MAX8790ETP+ Maxim Integrated Products, MAX8790ETP+ Datasheet - Page 20

• Manufacturer Part Number: MAX8790ETP+
• Description: IC LED DRVR WHITE BCKLGT 20-TQFN
• Manufacturer: Maxim Integrated Products
• Type: Backlight, White LEDr

Specifications of MAX8790ETP+

• 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: 25mA
• Internal Switch(s): Yes
• Low Level Output Current: 1000000 uA (Typ)
• High Level Output Current: 25.75 mA
• Operating Supply Voltage: 4.5 V to 26 V
• Maximum Supply Current: 2 mA
• Maximum Power Dissipation: 1349 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Efficiency: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The input capacitor (C
from the input supply and reduces noise injection into
the IC. A 10µF ceramic capacitor is used in the typical
operating circuit (Figure 1) because of the high source
impedance seen in typical lab setups. Actual applica-
tions usually have much lower source impe-dance since
the step-up regulator often runs directly from the output
of another regulated supply. In some applications, C
can be reduced below the values used in the typical
operating circuit. Ensure a low noise supply at IN by
using adequate C
tion can be tolerated on C
using an RC lowpass filter.
Select C
thermal rise is less than approximately 10°C:
The series/parallel configuration of the LED load and the
full-scale bias current have a significant effect on regu-
lator performance. LED characteristics vary significantly
from manufacturer to manufacturer. Consult the respec-
tive LED data sheets to determine the range of output
voltages for a given brightness and LED current. In ge-
neral, brightness increases as a function of bias current.
This suggests that the number of LEDs could be
decreased if higher bias current is chosen; however,
high current increases LED temperature and reduces
operating life. Improvements in LED technology are
resulting in devices with lower forward voltage while
increasing the bias current and light output.
LED manufacturers specify LED color at a given LED
current. With lower LED current, the color of the emitted
light tends to shift toward the blue range of the spec-
trum. A blue bias is often acceptable for business appli-
cations but not for high-image-quality applications such
as DVD players. Direct DPWM dimming is a viable solu-
tion for reducing power dissipation while maintaining
LED color integrity. Careful attention should be paid to
switching noise to avoid other display quality problems.
Using fewer LEDs in a string improves step-up
converter efficiency, and lowers breakdown voltage
requirements of the external MOSFET and diode. The
minimum number of LEDs in series should always be
greater than the maximum input voltage. If the diode
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
20
______________________________________________________________________________________
IN
’s RMS ripple current rating to ensure that its
IN
. Alternatively, greater voltage varia-
I
RMS
IN
Input Capacitor Selection
) filters the current peaks drawn
LED Selection and Bias
IN
=
2
if IN is decoupled from C
dI
×
L
3
IN
IN
voltage drop is lower than the maximum input voltage,
the voltage drop across the current-sense inputs (FB_)
increases and causes excess heating in the IC.
Between 8 and 12 LEDs in series is ideal for input volt-
ages up to 20V.
The MAX8790 has accurate (±1.5%) matching for each
current source. However, the forward voltage of each
white LED can vary up to ±5% from part to part. The
accumulated voltage difference in each string equates
to additional power loss within the IC. For the best effi-
ciency, the voltage difference between strings should
be minimized. The difference between lowest voltage
string and highest voltage string should be less than
4.5V. Otherwise, the internal LED short-circuit protection
shuts the part off.
Analog dimming mode allows lower peak LED current
and results in higher converter efficiency and lower
noise compared to direct DPWM mode. Unfortunately,
the LED color spectrum can shift as a function of DC
current so DPWM mode is often used to achieve more
consistent display characteristics. (See the LED manu-
facturer’s data sheet to determine the extent of the
color shift.) When the MAX8790 is configured with an
FSET resistor and CPLL capacitor, the ENA signal can
toggle between modes on the fly. Care should be exer-
cised when switching between modes to prevent the
current from becoming unstable during the PLL lock-in
time. To avoid such problems, force the controller into
DPWM mode between transitions.
Some LCD panels include a capacitor in parallel with
LED string to improve ESD immunity. Because of the
10µA pullup current source in each FB_ input for string
detection, the MAX8790 can start up with less than
470pF capacitance on each FB_ pin. If the string
capacitance C
pullup resistors to V
startup problems (see Figure 6). A delay of 3 x 1MΩ x
C
enabling the MAX8790 to ensure the FB_ voltage
exceeds the 3V internal threshold. A similar delay
should be added after the part is shut down to ensure
proper restart.
LED
Choosing the Appropriate Dimming Mode
should be added after V
Applications Information
LED
is greater than 470pF, a bank of
IN
should be added to prevent
LCD Panel Capacitance
LED V
IN
was settled before
FB_
Variation