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LTC3851EGN-PBF 数据表(PDF) 13 Page - Linear Technology |
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LTC3851EGN-PBF 数据表(HTML) 13 Page - Linear Technology |
13 / 28 page LTC3851 13 3851f APPLICATIONS INFORMATION VIN VIN INTVCC BOOST TG SW BG GND INDUCTOR DCR L SENSE+ SENSE– LTC3851 VOUT 3851 F02 R1 R2 *PLACE C1 NEAR SENSE+, SENSE– PINS C1* R1||R2 • C1 = RSENSE(EQ) = DCR L DCR R2 R1 + R2 Figure 2. Current Mode Control Using the Inductor DCR The DCR of the inductor can also be measured using a good RLC meter. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant fre- quency architectures by preventing sub-harmonic oscil- lations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal. Normally, this results in a reduction of maximum inductor peak current for duty cycles >40%. However, the LTC3851 uses a novel scheme that allows the maximum inductor peak current to remain unaffected throughout all duty cycles. Inductor Value Calculation The operating frequency and inductor selection are inter- related in that higher operating frequencies allow the use of smaller inductor and capacitor values. A higher frequency generally results in lower efficiency because of MOSFET gate charge losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. The inductor value has a direct effect on ripple current. The inductor ripple current ΔIL decreases with higher inductance or frequency and increases with higher VIN: ΔI fL V V V L OUT OUT IN = ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 • – Accepting larger values of ΔIL 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 ΔIL = 0.3(IMAX). The maximum ΔIL occurs at the maximum input voltage. The inductor value also has secondary effects. The tran- sition to Burst Mode operation begins when the average inductor current required results in a peak current below ≈10% of the current limit determined by RSENSE. Lower inductor values (higher ΔIL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or molypermalloy cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can con- centrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that induc- tance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Power MOSFET and Schottky Diode (Optional) Selection Two external power MOSFETs must be selected for the LTC3851 controller: one N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. |
类似零件编号 - LTC3851EGN-PBF |
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类似说明 - LTC3851EGN-PBF |
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