Datasheet LTM8040 (Analog Devices) - 9

LTM8040
APPLICATION INFORMATION
For most applications, the design process is straight Choose resistors according to the following formula: forward, summarized as follows: 1. Look at Table 2 and fi nd the row that has the desired R2 = 2.65V V −2.65V input voltage range LED string voltage range and output UVLO – 10.3µA current. R1 2. Apply the recommended C where V IN, RT and RADJ values. UVLO is the desired UVLO threshold 3. Connect BIAS as indicated. Suppose that the output needs to stay off until the input is above 8V. 4. Connect LEDA to the anode of the LED string. V 5. Connect the remaining pins as needed by the system UVLO = 8V requirements. Let R1 = 100k While these component combinations have been tested for proper operation, it is incumbent upon the user to R2 = 2.65V = 61.9k 8V − 2.65V verify proper operation over the intended system’s line, – 10.3µA 100k load and environmental conditions.
Open LED Protection
V V IN IN The LTM8040 has internal open LED circuit protection. If R1 LTM8040 SHDN the LED is absent or fails open, the LTM8040 clamps the voltage on the LEDA pin to 14V. The switching regulator C1 R2 GND then skips cycles to limit the input current. 8040 F01
Undervoltage Lockout Figure 1. Undervoltage Lockout
Undervoltage lockout (UVLO) is typically used in situations where the input supply is current limited, or has high source Keep the connections from the resistors to the SHDN pin resistance. A switching regulator draws constant power short. If high resistance values are used, the SHDN pin from the source, so the source current increases as the should be bypassed with a 1nF capacitor to prevent cou- source voltage drops. This looks like a negative resistance pling problems from switching nodes. load to the source and can cause the source to current limit or latch low under low source voltage conditions.
Setting the Switching Frequency
UVLO prevents the regulator from operating at source volt- The LTM8040 uses a constant frequency architecture that ages where this might occur. An internal comparator will can be programmed over a 500kHz to 2MHz range with a force the part to stop switching when V single external timing resistor from the RT pin to ground. IN falls below 3.5V. An adjustable UVLO threshold is also realized through the The current that fl ows into the timing resistor is used to SHDN pin, as the internal comparator that performs this charge an internal oscillator capacitor. The LTM8040 is function has a 2.65V threshold. If SHDN is below 0.3V all confi gured such that the default frequency is 500kHz internal circuitry is off. An internal resistor pulls 10.3μA without adding a resistor. Many applications use this value. to ground from the SHDN pin at the UVLO threshold in order to provide hysteresis. 8040fa 9