TS1518

Step-Up DC-DC Converter, White LED Driver

4/7

Version: B07

Application Information

Operation

The TS1518 uses a constant frequency, current mode control scheme to regulate the output LED current. Its operation

can be understood by referring to the block diagram in Figure 3. At the start of each oscillator cycle, a voltage

proportional to the switch current is added to a ramp output and the resulting sum is fed into the positive terminal of the

PWM comparator (comparator-1). When this voltage exceeds the level of the comparator negative input, the peak

current has been reached, and the SR latch (in Control Logic) is reset and turns off the power switch. The voltage at

the negative input of the comparator comes from the output of the error amplifier. The error amplifier sets the correct

peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to

the output; if it decreases, less current is delivered.

Soft Start and Current Limit

The internal soft start circuit minimizes the inrush current during turning on TS1518. The maximum switch current is

limited to about 450 mA by the chip.

Open Circuit Protection

The TS1518 has an internal latched off open-circuit protection circuit. When the LEDs are disconnected from the circuit

or fail open, the TS1518 will shutdown automatically until input condition changes to bring it out of the shutdown mode.

Inductor Selection

A 22uH inductor is recommended for most applications to drive more than 5 LEDs. Although small size and high

efficiency are major concerns, the inductor should have low core losses at 1MHz and low DCR (copper wire

resistance).

Diode Selection

To maintain high efficiency, the average current rating of the Schottky diode should be large than the peak inductor

current, IPK. Schottky diode with a low forward drop and fast switching speeds are ideal for increase efficiency in

portable application. Choose a reverse breakdown of the Schottky diode large than the output voltage.

Capacitor Selection

Choose low ESR capacitors for the output to minimize output voltage ripple. Multilayer capacitors are a good choice for

this as well. A 1uF capacitor is sufficient for 2~6 LEDs, 2.2uF is for 7~8 LEDs and 4.7uF is for 9 LEDs applications.

For additional bypassing, a 100nF ceramic capacitor can be used to shunt high frequency ripple on the input.

The input bypass capacitor Cin, as shown in Figure 1, must be placed close to the IC. This will reduce copper trace

resistance which affects input voltage ripple of the IC. For additional input voltage filtering, a 100nF bypass capacitor

can be placed in parallel with Cin to shunt any high frequency noise to ground. The output capacitor, Cout, should also

be placed close to the IC. Any copper trace connections for the Cout capacitor can increase the series resistance,

which directly effect output voltage ripple.

The feedback network, resister R2 should be kept close to the FB pin to minimize copper trace connections that can

inject noise into the system. The ground connection for the feedback resistor network should connect directly to an

analog ground plane. The analog ground plane should tie directly to the GND pin. If no analog ground plane is

available, the ground connection for the feedback network should tie directly to the GND pin. Trace connections made

to the inductor and Schottky diode should be minimized to reduce power dissipation and increase overall efficiency.