Simple 3V LED Driver Circuit using IC LT1937

This circuit is a tiny power manager for your LED!

It takes a higher voltage like from a battery and steps it down to the perfect 3V your LED needs.

The special chip, the LT1937 is super efficient and keeps the power flowing smoothly.

This ensures your LED gets just the right amount of juice to shine brightly and consistently.

Block Diagram:

To understand its operation better refer to the block diagram:

The LT1937 employs a constant frequency current mode control method for effective line and load regulation.

At the beginning of each oscillator cycle the internal SR latch is activated turning on the internal power switch Q1.

A voltage proportional to the switch current is added to a stabilizing ramp and the 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 SR latch is reset turning off the power switch.

The level at the negative input of A2 is determined by the error amplifier A1 which amplifies the difference between the feedback voltage and the reference voltage of 95mV.

This process allows the error amplifier to set the correct peak current level to maintain output regulation.

An increase in the error amplifiers output results in more current being delivered to the output while a decrease leads to less current being delivered.

Circuit Description:

Parts List:

The LT1937 can regulate four series LEDs connected at low output currents down to approximately 4mA from a 4.2V supply without pulse skipping using the same external components specified for 15mA operation.

However, as the current is further reduced the device will start skipping pulses resulting in some low-frequency ripple.

Nevertheless, the LED current remains regulated on an average basis down to zero.

For most LT1937 applications, a 22µH inductor is recommended.

It should have low core losses at 1.2MHz and low DCR copper wire resistance.

Ceramic capacitors are ideal for LT1937 applications due to their small size.

X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges compared to other types such as Y5V or Z5U.

A C1 2.2µF input capacitor and a C2 0.47µF output capacitor are sufficient for most LT1937 applications.

Schottky diodes are the ideal choice for LT1937 applications due to their low forward voltage drop and fast reverse recovery.

The forward voltage drop represents the conduction losses while the diode capacitance CT or CD represents the switching losses.

When selecting a diode consider both the forward voltage drop and the diode capacitance.

Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance which can cause significant switching losses at the LT1937s 1.2MHz switching frequency.

A Schottky diode rated at 100mA to 200mA is sufficient for most LT1937 applications.

The LED current is controlled by the feedback resistor R2.

The feedback reference is 95mV so the LED current is 95mV R1.

To achieve accurate LED current precision resistors are preferred 1% is recommended.

Formulas and Calculations:

The below formula refers to utilizing the LT1937 DC/DC converter IC to set the current limit for controlling LEDs in a circuit.

From the above diagram the formula R2 are provided below:

R2 = 95mV / I_LED

The LT1937 datasheet states that choosing a suitable resistor R2 to connect to the ISET pin will control the current flowing through LED I.

The connection is provided by:

I_LED​ = 95mV​ / R2

When this equation is rearranged to find R2, the result is:

R2 = ​95mV / I_LED​

Example:

To set the LED current VLED to 20mA, which is the usual current for many standard LEDs, for example, you would compute R2 as follows:

R2 = 95mV / 20mA​=0.095V / 0.02A​ = 4.75Ω

Depending on what is available, you may really select a typical resistor value of 4.7Ω or 5.1Ω, which are near to 4.75Ω.

To prevent overheating, make sure that P = I²_LED R2 can manage the power dissipation R2.

Select a resistor with a power rating that is enough (usually 1/4 watt or more for normal applications).

To summarise: R2 = ​95mV / ILED

Based on the internal reference voltage at the ICs ISET pin, the resistor value that sets the LED current in a circuit utilizing the LT1937 DC/DC converter IC is determined to be 95mV.

How to Build:

To build a Simple 3V LED Driver Circuit using IC LT1937 follow the below mentioned steps for connection purpose:

Power Supply:

• Provide a stable power supply within the recommended input voltage range for the LT1937 check the datasheet for details.

Connect Components:

• Connect the LT1937 IC as per the typical application circuit in the datasheet, which includes connections for the inductor, input and output capacitors, schottky diode and feedback resistor.
• Ensure correct polarity for capacitors and diodes.

Inductor Selection:

• Use a 22µH inductor with low core losses at 1.2MHz and low DCR.

Capacitor Selection:

• Use a 2.2µF input capacitor and a 0.47µF output capacitor, preferably ceramic X5R or X7R types are recommended.

Diode Selection:

• Choose a schottky diode with a forward current rating of 100mA to 200mA considering both forward voltage drop and diode capacitance.

Feedback Resistor R1:

• Calculate the value of R2 using the formula R1 = 95mV/ILED, where ILED is the desired LED current.
• Use a precision resistor 1% tolerance recommended.

Testing:

• Power up the circuit and measure the output voltage and current to ensure it meets your requirements.
• Observe the circuit’s performance under different load conditions to verify stability and regulation.

Adjustments:

• If necessary, adjust component values or the feedback resistor to achieve the desired output voltage and current regulation.

Safety Precautions:

• Ensure proper insulation and grounding to prevent electrical hazards.
• Follow best practices for soldering and circuit assembly.

Further Optimization:

• Consider additional circuit optimizations based on the specific requirements of your application and the recommendations in the LT1937 datasheet.

Note:

• It is important to refer to the LT1937 datasheet for detailed specifications application notes and recommended circuit configurations for your specific project requirements.

Conclusion:

The 3V LED driver circuit using the LT1937 IC is a compact and efficient solution for powering 3V LEDs from higher voltage sources.

It offers stable and controlled brightness levels making it suitable for various applications requiring reliable LED illumination.

References:

Datasheet IC LT1937