MAX16833 Datasheet, PDF(12/22 Page) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

◁

High-Voltage HB LED Drivers with

Integrated High-Side Current Sense

Analog Dimming (ICTRL)

The MAX16833/MAX16833B offer an analog dimming

control input (ICTRL). The voltage at ICTRL sets the LED

current level when VICTRL < 1.2V. The LED current can be

linearly adjusted from zero with the voltage on ICTRL. For

VICTRL > 1.4V, an internal reference sets the LED current.

The maximum withstand voltage of this input is 5.5V.

Low-Side Linear Regulator (VCC)

The MAX16833/MAX16833B feature a 7V low-side linear

regulator (VCC). VCC powers up the switching MOSFET

driver with sourcing capability of up to 50mA. Use a 1FF

(min) low-ESR ceramic capacitor from VCC to PGND for

stable operation. The VCC regulator goes below 7V if the

input voltage falls below 7V. The dropout voltage for this

regulator at 50mA is 0.2V. This means that for an input

voltage of 5V, the VCC voltage is 4.8V. The short-circuit

current on the VCC regulator is 100mA (typ). Connect

VCC to IN if VIN is always less than 7V.

LED Current-Sense Inputs (ISENSEÂ±)

The differential voltage from ISENSE+ to ISENSE- is fed

to an internal current-sense amplifier. This amplified sig-

nal is then connected to the negative input of the trans-

conductance error amplifier. The voltage-gain factor of

this amplifier is 6.15.

The offset voltage for this amplifier is P 1mV.

Internal Transconductance Error Amplifier

The MAX16833/MAX16833B have a built-in transconduc-

tance amplifier used to amplify the error signal inside the

feedback loop. When the dimming signal is low, COMP

is disconnected from the output of the error amplifier and

DIMOUT goes high. When the dimming signal is high,

the output of the error amplifier is connected to COMP

and DIMOUT goes low. This enables the compensation

capacitor to hold the charge when the dimming signal

has turned off the internal switching MOSFET gate drive.

To maintain the charge on the compensation capaci-

tor CCOMP (C4 in the Typical Operating Circuits), the

capacitor should be a low-leakage ceramic type. When

the internal dimming signal is enabled, the voltage on the

compensation capacitor forces the converter into steady

state almost instantaneously.

Internal Oscillator (RT/SYNC)

The internal oscillators of the MAX16833/MAX16833B

are programmable from 100kHz to 1MHz using a single

resistor at RT/SYNC. Use the following formula to calcu-

late the switching frequency:

7350 (kâ¦)

fOSC (kHz) = RRT (kâ¦)

where RRT is the resistor from RT/SYNC to SGND.

Synchronize the oscillator with an external clock by

AC-coupling the external clock to the RT/SYNC input.

For fOSC between 200kHz and 1MHz, the capacitor used

for the AC-coupling should satisfy the following relation:

C SYNC

â¤

9.8624 Ã 10-6

RRT

0.144

Ã 10 -9

farads

where RRT is in kÏ. For fOSC below 200GHz, CSYNC â¤

268nF.

The pulse width for the synchronization pulse should

satisfy the following relations:

tPW < 0.5

t CLK VS

and

t PW

t CLK

ï£«

ï£¬1-

ï£

1.05 Ã t CLK

t OSC

ï£¶

ï£·

ï£¸

3.4V

ï£«

ï£¬0.8

ï£

t PW

t CLK

ï£¶

ï£·

ï£¸

where tPW is the synchronization source pulse width,

tCLK is the synchronization clock time period, tOSC is

the free-running oscillator time period, and VS is the syn-

chronization pulse-voltage level.

Ensure that the external clock signal frequency is at least

1.1 x fOSC, where fOSC is the oscillator frequency set

by RRT. A typical pulse width of 200ns can be used for

proper synchronization of a frequency up to 250kHz. A

rising external clock edge (sync) is interpreted as a syn-

chronization input. If the sync signal is lost, the internal

oscillator takes control of the switching rate returning the

switching frequency to that set by RRT. This maintains

output regulation even with intermittent sync signals.

▷