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TD6817 Datasheet(PDF) 11 Page - TECHCODE SEMICONDUCTOR, INC. |
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TD6817 Datasheet(HTML) 11 Page - TECHCODE SEMICONDUCTOR, INC. |
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11 / 17 page  December, 20, 2009 America Techcode Semiconductor, Inc. www.techcodesemi.c om 11 Techcode® 1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817 DATASHEET Function Description(Cont.) Low Supply Operation The TD6817 will operate with input supply voltages as low as 2.5V, but the maximum allowable output current is reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input voltage for various output voltages. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current for duty cycles >40%. However, the TD6817 uses a patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. Maximum Output Current vs Input Voltag The basic TD6817 application circuit is shown in Figure 3. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Selection For most applications, the value of the inductor will fall in the range of 1uH to 4.7uH. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is DIL = 800mA (40% of 2000mA). The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 2200mA rated inductor should be enough for most applications (2000mA + 200mA). For better efficiency, choose a low DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher DIL) 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. |
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