MAX16838_11 Datasheet, PDF(14/21 Page) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

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Integrated, 2-Channel, High-Brightness LED Driver

with High-Voltage Boost and SEPIC Controller

Connect a 4.7I resistor from VCC to DRV to power the

rest of the chip from the VCC pin with the 5V internal

regulator. Bypass DRV to PGND with a minimum of 1FF

ceramic capacitor as close as possible to the device. For

input voltage range of 4.5V to 5.5V, connect IN to VCC.

LED Current Control (ISET)

The MAX16838 features two identical constant-current

sources used to drive multiple HB LED strings. The

current through each of the channels is adjustable

between 20mA and 150mA using an external resistor

(RISET) connected between ISET and SGND. Select

RISET using the following formula:

RISET

1512

IOUT _

(â¦)

where IOUT_ is the desired output current for both

channels in amps.

For single-channel operation, connect channel 1 and

channel 2 together. See Figure 2.

LED Dimming Control

The MAX16838 features LED brightness control using an

external PWM signal applied at DIM. The device accepts

a minimum pulse width of 1Fs. Therefore, a 5000:1 dim-

ming ratio is achieved when using a PWM frequency of

200Hz. Drive DIM high to enable both LED current sinks

and drive DIM low to disable both LED current sinks.

BOOST

CONVERTER

OUTPUT

40mA TO 300mA

MAX16838

OUT1

OUT2

Figure 2. Configuration for Higher LED String Current

The duty cycle of the PWM signal applied to DIM also

controls the DC-DC converterâs output voltage. If the

turn-on duration of the PWM signal is less than five

oscillator clock cycles, then the boost converter regu-

lates its output based on feedback from the OV input.

During this mode, the converter output voltage is regu-

lated to 95% of the OV threshold voltage. If the turn-on

duration of the PWM signal is greater than or equal to

six oscillator clock cycles, then the converter regulates

its output such that the minimum voltage at OUT_ is 1V.

Fault Protections

The MAX16838 fault protections include cycle-by-cycle

current limiting, DC-DC converter output overvoltage

protection, open-LED detection, short-LED detection,

and overtemperature detection. An open-drain LED fault

flag output (FLT) goes low when an open-LED/short-LED

or overtemperature condition is detected.

Open-LED Management and Overvoltage Protection

The MAX16838 monitors the drains of the current sinks

(OUT_) to detect any open string. If the voltage at

any output falls below 300mV and the OV threshold is

triggered (i.e., even with OUT_ at the OV voltage the

string is not able to regulate above 300mV), then the

MAX16838 interprets that string to be open, asserts FLT,

and disconnects that string from the operation loop. The

MAX16838 features an adjustable overvoltage threshold

input (OV). Connect a resistor-divider from the switching

converter output to OV and SGND to set the overvoltage

threshold level. Use the following formula to program the

overvoltage threshold:

VOV

1.23V

ï£«

ï£¬1+

ï£

R2OV

R1OV

ï£¶

ï£·

ï£¸

Short-LED Detection

The MAX16838 features a two-level short-LED detection

circuitry. If a level 1 short is detected on any one of the

strings, FLT is asserted. A level 1 short is detected if

the difference between the total forward LED voltages

of the two strings exceeds 4.2V (typ). If a level 2 short

is detected on any one of the strings, the particular LED

string with the short is turned off after 6Fs and FLT is

asserted. A level 2 short is detected if the difference

between the total forward LED voltages of the two strings

exceeds 7.8V (typ). The strings are reevaluated on each

DIM rising edge and FLT is deasserted if the short is

removed.

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