clamped to a level set with the OV feedback input.

When this elevated output voltage is applied to the

undamaged strings, excessive voltage drop develops

across the FB_ pins. If the resulting HVC signal

latch is triggered to protect the circuit.

LED-Short and String Mismatch Protection

Normally, white LEDs have variations in forward-voltage

drop of 3.1V to 3.6V. The MAX8790 can tolerate slight

mismatches between LED strings. When the sum of the

LED forward voltages creates a mismatch in the strings

65ms, the fault latch is triggered in much the same way

as the circuit responds to open string faults. Similar pro-

tection is activated when an LED is shorted.

The larger the number of series-connected LEDs (N),

the smaller the tolerable mismatch between LEDs:

For N = 8, the average error per LED = 644mV.

For N = 10, the average error per LED = 510mV.

The larger the total mismatch, the larger the voltage

drop required across each current source to correct for

the error, and therefore the larger the dissipation within

the MAX8790.

Dimming Control

The MAX8790 features both analog and digital dim-

ming control. Analog dimming can provide potentially

higher converter efficiency because of low voltage drop

across each WLED when the current is low. Digital dim-

ming (DPWM) provides less WLED color distortion

since the WLED current is held at full scale when the

WLED is on.

The MAX8790’s dimming control circuit consists of a

PLL, a digital comparator, and a DAC. The controller

provides 100:1 dimming range through either analog or

digital control methods. Both methods translate the

duty cycle of the BRT input into a control signal for the

LED current sources. In analog dimming mode, the cur-

rent-source outputs are DC and the BRT duty cycle

tally modulated to reduce the average LED current

down to 1% of full scale. The PLL detects the BRT fre-

quency and phase, and adjusts the current-source

amplitude and duty cycle synchronously (see Figure 4).

Average Error Per LED

V

N

=

5 150

.

Error

V

N

∑

< 5 150

.

Error V

V V

N

CC

SAT

∑

<+

−

06

.

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

14

______________________________________________________________________________________

BRT

0A

ANALOG DIMMING MODE

D = 50%

D = 30%

D = 12.5%

D = 6.25%

D =

Figure 4. LED Current Control Using Analog Dimming Mode