MAX16838_11 Datasheet, PDF(12/21 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

Detailed Description

The MAX16838 high-efficiency, HB LED driver inte-

grates all the necessary features to implement a high-

performance backlight driver to power LEDs in small-to-

medium-sized displays for automotive as well as general

applications. The device provides load-dump voltage

protection up to 40V in automotive applications. The

MAX16838 incorporates a DC-DC controller with peak

current-mode control to implement a boost, coupled

inductor boost-buck, or SEPIC-type switched-mode

power supply and a 2-channel LED driver with 20mA

to 150mA constant-current-sink capability per channel.

The MAX16838 can be combined with the MAX15054 to

achieve boost-buck topology without a coupled inductor

(see Figure 5).

The MAX16838 features a constant-frequency peak

current-mode control with internal slope compensation

to control the duty cycle of the PWM controller. The

DC-DC converter generates the required supply volt-

age for the LED strings from a wide input supply range.

Connect LED strings from the DC-DC converter output

to the 2-channel constant current sinks that control the

current through the LED strings. A single resistor

connected from ISET to ground sets the forward current

through both LED strings.

The MAX16838 features adaptive LED voltage control

that adjusts the converter output voltage depending

on the forward voltage of the LED strings. This feature

minimizes the voltage drops across the constant-current

sinks and reduces power dissipation in the device. The

MAX16838 provides a very wide PWM dimming range

where a dimming pulse as narrow as 1Fs is possible at

a 200Hz dimming frequency.

A logic input (EN) shuts down the device when pulled low.

The device includes an internal 5V LDO to power up the

internal circuitry and drive the internal switching MOSFET.

The MAX16838 includes output overvoltage protec-

tion that limits the converter output voltage to the pro-

grammed OV threshold in the event of an open-LED

condition. The device also features an overtemperature

protection that shuts down the controller if the die tem-

perature exceeds +165Â°C. In addition, the MAX16838

has a shorted LED string detection and an open-drain

FLT signal to indicate open LED, shorted LED, and over-

temperature conditions.

Current-Mode DC-DC Controller

The MAX16838 uses current-mode control to provide

the required supply voltage for the LED strings. The

internal MOSFET is turned on at the beginning of every

switching cycle. The inductor current ramps up linearly

until it is turned off at the peak current level set by the

feedback loop. The peak inductor current is sensedfrom

the voltage across the current-sense resistor (RCS)

connected from the source of the internal MOSFET to

PGND. A PWM comparator compares the current-sense

voltage plus the internal slope compensation signal with

the output of the transconductance error amplifier. The

controller turns off the internal MOSFET when the voltage

at CS exceeds the error amplifierâs output voltage. This

process repeats every switching cycle to achieve peak

current-mode control.

Error Amplifier

The internal error amplifier compares an internal feed-

back (FB) signal with an internal reference voltage

(VREF) and regulates its output to adjust the inductor

current. An internal minimum string detector measures

the minimum LED string cathode voltage with respect

to SGND. During normal operation, this minimum VOUT_

voltage is regulated to 1V through feedback. The result-

ing DC-DC converter output voltage is 1V above the

maximum required total LED voltage.

The converter stops switching when LED strings are

turned off during PWM dimming. The error amplifier is

disconnected from the COMP output to retain the com-

pensation capacitor charge. This allows the converter to

settle to a steady-state level immediately when the LED

strings are turned on again. This unique feature provides

fast dimming response without having to use large out-

put capacitors. If the PWM dimming on-pulse is less than

five switching cycles, the feedback controls the voltage

on OV such that the converter output voltage is regu-

lated at 95% of the OV threshold. This mode ensures

that narrow PWM dimming pulses are not affected by

the response time of the converter. During this mode,

the error amplifier remains continuously connected to

the COMP output.

Adaptive LED Voltage Control

The MAX16838 reduces power dissipation using an

adaptive LED voltage control scheme. The adaptive LED

voltage control regulates the DC-DC converter output

based on the operating voltage of the LED strings.

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