MAX8790AETP+T Maxim Integrated Products, MAX8790AETP+T Datasheet - Page 14

• Manufacturer Part Number: MAX8790AETP+T
• Description: IC LED DRVR WHITE BCKLGT 20-TQFN
• Manufacturer: Maxim Integrated Products
• Type: Backlight, White LEDr

Specifications of MAX8790AETP+T

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: No
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 1MHz
• Voltage - Supply: 4.5 V ~ 5.5 V, 5.5 V ~ 26 V
• Voltage - Output: 5V
• Mounting Type: Surface Mount
• Package / Case: 20-TQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 27mA
• Internal Switch(s): Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Efficiency: -
• Lead Free Status / Rohs Status: Details

The MAX8790A step-up converter output voltage is reg-
ulated according to the minimum value of the enable
FB_ voltages. If an individual LED string is open, the
respective FB_ is pulled down to near ground. In this sit-
uation, 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 devel-
ops across the FB_ pins. If the resulting HVC signal
exceeds V
latch is triggered to protect the circuit.
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 V
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:
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Figure 4. LED Current Control Using Analog Dimming Mode
14
D =
BRT
I
I
0A
LEDMAX
LED
______________________________________________________________________________________
t
t
BRT
ON
LED-Short and String Mismatch Protection
CC
D = 50%
t
ON
t
BRT
V
SAT
+ 0.6V for greater than 65ms, the fault
∑
N
Error V
≈ 450mV and V
∑
N
Open-Current Source Protection
<
Error
CC
< 5 150
+
CC
0 6 .
.
+ 0.6V for greater than
V V
CC
V
−
SAT
= 5V
D = 30%
ANALOG DIMMING MODE
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.
The MAX8790A 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 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 cur-
rent-source outputs are DC and the BRT duty cycle
(12.5% < D
the currents. For D
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).
D = 12.5%
Average Error Per LED
BRT
< 100%) modulates the amplitude of
BRT
< 12.5%, the LED current is digi-
Dimming Control
D = 6.25%
=
5 150
.
N
V