LM3509SDX National Semiconductor Corporation, LM3509SDX Datasheet - Page 12

• Manufacturer Part Number: LM3509SDX
• Description: High Efficiency Boost For White Led S And/or Oled Displays With Dual Current Sinks And I2c Compatible Brightness Control
• Manufacturer: National Semiconductor Corporation

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Block Diagram
Operation Description
The LM3509 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 5 series white LED’s. The second output, SUB/FB, can be
configured as a current source for up to 5 series white LED’s
at at 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
patible interface. When MAIN is configured for white LED bias
and SUB/FB is configured as a feedback voltage pin, the cur-
rent 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 LM3509 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. LM3509 Block Diagram
2
C com-
12
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
LM3509 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
FB
MAIN
are at least 500mV, thus providing enough voltage head-
or V
SUB/FB
is then applied to the inverting input of the
30004333
MAIN
and V
SUB/