MAX8790A データシートの表示（PDF） - Maxim Integrated

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MAX8790A

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

Maxim Integrated 

MAX8790A

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Open-Current Source Protection

The MAX8790A 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

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 exceeds VCC + 0.6V for

greater than 65ms, the fault 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 MAX8790A can tolerate slight

mismatches between LED strings. When the sum of the

LED forward voltages creates a mismatch in the strings

so the HVC signal exceeds VCC + 0.6V for greater than

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

the circuit responds to open string faults. Similar protec-

tion is activated when an LED is shorted.

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

smaller the tolerable mismatch between LEDs:

∑ Error < VCC + 0.6V − VSAT

N

VSAT ≈ 450mV and VCC = 5V

∑ Error < 5.150V

N

Average Error Per LED = 5.150V

N

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

MAX8790A.

Dimming Control

The MAX8790A features both analog and digital dimming

control. Analog dimming can provide potentially higher

converter efficiency because of low voltage drop across

each WLED when the current is low. Digital dimming

(DPWM) provides less WLED color distortion since the

WLED current is held at full scale when the WLED is on.

The MAX8790A’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 current-

source outputs are DC and the BRT duty cycle (12.5% <

DBRT < 100%) modulates the amplitude of the currents.

For DBRT < 12.5%, the LED current is digitally modulated

to reduce the average LED current down to 1% of full

scale. The PLL detects the BRT frequency and phase,

and adjusts the current-source amplitude and duty cycle

synchronously (see Figure 4).

D = tON

tBRT

BRT

D = 50%

tON

tBRT

ILEDMAX

ANALOG DIMMING MODE

D = 30%

D = 12.5%

D = 6.25%

ILED

0A

Figure 4. LED Current Control Using Analog Dimming Mode

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