LM3528TMX National Semiconductor, LM3528TMX Datasheet - Page 26

• Manufacturer Part Number: LM3528TMX
• Description: LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and Integrated OLED Power Supply in a 1.2mm - 1.6mm uSMD Package; Package: MICRO SMD; No of Pins: 12; Qty per Container: 3000; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
Application Information
LED CURRENT SETTING/MAXIMUM LED CURRENT
Connect a resistor (R
maximum LED current (I
R
where SET provides the constant 1.244V output.
OUTPUT VOLTAGE SETTING (OLED MODE)
Connect Feedback resistors from the converters output to
SUB/FB to GND to set the output voltage in OLED mode (see
R1 and R2 in the Typical Application Circuit (OLED Panel
Power Supply). First select R2 < 100kΩ then calculate R1
such that:
In OLED mode the MAIN current sink continues to regulate
the current through MAIN, however, V
lated. To avoid dropout and ensure proper current regulation
the application must ensure that V
OUTPUT CAPACITOR SELECTION
The LM3528’s output capacitor supplies the LED current dur-
ing the boost converters on time. When the switch turns off
the inductor energy is discharged through the diode supplying
power to the LED’s and restoring charge to the output capac-
itor. This causes a sag in the output voltage during the on time
and a rise in the output voltage during the off time. The output
capacitor is therefore chosen to limit the output ripple to an
acceptable level depending on LED or OLED panel current
requirements and input/output voltage differentials. For prop-
er operation ceramic output capacitors ranging from 1µF to
2.2µF are required.
As with the input capacitor, the output voltage ripple is com-
posed of two parts, the ripple due to capacitor discharge (delta
V
continuous conduction mode, the ripple components are
found by:
SET
Q
) and the ripple due to the capacitors ESR (delta V
to I
LED_MAX
relationship is:
SET
LED_MAX
) from SET to GND to program the
) into MAIN or SUB/FB. The
MAIN
MAIN
> 0.3V.
is no longer regu-
ESR
). For
26
INPUT CAPACITOR SELECTION
Choosing the correct size and type of input capacitor helps
minimize the input voltage ripple caused by the switching of
the LM3528’s boost converter. For continuous inductor cur-
rent operation the input voltage ripple is composed of 2 pri-
mary components, the capacitor discharge (delta V
capacitor’s equivalent series resistance (delta V
ripple components are found by:
In the typical application circuit a 1µF ceramic input capacitor
works well. Since the ESR in ceramic capacitors is typically
less than 5mΩ and the capacitance value is usually small, the
input voltage ripple is primarily due to the capacitive dis-
charge. With larger value capacitors such as tantalum or
aluminum electrolytic the ESR can be greater than 0.5Ω. In
this case the input ripple will primarily be due to the ESR.
Table 7 lists different manufacturers for various capacitors
and their case sizes that are suitable for use with the LM3528.
When configured as a dual output LED driver a 1µF output
capacitor is adequate. In OLED mode for output voltages
above 12V a 2.2µF output capacitor is required (see Low
Output Voltage Operation (OLED) Section).
ESR
Q
) and the
). These