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LTC3801B Datasheet(PDF) 7 Page - Linear Technology |
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LTC3801B Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC3801/LTC3801B sn3801 3801fs OPERATIO (Refer to the Functional Diagram) inductor current. The amount of reduction is given by the curve in Figure 3. Soft-Start An internal default soft-start circuit is employed at power- up and/or when coming out of shutdown. The soft-start circuit works by internally clamping the voltage at the ITH/RUN pin to the corresponding zero current level and gradually raising the clamp voltage such that the minimum time required for the programmed switch current to reach its maximum is approximately 0.6ms. After the soft-start circuit has timed out, it is disabled until the part is put in shutdown again or the input supply is cycled. Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle DUTY CYCLE (%) 20 75 95 100 3801 F03 55 35 40 60 80 30 50 70 90 115 65 85 45 105 70 50 30 100 60 80 40 90 VIN = 4.2V TA = 25°C LTC3801 LTC3801B APPLICATIO S I FOR ATIO The basic LTC3801/LTC3801B application circuit is shown on the front page of this data sheet. External component selection is driven by the load requirement and begins with the selection of the inductor and RSENSE. Next, the power MOSFET and the output diode are selected followed by the input bypass capacitor CIN and output bypass capacitor COUT. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage developed across RSENSE, the threshold of the compara- tor determines the inductor’s peak current. The output current the LTC3801 can provide is given by: I R I OUT SENSE RIPPLE =− 0 117 2 . where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation section). For the LTC3801B use 104mV in the previous equation and follow through the analysis using that number. A reasonable starting point for setting ripple current is IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it becomes: R I SENSE OUT =< 1 10 ()( ) for Duty Cycle 40% However, for operation that is above 40% duty cycle, slope compensation effect has to be taken into consideration to select the appropriate value to provide the required amount of current. Using Figure 3, the value of RSENSE is: R SF I SENSE OUT = ()( )( ) 10 100 where SF is the “Slope Factor.” Inductor Value Calculation The operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use of a smaller inductor for the same amount of inductor ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. The inductance value also has a direct effect on ripple cur- rent. The ripple current, IRIPPLE, decreases with higher in- ductance or frequency and increases with higher VIN or VOUT.Theinductor’speak-to-peakripplecurrentisgivenby: I VV fL VV VV RIPPLE IN OUT OUT D IN D = −+ + () where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is |
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