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TC3684 Datasheet(PDF) 5 Page - Microchip Technology |
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TC3684 Datasheet(HTML) 5 Page - Microchip Technology |
5 / 10 page © 2005 Microchip Technology Inc. DS21556C-page 5 TC3682/TC3683/TC3684 4.0 APPLICATIONS INFORMATION 4.1 Output Voltage Considerations The TC3682/TC3683/TC3684 perform inverting voltage conversions but do not provide any type of regulation. The output voltage will droop in a linear manner with respect to the output load current. The value of the equivalent output resistance of the ‘-VIN’ output is approximately 50 Ω nominal at +25°C and VIN = +5V. The value of the ‘-2VIN’ output is approxi- mately 140 Ω nominal at +25°C and VIN = +5V. In this particular case, ‘-VIN’ is approximately -5V and ‘-2VIN’ is approximately -10V at very light loads and each stage will droop according to the equation below: VDROOP = IOUT x ROUT 4.2 Capacitor Selection In order to maintain the lowest output resistance and output ripple voltage, it is recommended that low ESR capacitors be used. Additionally, larger values of C1 and C2 will lower the output resistance and larger values of COUT will reduce output ripple. Note: For proper charge pump operation, C1 must have a voltage rating greater than or equal to VIN, while C2 and COUT must have a voltage rating greater than or equal to 2VIN. Table 4-1 shows various values of C1/C2 and the corresponding output resistance values for VIN = 5V @ +25°C. Table 4-2 shows the output voltage ripple for various values of COUT (again assuming VIN = 5V @ +25°C). The VRIPPLE values assume a 1mA output load current and a 0.1 Ω ESRCOUT. TABLE 4-1: OUTPUT RESISTANCE VS. C1/C2 (ESR = 0.1 Ω) TABLE 4-2: OUTPUT VOLTAGE RIPPLE VS. COUT (ESR = 0.1Ω) IOUT = 1mA 4.3 Input Supply Bypassing The VIN input should be capacitively bypassed to reduce AC impedance and minimize noise effects due to the switching internal to the device. It is recommended that a large value capacitor (at least equal to C1) be connected from VIN to GND for optimal circuit performance. 4.4 Shutdown Input The TC3682/TC3683/TC3684 is enabled when SHDN is high, and disabled when SHDN is low. This input cannot be allowed to float. (If SHDN is not required, see the TC2682/TC2683/TC2684 data sheet.) The SHDN input should be limited to 0.3V above VIN. 4.5 Inverting Voltage Doubler The most common application for the TC3682/TC3683/ TC3684 devices is the inverting voltage doubler (Figure 4-1). This application uses three external capacitors: C1, C2 and COUT. Note: A power supply bypass capacitor is recommended. The output is equal to -2VIN plus any voltage drops due to loading. Refer to Table 4-1 and Table 4-2 for capacitor selection guidelines. FIGURE 4-1: INVERTING VOLTAGE DOUBLER TEST CIRCUIT 4.6 Layout Considerations As with any switching power supply circuit, good layout practice is recommended. Mount components as close together as possible to minimize stray inductance and capacitance. Also use a large ground plane to minimize noise leakage into other circuitry. C1, C2 ( μF) TC3682 ROUT(Ω) TC3683 ROUT(Ω) TC3684 ROUT(Ω) 0.33 633 184 120 1 262 120 102 3.3 120 95 84 COUT ( μF) TC3682 VRIPPLE (mV) TC3683 VRIPPLE (mV) TC3684 VRIPPLE (mV) 0.33 192 60 27 1 632116 3.3 17 8 7 C1 C2 VIN CIN VOUT RL VIN COUT TC3682 TC3683 TC3684 C1– C2– 2 1 7 6 8 4 5 3 C1+ C2+ GND + + + + Device CIN C1 C2 COUT TC3682 3.3 μF 3.3 μF 3.3 μF 3.3 μF TC3683 1 μF 1 μF 1 μF 1 μF TC3684 0.33 μF 0.33 μF 0.33 μF 0.33 μF SHDN -2VIN |
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