LM3508 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – Synchronous Magnetic Constant Current White LED Driver

Operation
FIGURE 1. LM3508 Block Diagram
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The LM3508 utilizes a synchronous step-up current mode
PWM controller and a regulated current sink to provide a
highly efficient and accurate LED current for white LED bias.
The internal synchronous rectifier increases efficiency and
eliminates the need for an external diode. Additionally, inter-
nal compensation eliminates the need for external compen-
sation components resulting in a compact overall solution.
Figure 1 shows the detailed block diagram of the LM3508.
The output of the boost converter (OUT) provides power to
the series string of white LED’s connected between OUT and
ILED. The boost converter regulates the voltage at ILED to
500mV. This voltage is then used to power the internal current
source whose output is at SET.
The first stage of the LM3508 consists of the synchronous
boost converter. Operation is as follows: At the start of each
switching cycle the oscillator sets the PWM controller. The
controller turns the low side (NMOS) switch on and the syn-
chronous rectifier (PMOS) switch off. During this time current
ramps up in the inductor while the output capacitor supplies
the current to the LED’s. The error signal at the output of the
error amplifier is compared against the sensed inductor cur-
rent. When the sensed inductor current equals the error sig-
nal, or when the maximum duty cycle is reached, the NMOS
switch turns off and the PMOS switch turns on. When the
PMOS turns on, the inductor current ramps down, restoring
energy to the output capacitor and supplying current to the
LED’s. At the end of the clock period the PWM controller is
again set and the process repeats itself. This action regulates
ILED to 500mV.
The second stage of the LM3508 consists of an internal cur-
rent source powered by the ILED voltage and providing a
regulated current at SET. The regulated LED current is set by
connecting an external resistor from SET to PGND. V_SET is
adjusted from 0 to 200mV by applying a PWM signal of up to
typically 100kHz at DIM (see Typical Performance Charac-
teristic of SET voltage vs DIM frequency). The PWM signal at
DIM modulates the internal 200mV reference and applies it to
an internal RC filter resulting in an adjustable SET voltage and
thus an adjustable LED current.
Start-Up
The LM3508 features a soft-start to prevent large inrush cur-
rents during start-up that can cause excessive voltage ripple
on VIN. During start-up the average input current is ramped
up at a controlled rate. For the typical application circuit, driv-
ing 4LED’s from a 3.6V lithium battery at 30mA, when EN is
driven high the average input current ramps from zero to 160-
mA in 470µs. See plot of Soft Start functionality in the Typical
Performance Characteristics.
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