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TC7129CPL Datasheet(PDF) 9 Page - Microchip Technology |
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TC7129CPL Datasheet(HTML) 9 Page - Microchip Technology |
9 / 24 page © 2002 Microchip Technology Inc. DS21459B-page 9 TC7129 3.5 Voltage Reference (DREF,RREF,RBIAS,CRF) The reference potentiometer (RREF)provides an adjustment for adjusting the reference voltage; any value above 20k Ω is adequate. The bias resistor (RBIAS) limits the current through DREF to less than 150 µA. The reference filter capacitor (CRF)forms an RC filter with RBIAS to help eliminate noise. 3.6 Input Filter (RIF,CIF) For added stability, an RC input noise filter is usually included in the circuit. The input filter resistor value should not exceed 100k Ω. A typical RC time constant value is 16.7msec to help reject line frequency noise. The input filter capacitor should have low leakage for a high-impedance input. 3.7 Battery The typical circuit uses a 9V battery as a power source. Any value between 6V and 12V can be used. For oper- ation from batteries with voltages lower than 6V and for operation from power supplies, see Section 4.2, Powering the TC7129. 4.0 TYPICAL APPLICATIONS 4.1 TC7129 as a Replacement Part The TC7129 is a direct pin-for-pin replacement part for the ICL7129. Note, however, that part requires a capacitor and resistor between Pins 26 and 28 for phase compensation. Since the TC7129 uses internal phase compensation, these parts are not required and, in fact, must be removed from the circuit for stable operation. 4.2 Powering the TC7129 While the most common power source for the TC7129 is a 9V battery, there are other possibilities. Some of the more common ones are explained below. 4.3 ±5V Power Supply Measurements are made with respect to power supply ground. DGND (Pin 36) is set internally to about 5V less than V+ (Pin 24); it is not intended as a power sup- ply input and must not be tied directly to power supply ground. It can be used as a reference for external logic, as explained in Section 4.6, Connecting to External Logic (see Figure 4-1). FIGURE 4-1: POWERING THE TC7129 FROM A ±5V POWER SUPPLY 4.4 Low Voltage Battery Source A battery with voltage between 3.8V and 6V can be used to power the TC7129, when used with a voltage doubler circuit, as shown in Figure 4-2. The voltage doubler uses the TC7660 DC-to-DC voltage converter and two external capacitors. FIGURE 4-2: POWERING THE TC7129 FROM A LOW VOLTAGE BATTERY V- V+ REF HI REF LO IN HI COMMON IN LO DGND VIN + – -5V 0.1 µF +5V 0.1 µF 24 34 35 28 33 32 23 36 TC7129 0.1 µF V- TC7129 V+ REF HI REF LO IN HI COMMON IN LO DGND 3.8V to 6V + + 10 µF + 8 2 4 10 µF + – 3 TC7660 VIN 5 24 34 35 28 33 32 23 36 |
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