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TK65020 Datasheet(PDF) 8 Page - TOKO, Inc |
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TK65020 Datasheet(HTML) 8 Page - TOKO, Inc |
8 / 14 page ADVANCED INFORMATION Page 8 January 1999 TOKO, Inc. TK65020 ESR of the output capacitor can increase the voltage drop across the inductor by the additional voltage dropped across the ESR when the peak current flows in it. For example, the voltage across a capacitor with an ESR of 2 Ohms (not unusual at cold temperature) would jump by 200 mV when 100 mA peak current began to flow in it. This extra voltage drop would cause the inductor current to ramp down more quickly, thus depleting the available output current. Possible choices for low ESR capacitors are: Panasonic TE series (surface mount); AVX TPS series (surface mount); Matsuo 267 series (surface mount); Sanyo OS-CON series. RIPPLE AND NOISE CONSIDERATIONS The filtered test circuit of the TK65020 is shown below in Figure 2. VIN GND GND NC VOUT SW IB L = 95 µF D IOUT VOUT RN 1 K RS 1 K VIN CN 10 µF CD 10 µF CS 220 pF ROF 15 ++ CU 10 µF FIGURE 2: FILTERED TEST CIRCUIT SINGLE-CELL APPLICATION (CONT.) normal case), then the IC may become inoperable due to insufficient V IN. This is why the application test circuit features an RC filter into the V IN pin. The current draw is very small, so the voltage drop across this filter resistor is negligible. The filter serves to average out the input ripple caused by the battery resistance. Note that this filter is optional, and the net effect of its use is the extension of battery life by allowing the battery to be discharged more deeply. A more power-efficient method comes at the price of a large capacitor. This can be placed in parallel with the battery to help channel the converter current pulses away from the battery. The capacitor must have low ESR compared to the battery resistance in order to accomplish this effectively. Still another solution is to filter the DC input with an LC filter. However, it is more likely that the filter will be either too large or too lossy. It is of questionable benefit to smooth the input if the DC loss through the filter is large. Assuming that input ripple voltage at the battery terminal and converter input is large, and that we filter the V IN pin of the IC as in the test circuit, then the parameter “V IN” in the previous equations is not usable, and we will need to use parameters to represent both the source voltage and the source resistance. SWITCH ON-RESISTANCE, INDUCTOR WINDING RESISTANCE, AND CAPACITANCE ESR The on-resistance of the TK65020’s internal switch is about 1 Ohm maximum. Using the previously stated example of 100 mA peak current, the voltage drop across the switch would reach 100 mV during the on-time. This subtracts from the voltage which is impressed across the inductor to store energy during the on-time. As a result, less energy is delivered to the output during the off-time. If the winding resistance of the inductor increases to 1 Ohm or greater, the voltage drop across the winding resistance also subtracts from the voltage used to store energy in the core. This causes a degradation in efficiency. As the inductor delivers energy into the output capacitor during the off-time, its current decays at a rate proportional to the voltage drop across it. The idealized equations assume that the voltage at the switching node is clamped at a diode drop above the output voltage. However, the |
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