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TK65020 Datasheet(PDF) 11 Page - TOKO, Inc |
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TK65020 Datasheet(HTML) 11 Page - TOKO, Inc |
11 / 14 page ADVANCED INFORMATION January 1999 TOKO, Inc. Page 11 TK65020 HOW TO MAKE A STEP-DOWN CONVERTER USING THE TK65020 AND AN IRF7524D1 “FETKY” PART The TK65020 can be used as a controller in a step-down converter with the following two additional changes. See Fig 3. 1) Change the main switch orientation for use in a step-down converter. An external P-channel power MOSFET is used as the main switch in a step-down converter configuration. The gate of FET is turned on through a resistor divider being pulled down to GND by the internal output transistor of the TK65020. This application requires both a logic level P-channel MOSFET and a Schottky diode. An IRF7524D1 “FETKY” part contains both in a small micro 8 package. 2) Change the voltage seen at the V IN pin of the TK65020 to below the regulation voltage at the VOUT pin. A resistor divider between the converter V IN and the chip VIN pin drops the voltage seen at the VIN pin. If the VIN pin is a higher voltage than the V OUT pin, the TK65020 will not regulate the output, but will continue to pulse its output transistor. WHERE TO USE THIS STEP-DOWN CONVERTER The TK65020 is a Pulse Burst Modulation (PBM) controller with a fixed duty cycle of approximately 50%. Therefore, only if V BATT is more than twice the voltage of VOUT can the converter run in Continuous Current Mode (CCM). The converter can and does regulate in Discontinuous Current Mode (DCM) for lighter output current loads with V IN less than twice the voltage of V OUT. But DCM produces more peak current and more ripple current than CCM. Below is a table giving some examples of where this type of step-down converter might be used. 1 k + R3 150 R4 10 k R1 3.9 k R2 TK65125 U1 10 µH C2 U2 L1 3 1 4 2 5 VIN VOUT GND SW IRF7524D1 1,2 4 3 R7 1 k C1 5,6, 7,8 + 47 µF C3 47 µF 6 NC VBATT VOUT 220 pF GND STEP-DOWN CONVERTER APPLICATION FIGURE 3: STEP-DOWN CONVERTER USING THE TK65020 SCHEMATIC Note 1: Li-ion cell voltage range 2.7 V to 4.2 V Note 2: NiMH cell voltage range 1.1 V to 1.3 V Note: L = 10 µH Toko P/N: 636CY-100M D73C Coil Type Battery # of Cells VBATT Range VOUT Typ. Max IOUT Oper Mode Inductor Li-ion 2 (Note 1) 5.4 to 8.4 V 2.5 V 500 mA DCM 10 µH NiMH 4 (Note 2) 4.4 to 5.2 V 2.5 V 500 mA DCM 10 µH NiMH 6 (Note 2) 6.6 to 7.8 V 2.5 V 500 mA CCM 120 µH |
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