LM3528TMX National Semiconductor, LM3528TMX Datasheet - Page 14

• 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
Block Diagram
Operation Description
The LM3528 Current Mode PWM boost converter operates
from a 2.7V to 5.5V input and provides two regulated outputs
for White LED and OLED display biasing. The first output,
MAIN, provides a constant current of up to 30mA to bias up
to 6 series white LED’s. The second output, SUB/FB, can be
configured as a current source for up to 6 series white LED’s
at at up to 30mA, or as a feedback voltage pin to regulate a
constant output voltage of up to 21V. When both MAIN and
SUB/FB are configured for white LED bias the current for each
LED string is controlled independently or in unison via an I
compatible interface. When MAIN is configured for white LED
bias and SUB/FB is configured as a feedback voltage pin, the
current into MAIN is controlled via the I
and SUB/FB becomes the middle tap of a resistive divider
used to regulate the output voltage of the boost converter.
The core of the LM3528 is a Current Mode Boost converter.
Operation is as follows. At the start of each switching cycle
the internal oscillator sets the PWM converter. The converter
turns the NMOS switch on, allowing the inductor current to
2
C compatible interface
FIGURE 2. LM3528 Block Diagram
2
C
14
ramp while the output capacitor supplies power to the white
LED’s and/or OLED panel. The error signal at the output of
the error amplifier is compared against the sensed inductor
current. When the sensed inductor current equals the error
signal, or when the maximum duty cycle is reached, the
NMOS switch turns off causing the external Schottky diode to
pick up the inductor current. This allows the inductor current
to ramp down causing its stored energy to charge the output
capacitor and supply power to the load. At the end of the clock
period the PWM controller is again set and the process re-
peats itself.
ADAPTIVE REGULATION
When biasing dual white led strings (White LED mode) the
LM3528 maximizes efficiency by adaptively regulating the
output voltage. In this configuration the 500mV reference is
connected to the non-inverting input of the error amplifier via
mux S2 (see Figure 2, Block Diagram). The lowest of either
V
error amplifier via mux S1. This ensures that V
MAIN
or V
SUB/FB
is then applied to the inverting input of the
MAIN
and V
30020533
SUB/