MAX16833_11 Datasheet, PDF(13/22 Page) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

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MAX16833_11 Datasheet, PDF (13/22 Pages) 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

SYNC

680pF

22I

1000pF

SD103AWS

RT PIN

RRT

24.9I

GND

Figure 1. SYNC Circuit

Figure 1 shows the frequency-synchronization circuit

suitable for applications where a 5V amplitude pulse with

20% to 80% duty cycle is available as the synchronization

source. This circuit can be used for SYNC frequencies

in the 100kHz to 1MHz range. C1 and R2 act as a dif-

ferentiator that reduces the input pulse width to suit the

ICsâ RT/SYNC input. D2 bypasses the negative current

through C1 at the falling edge of the SYNC source to limit

the minimum voltage at the RT/SYNC pin. The differentia-

tor output is AC-coupled to the RT/SYNC pin through C2.

The output impedance of the SYNC source should be

low enough to drive the current through R2 on the rising

edge. The rise/fall times of the SYNC source should be

less than 50ns to avoid excessive voltage drop across C1

during the rise time. The amplitude of the SYNC source

can be between 4V and 5V. If the SYNC source amplitude

is 5V and the rise time is less than 20ns, then the maxi-

mum peak voltage at RT/SYNC pin can get close to 6V.

Under such conditions, it is desirable to use a resistor in

series with C1 to reduce the maximum voltage at the RT/

SYNC pin. For proper synchronization, the peak SYNC

pulse voltage at RT/SYNC pin should exceed 3.8V.

Frequency Dithering (LFRAMP/MAX16833/

MAX16833C/MAX16833E)

The MAX16833/MAX16833C/MAX16833E feature a

low-frequency ramp output. Connect a capacitor from

LFRAMP to ground to program the ramp frequency.

Connect to SGND if not used. A resistor can be con-

nected between LFRAMP and RT/SYNC to dither the

PWM switching frequency to achieve spread spectrum.

A lower value resistor provides a larger amount of fre-

quency dithering. The LFRAMP voltage is a triangular

waveform between 1V (typ) and 2V (typ). The ramp fre-

quency is given by:

Voltage-Reference Output

(REF/MAX16833B/MAX16833D)

The MAX16833B/MAX16833D have a 2% accurate 1.64V

reference voltage on the REF output. Connect a 1FF

ceramic capacitor from REF to SGND to provide a stable

reference voltage. This reference can supply up to 80ÂµA.

This output can drive a resistive divider to the ICTRL

input for analog dimming. The resistance from REF to

ground should be greater than 20.5kI.

Switching MOSFET

Current-Sense Input (CS)

CS is part of the current-mode control loop. The switch-

ing control uses the voltage on CS, set by RCS (R4 in the

Typical Operating Circuits) and RSLOPE (R1 in the Typical

Operating Circuits), to terminate the on pulse width of the

switching cycle, thus achieving peak current-mode con-

trol. Internal leading-edge blanking of 50ns is provided

to prevent premature turn-off of the switching MOSFET

in each switching cycle. Resistor RCS is connected

between the source of the n-channel switching MOSFET

and PGND.

During switching, a current ramp with a slope of 50FA

x fSW is sourced from the CS input. This current ramp,

along with resistor RSLOPE, programs the amount of slope

compensation.

Overvoltage-Protection Input (OVP)

OVP sets the overvoltage-threshold limit across the

LEDs. Use a resistive divider between ISENSE+ to OVP

and SGND to set the overvoltage-threshold limit. An

internal overvoltage-protection comparator senses the

differential voltage across OVP and SGND. If the dif-

ferential voltage is greater than 1.23V, NDRV goes low,

DIMOUT goes high, and FLT asserts. When the differen-

tial voltage drops by 70mV, NDRV is enabled, DIMOUT

goes low, and FLT deasserts.

Fault Indicator (FLT)

The ICs feature an active-low, open-drain fault indicator

(FLT). FLT goes low when one of the following conditions

occur:

U Overvoltage across the LED string

U Short-circuit condition across the LED string

U Overtemperature condition

FLT goes high when the fault condition ends.

fLFRAMP(Hz)

= 50FA

C LFRAMP (F)

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