Datasheet LTC3854 (Analog Devices) - 10

LTC3854
APPLICATIONS INFORMATION
The LTC3854 can be configured to use either DCR (inductor
Using a Sense Resistor for Current Sensing
winding resistance) sensing or low value resistor sensing. A typical sensing circuit using a discrete resistor is shown The choice of the two current sensing schemes is largely in Figure 1. R a design tradeoff between cost, power consumption, and SENSE is chosen based on the required output current. accuracy. DCR sensing is becoming popular because it eliminates expensive current sensing resistors and is The current comparator has a maximum threshold of 50mV. more power efficient, especially in high current applica- The input common mode range of the current comparator tions. However, current sensing resistors provide the most is 0V to 5.5V. The current comparator threshold sets the accurate current limits for the controller. Other external peak of the inductor current, yielding a maximum aver- component selection is driven by the load requirement, age output current IMAX equal to the peak value less half and begins with the selection of RSENSE (if RSENSE is used) the peak-to-peak ripple current, ∆IL. Allowing a margin and inductor value. Next, the power MOSFETs and Schottky of 20% for variations in the IC and external component diodes are selected. Finally, input and output capacitors are values yields: selected. The Typical Application shown on the first page V can be configured for operation up to 38V on V SENSE(MAX) IN. R SENSE = 0.8 • ∆I I L
SENSE+ and SENSE– Pins
MAX + 2
Inductor DCR Sensing
The SENSE+ and SENSE– pins are the inputs to the cur- rent comparator. The common mode input voltage range For applications requiring the highest possible efficiency, of the current comparator is 0V to 5.5V. Both SENSE pins the LTC3854 is capable of sensing the voltage drop across are high impedance inputs with small input bias currents the inductor DCR, as shown in Figure 2. The DCR of the of less than 1μA. When the SENSE pins ramp up from 0V inductor represents the small amount of DC copper wind- to 1.4V, small bias currents flow out of the SENSE pins. ing resistance, which can be less than 1mΩ for today’s When the SENSE pins ramp down from 5.5V to 1.1V, the low value, high current inductors. When the external small bias currents flow into the SENSE pins. The high R1||R2•C1 time constant is chosen to be equal to the impedance inputs to the current comparator allow accurate L/DCR time constant, the voltage drop across the external DCR sensing. VIN INTVCC BOOST TG LTC3854 RSENSE VOUT SW BG GND SENSE+ SENSE– 3854 F01 FILTER COMPONENTS PLACED NEAR SENSE PINS
Figure 1. Using a Resistor to Sense Current with the LTC3854
3854fb 10 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts