LM1937 HTC | Alldatasheet
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Technical content
WHITE LED STEP-UP CONVERTER LM1937
FEATURES
- Inherently Matched LED Current
- High Efficiency: 84% Typical
- Drives Up to Four LEDs from a 3.2V Supply
- Drives Up to Six LEDs from a 5V Supply
- 36V Rugged Bipolar Switch
- Fast 1.2MHz Switching Frequency
- Uses Tiny 1mm Tall Inductors
- Requires Only 0.22µF Output Capacitor
- Moisture Sensitivity Level 3
APPLICATIONS
- Cellular Phones
- PDAs, Handheld Computers
- Digital Cameras
- MP3 Players
- GPS Receivers ORDERING INFORMATION
DESCRIPTION
TYPICAL APPLICATION CIRCUIT Figure 1. Li-Ion Powered Driver for Three White LEDs The LM1937 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. feedback voltage minimizes power loss for better efficiency.
WHITE LED STEP-UP CONVERTER LM1937 PIN DESCRIPTION BLOCK DIAGRAM HTC - 2 - ABSOLUTE MAXIMUM RATING (Note 1) Input Voltage (Vin) 10 V SW Voltage 36 V FB Voltage 10 V SHDN Voltage 10 V Operating Temperature (Topr) -40 ~ +85 °C Maximum Junction Temperature 125 °C Storage Temperature (Tstg) -65 ~+125 °C Lead Temperature (soldering, 5 sec) 260 °C
WHITE LED STEP-UP CONVERTER LM1937 HTC - 3 - OPERATION The LM1937 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 in Figure 2. At the start of each oscillator cycle, the SR 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 SR 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 LM1937 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 15mA operation. As 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. SHDN SHDN SHDN SHDN SHDN
losses at 1.2MHz and low DCR (copper wire resistance). inductors is shown in Figure 3. Table 1. Recommended Inductors Figure 3. Efficiency Comparison of Different Inductors Table 2. Recommended Ceramic Capacitor 200mA is sufficient for most LM1937 applications. Table 3. Recommended Schottky Diodes
Figure 6. Dimming Control Using a Filtered PWM