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TS1937

Boost (Step-up) WLED Driver

TSC Corporation 

TS1937

Boost (Step-up) WLED Driver

Application Information

Operation

The TS1937 uses a constant frequency, current mode control scheme to provide excellent line and load regulation.

Operation can be best understood by referring to the block diagram. At the start of each oscillator cycle, the RS latch is

set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and

the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at

the negative input of A2, the RS latch is reset turning off the power switch. The level at the negative input of A2 is set

by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the

reference voltage of 95mV. In this manner, 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.

Minimum Output Current

The TS1937 can regulate three series LEDs connected at low output currents, down to approximately 4mA from a 4.2V

supply, without pulse skipping, using the same external components as specified for 20mA operation. As the current is

further reduced, the device will begin skipping pulses. This will result in some low frequency ripple, although the LED

current remains regulated on an average basis down to zero.

Soft Start and Current Limit

The internal soft start circuit minimizes the inrush current during turning on TS1937. The Typical switch current is

limited to about 320mA by the chip.

Over Voltage Protection

The TS1937 has design an internal latched off open-circuit protection circuit, the additional sense pin to detect the

voltage when the LEDs are disconnected from the circuit or fail open, the TS1937 will shutdown until input condition

changes to bring it out of the shutdown mode.

Inductor Selection

A 22mH inductor is recommended for most TS1937 applications. Although small size and high efficiency are major

concerns, the inductor should have low core losses at 1.2MHz 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. A Schottky

diode rated at 100mA to 200mA is sufficient for most TS1937 applications.

Capacitor Selection

The small size of ceramic capacitors makes them ideal for TS1937 applications. X5R and X7R types are

recommended because they retain their capacitance over wider voltage and temperature ranges than other types such

as Y5V or Z5U. A 1µF input capacitor and a 0.22µF output capacitor are sufficient for most.

4/8

Version: C07

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