Datasheet LT3477 (Analog Devices) - 10

LT3477
applicaTions inForMaTion Setting Positive Output Voltages
For designs needing an adjustable current level, the IADJ1 To set a positive output voltage, select the values of R1 and IADJ2 pins are provided for the first and the second and R2 (see Figure 2) according to the following equation: current sense amplifiers, respectively. With the IADJ1 and IADJ2 pins tied higher than 650mV, the nominal current V ⎛ R1⎞ sense voltage is 100mV (appearing between the I OUT = 1.235V⎜1+ ⎟ SP1 and ⎝ R2⎠ ISN2 or ISP2 and ISN2 pins). Applying a positive DC voltage less than 600mV to the IADJ1 and IADJ2 pins will decrease the FBP current sense voltage according to the following formula: LT3477 VOUT VREF I 100mV SENSE = • VADJ R1 RSENSE 618mV FBN R2 For example, if 309mV is applied to the IADJ1 pin and 3477 F02 RSENSE is 0.5Ω, the current sense will be reduced from 200mA to 100mA. The adjustability allows the regulated
Figure 2. Positive Output Voltage Feedback Connections
current to be reduced without changing the current sense resistor (e.g., to adjust brightness in an LED driver or to
Setting Negative Output Voltages
reduce the charge current in a battery charger). To set a negative output voltage, select the values of R3
Considerations When Sensing Input Current
and R4 (see Figure 3) according to the following equation: In addition to regulating the DC output current for current- V ⎛ R3⎞ source applications, the constant-current loop of the OUT = 1.235V⎜1+ ⎟ ⎝ R4⎠ LT3477 can also be used to provide an accurate input current limit. Boost converters cannot provide output –VOUT short-circuit protection, but the surge turn-on current can R3 be drastically reduced using the LT3477 current sense at FBP the input. SEPICs, however, have an output that is DC- LT3477 R4 isolated from the input, so an input current limit not only VREF helps soft-start the output but also provides excellent FBN short-circuit protection. 3477 F03 When sensing input current, the sense resistor should be
Figure 3. Negative Output Voltage Feedback Connections
placed in front of the inductor (between the decoupling capacitor and the inductor). This will regulate the average inductor current and maintain a consistent inductor ripple
Selecting RSENSE/Current Sense Adjustment
current, which will, in turn, maintain a well regulated input Using the following formula to choose the correct cur- current. Do not place the sense resistor between the input rent sense resistor value (for constant current or fail-safe source and the input decoupling capacitor, as this may allow operation). the inductor ripple current to vary widely (even though the average input current and the average inductor current will R 100mV SENSE = still be regulated). Since the inductor current is a triangular ISENSE waveform (not a DC waveform like the output current) some tweaking of the compensation values (RC and CC 3477fd 10 For more information www.linear.com/LT3477 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Package Description Typical Application Related Parts