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MAX8790
(Rev.:2006)

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

Maxim Integrated 

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 corre-

sponding string current source is disabled. This feed-

back 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

current 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-fre-

quency (1MHz) operation optimizes the regulator for the

smallest component size, at the expense of efficiency

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 rea-

son, 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 high-

voltage 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 com-

parator turns off N1. The step-up converter switch is

reenabled after the output voltage drops below the pro-

tection threshold.

LED Current Sources

Maintaining uniform LED brightness and dimming

capability 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 pow-

ered 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

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

is grounded, the controller disables the corresponding

current regulator. The current regulator cannot be dis-

abled 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 converter’s output voltage.

Current-Source Fault Protection

The LED current sources are protected against string

open, short, and gross mismatch faults, using overvolt-

age 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:

t TIMEOUT_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:

t TIMEOUT_ANALOG = 8.125ms/DBRT

The fault latch can be cleared by cycling the power or

toggling 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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