MAX16833_11 Datasheet, PDF(14/22 Page) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

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High-Voltage HB LED Drivers with

Integrated High-Side Current Sense

Thermal Protection

The ICs feature thermal protection. When the junction

temperature exceeds +160NC, the ICs turn off the external

power MOSFETs by pulling the NDRV low and DIMOUT

high. External MOSFETs are enabled again after the junc-

tion temperature has cooled by 10Â°C. This results in a

cycled output during continuous thermal-overload condi-

tions. Thermal protection protects the ICs in the event of

fault conditions.

Short-Circuit Protection

Boost Configuration

In the boost configuration, if the LED string is shorted

it causes the (ISENSE+ to ISENSE-) voltage to exceed

300mV. If this condition occurs for R 1Fs, the ICs activates

the hiccup timer for 8192 clock cycles during which:

U NDRV goes low and DIMOUT goes high.

U The error amplifier is disconnected from COMP.

U FLT is pulled to SGND.

After the hiccup time has elapsed, the ICs retry. During

this retry period, FLT is latched and is reset only if there is

no short detected after 20Fs of retrying.

Buck-Boost Configuration

In the case of the buck-boost configuration, once an

LED string short occurs the behavior is different. The ICs

maintain the programmed current across the short. In this

case, the short is detected when the voltage between

ISENSE+ and IN falls below 1.5V. For all MAX16833 ver-

sions except MAX16833E, a buck-boost short fault starts

an up counter and FLT is asserted only after the counter

has reached 8192 clock cycles consecutively. If for

any reason (VISENSE+ - VIN > 1.5V), the counter starts

down counting, resulting in FLT being deasserted only

after 8192 consecutive clock cycles of (VISENSE+ - VIN

> 1.5V) condition. For MAX16833E, there is no counter

for FLT assertion and deassertion, so FLT is asserted

immediately when the voltage between ISENSE+ and IN

falls below 1.5V, and is deasserted immediately when

this condition terminates.

Exposed Pad

The ICsâ package features an exposed thermal pad on

its underside that should be used as a heatsink. This

pad lowers the packageâs thermal resistance by provid-

ing a direct heat-conduction path from the die to the

PCB. Connect the exposed pad and GND to the system

ground using a large pad or ground plane, or multiple

vias to the ground plane layer.

Applications Information

Setting the Overvoltage Threshold

The overvoltage threshold is set by resistors R5 and R11

(see the Typical Operating Circuits). The overvoltage

circuit in the ICs is activated when the voltage on OVP

with respect to GND exceeds 1.23V. Use the following

equation to set the desired overvoltage threshold:

VOV = 1.23V (R5 + R11)/R11

Programming the LED Current

Normal sensing of the LED current should be done on

the high side where the LED current-sense resistor is

connected to the boost output. The other side of the LED

current-sense resistor goes to the source of the p-channel

dimming MOSFET if PWM dimming is desired. The LED

current is programmed using R7. When VICTRL > 1.23V,

the internal reference regulates the voltage across R7 to

200mV:

ILED

200mV

The LED current can also be programmed using the

voltage on ICTRL when VICTRL < 1.2V (analog dimming).

The voltage on ICTRL can be set using a resistive divider

from the REF output in the case of the MAX16833B/

MAX16833D. The current is given by:

ILED

VICTRL

R7 Ã 6.15

where:

VICTRL

VREF Ã R8

(R8 + R9)

where VREF is 1.64V and resistors R8 and R9 are in

ohms. At higher LED currents there can be noticeable

ripple on the voltage across R7. High-ripple voltages can

cause a noticeable difference between the programmed

value of the LED current and the measured value of the

LED current. To minimize this error, the ripple voltage

across R7 should be less than 40mV.

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