

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