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MAX8790

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

Maxim Integrated 

MAX8790

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Startup

At startup, the MAX8790 checks each FB_ pin to deter-

mine if the respective current string is enabled. Each FB_

pin is internally pulled up with a 10μA current source. If

an FB_ pin is connected to GND, the corresponding string

current source is disabled. This feedback scan takes

approximately 264μs, after which the step-up converter

begins switching.

Shutdown

When the SHDN pin is less than 0.8V, the MAX8790

shuts down the internal LDO, the reference, current

sources, and all control circuitry. The resulting supply cur-

rent is less than 10μA. While the n-channel MOSFET is

turned off, the step-up regulator’s output is connected to

IN through the external inductor and rectifier diode.

Frequency Selection

A tri-level OSC input sets the internal oscillator frequency

for the step-up converter, as shown in Table 1. High-

frequency (1MHz) operation optimizes the regulator for

the smallest component size, at the expense of effi-

ciency due to increased switching losses. Low-frequency

(500kHz) operation offers the best overall efficiency, but

requires larger components and PCB area.

Table 1. Frequency Selection

OSC

GND

Open

VCC

SWITCHING FREQUENCY (kHz)

500

750

1000

Overvoltage Protection

To protect the step-up converter when the load is open,

or the output voltage becomes excessive for any reason,

the MAX8790 features a dedicated overvoltage feedback

input (OV). The OV pin is connected to the center tap of a

resistive voltage-divider from the highvoltage output (see

Figure 1). When the MAX8790 is powered up, if none of

the LED strings on FB1–FB6 are connected to the step-

up converter output, the step-up converter regulates the

output voltage to VOUT = 1.23V(1 + R1 / R2). When VOV

exceeds 1.23V, a comparator turns off N1. The step-up

converter switch is reenabled after the output voltage

drops below the protection threshold.

LED Current Sources

Maintaining uniform LED brightness and dimming capa-

bility are critical for LCD backlight applications. The

MAX8790 is equipped with a bank of six matched current

sources. These specialized current sources are accurate

to within ±1.5% and can be switched on and off within

10μs, enabling PWM frequencies of up to 2kHz. All LED

full-scale currents are identical and are set through the

ISET pin (15mA < ILED < 25mA).

The minimum voltage drop across each current source is

approximately 450mV at 20mA. The low voltage drop helps

reduce dissipation while maintaining sufficient compliance

to control the LED current within the required tolerances.

The LED current sources can be disabled by grounding

the respective FB_ pin at startup. When the IC is powered

up, the controller scans settings for all FB_ pins. If an

FB_ pin is not grounded, an internal circuit pulls this pin

high, and the controller enables the corresponding cur-

rent source to regulate the string current. If the FB_ pin is

grounded, the controller disables the corresponding cur-

rent regulator. The current regulator cannot be disabled by

grounding any of the FB_ pins after the IC is powered up.

All FB_ pins in use are measured and the highest signal

(HVC) and the lowest signal (LVC) are extracted for two

feedback loops. HVC is used to identify excessive dis-

sipation across the current-source inputs. When HVC is

greater than VCC + 0.6V (typ) for greater than 65ms (see

the Current-Source Fault Protection section), a fault latch

is set and the MAX8790 is shut down. The LDO output is

not affected by the fault latch. LVC is fed into the step-up

converter’s error amplifier to regulate the step-up con-

verter’s output voltage.

Current-Source Fault Protection

The LED current sources are protected against string

open, short, and gross mismatch faults, using overvoltage

detection circuitry on each FB_ pin. If any of these three

fault conditions persists for a preset duration, the MAX8790

is latched off. The duration of the fault time depends on the

dimming mode and the duty cycle of the BRT input (DBRT).

In the DPWM mode, the timeout interval is:

tTIMEOUT_DPWM = 65ms/DBRT

In analog dimming mode, the fault time is fixed at 65ms

for DBRT greater than 12.5%. When DBRT is less than

12.5%, the timeout interval is:

tTIMEOUT_ANALOG = 8.125ms/DBRT

The fault latch can be cleared by cycling the power or tog-

gling the shutdown pin SHDN.

Open-Current Source Protection

The MAX8790 step-up converter output voltage is regu-

lated according to the minimum value of the enable FB_

voltages. If an individual LED string is open, the respec-

tive FB_ is pulled down to near ground. In this situation,

the step-up converter output voltage increases but is

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