CS1937 Step-up DC/DC Converter for White LEDs Features Description ●Inherently Matched LED Current CS1937 is a step-up DC-DC converter ●High Efficiency: 84% Typical specially designed to drive white LEDs with ●Drives Up to 4 LEDs from a 3.2V Supply a constant current. The device can drive ●Drives Up to Six LEDs from a 5V Supply 2, 3 or 4 LEDs in series from a Li-lon cell. ●36V Rugged Bipolar Switch Series connection of the LEDs provides ●Fast 1.2MHz Switching Frequency identical LED currents resulting in uniform ●Requires Only 0.22uF Output Capacitor brightness and eliminating the need for ●Uses Tiny Inductors ballast resistors. The CS1937 switches at ●SOT23-5(6) Package 1.2MHz, allowing the use of tiny external components. The output capacitor can be as small as 0.22uF, saving space and cost. A low 95mV feedback voltage minimizes power loss in the current setting resistor for better efficiency. Applications ● Cellular Phones ● PDA, Notebook, Digital Cameras ● MP3, MP4, GPS Receiver Typical Application CS1937 Absolute Maximum Ratings Input Voltage (Vin) SW Voltage FB Voltage SHDN Voltage 10V 36V 10V 10V Operating Temperature Range -40℃ to +85℃ Maximum Junction Temperature 125℃ Storage Temperature Range -65℃ to +150℃ Lead Temperature(Soldering, 10s) 300℃ Package and Order Information Package Description CS1937 Electrical Characteristics (TA=25℃，VIN=3V，VSHDN=3V, unless otherwise note) Parameter Conditions Minimum Operating Voltage Min Typical 2.3 ISW=100mA,duty cycle=66% FB Pin Bias Current Unit V Maximum Operating Voltage Feedback Voltage Max 10 V 86 95 104 mV 10 45 100 nA 1.9 0.1 2.5 1.0 mA uA 1.6 MHz Supply Current Shutdown Pin = 0V Switching Frequency 0.8 1.2 Maximum Duty Cycle 85 90 % 320 mA mV Switch Current Limit Switch VCESAT ISW=250mA 350 Switch Leakage Current VSW=5V 0.01 SHDN Voltage High 1.5 uA V SHDN Voltage Low SHDN Pin Bias Current 5 0.4 65 V uA CS1937 Pin Function Pin 1(SW): Switch pin. Connect inductor and diode here. Minimize trace area at this pin to reduce EMI. Pin 2(GND): Ground pin. Connect this pin to local ground plane directly. Pin 3(FB): Feedback pin. Reference voltage is 95mV. Connect cathode of lowest LED and resistor here. Calculate resistor value according to following formula: RFB=95mV/ILED Pin 4(SHDN): Shutdown pin. Connect 1.5V or higher to enable device; 0.4V or less to disable device. Pin 5(GND for SOT23-6): Ground pin. Connect to Pin 2 and to local ground plane. Pin 5(Vin for SOT23-5): Pin 6(Vin for SOT23-6): Input supply pin. Must be locally bypassed. Board Layout Consideration As we know, for all switching regulators, we should pay attention to PCB board layout and component placement. To maximize efficiency, switching rise/fall time should be made as short as possible. To prevent EMI issue, proper layout of the high frequency switching path is essential. The signal of the SW pin has sharp rise/fall edges. Minimize the length and area of all traces connected to SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. In addition, the ground connection for the feedback resistor R1 should be tied to GND pin directly and not shared with any other component. Recommended component placement is shown in following figure. CS1937 Typical Performance Characteristics CS1937 Application Information 1. LED Current Setting The LED current is controlled by the feedback resistor R1. The formula is: R1=95mV/ILED For example, if you intend to set ILED=15mA, then R1 will be 6.34Ω. 2. Open-Circuit Protection In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The CS1937 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to limit the voltage on the SW pin . The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating of the zener should be larger than 0.1mA. 3. Dimming Control There are 4 different types of dimming control circuits: PWM control, DC voltage control, filtered PWM control and logic signal control.