MAX1579ETG+ Maxim Integrated Products, MAX1579ETG+ Datasheet - Page 10

• Manufacturer Part Number: MAX1579ETG+
• Description: IC PS BIAS/WHITE LED TFT 24-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX1579ETG+

• Applications: LCD Display
• Current - Supply: 3mA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TQFN Exposed Pad
• Number Of Segments: 8
• Operating Supply Voltage: 2.7 V to 5.5 V
• Maximum Power Dissipation: 1667 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The LED boost converter utilizes a soft-start function to
eliminate inrush current during startup. Once the boost
converter is enabled, LX begins switching at the mini-
mum duty cycle until C
this occurs, the duty cycle increases to further charge
the output until V
start time is adjustable using the capacitor from COMP
to GND. Calculate the required COMP capacitor as:
where t
The output of the LED boost converter is protected from
overvoltage conditions by internal overvoltage circuitry.
If V
Once V
soft-start is re-initiated.
The MAX1579 limits the maximum LED current depend-
ing on the die temperature. V
to +42°C. Once the temperature reaches +42°C, the
maximum V
40mV threshold is reached at +100°C. Due to the pack-
age’s exposed paddle, the die temperature is always
very close to the PC board temperature.
The temperature derating function allows the LED cur-
rent to be safely set higher at normal operating temper-
atures, thereby allowing either a brighter display or
fewer LEDs to be used for normal display brightness.
The MAX1578/MAX1579 include a low-quiescent-current
shutdown mode. To enter shutdown, drive CTRL below
0.1V for longer than 10.5ms and drive ONBIAS low. The
quiescent current is reduced to less than 1µA when the
boost converter and charge pumps are disabled.
To disable the LED boost converter, drive CTRL below
0.1V for longer than 10.5ms. During shutdown, the
internal boost switch from LX to PGND is high imped-
ance; however, a DC path exists from IN to OUT
through the external inductor and Schottky diode. Drive
CTRL with an analog voltage between 0.24V and 1.65V
or a 200Hz to 200kHz digital PWM dimming signal for
normal operation. The quiescent current is reduced to
870µA when the boost converter is shut down and the
charge pumps are enabled.
Complete Bias and White LED Power Supplies
for Small TFT Displays
10
Ambient Temperature Derating Function
OUT
______________________________________________________________________________________
SS
OUT
exceeds 34V, the LX switching terminates.
is the desired soft-start time in seconds.
CS
falls below 32V, LX switches normally and
C
declines by 6mV/°C until the minimum
COMP
CS
reaches 20% of V
COMP
=
12
Overvoltage Protection
μ
1 25
A t
.
is charged to 1.25V. Once
CS
×
V
SS
is limited to 340mV up
CTRL
(MAX1579)
Shutdown
Soft-Start
. The soft-
Drive ONBIAS low to shut down the internal POS and
NEG charge pumps and disable the MAIN LDO output.
On-chip pulldown resistors discharge these outputs
during shutdown. Drive ONBIAS high for normal opera-
tion. V
quiescent current is reduced to 430µA when the charge
pumps are shut down and the boost converter is
enabled.
Set the maximum LED current using a resistor from CS
to GND. Calculate the resistance as follows:
where I
LEDs in Amps when V
V
regulates to 20% of V
through the LEDs and, therefore, the brightness. Drive
CTRL using a DAC with an output voltage between
0.24V and 1.65V to control the brightness of the LEDs.
Increasing V
brightness increase. Hold CTRL below 100mV for
longer than 10.5ms to shut down the boost converter.
Another useful technique for LED dimming control is the
application of a logic-level PWM signal applied directly
to CTRL. LED current may be varied from zero to full
scale. The frequency range of the PWM signal is from
200Hz to 200kHz, while 0% duty cycle corresponds to
zero current and 100% duty cycle corresponds to full
current. The error amplifier and compensation capaci-
tor form a lowpass filter so PWM dimming results in DC
current to the LEDs without the need for any additional
RC filters. See the Typical Operating Characteristics.
For LED drivers, input and output ripple may be impor-
tant. Input ripple depends on the source supply’s output
impedance. Adding a lowpass filter to the input further
reduces input ripple. Alternately, increasing C
cuts input ripple in half. Likewise, an output filter or high-
er output capacitance value reduces output ripple.
LED Dimming Using Direct PWM into CTRL
CTRL
DD
controls the LED drive current. The voltage at CS
LED
is connected to IN when ONBIAS is low. The
LED Dimming Control Using a DAC
is the desired maximum current through the
CTRL
R
R
Applications Information
CS
CS
=
=
beyond 1.65V results in no further
330
340
CTRL
I
I
LED
LED
mV
mV
CTRL
Adjusting LED Current
is 1.65V.
for the MAX
for the MAX
Input/Output Ripple
to control the current
1578
1579
IN
to 10µF