APPLICATION NOTE 2 USING THE TELCOM TC54 VOLTAGE DETECTOR USING THE TELCOM TC54 VOLTAGE DETECTOR AN-2 INTRODUCTION OPERATION TelCom's TC54 series of voltage detectors are designed to supersede a variety of discrete comparator circuits and bipolar technology voltage detectors. These voltage detectors are especially suited for application in battery powered systems because of their extremely low 1A operating current. Each part is laser programmed to the desired trip point voltage over the range of 0.8V to 6.0V in 100mV steps. For complete design flexibility, complementary or open-drain output versions are available. TC54 package options include surface mount SOT-23 and SOT-89 or through-hole TO-92 types. The TC54VN equivalent circuit is shown in Figure 1. The internal precision temperature-compensated reference and comparator circuit consistently monitor VIN for an undervoltage condition. When such a condition occurs, the output is forced to the active (LOW) state and remains there until VIN is once again within tolerance. As shown in the timing diagram of Figure 1, the output is forced active when VIN < (-VDET) (Where (-VDET) is the threshold voltage) and returns HIGH when VIN exceeds (-VDET) plus 5% hysteresis (example: if (-VDET) = 2.0V, the output will go HIGH when VIN > 2.1V). TC54 BLOCK DIAGRAM AND TIMING TC54VC only INPUT VOLTAGE 2 VIN HYSTERESIS VOLTAGE RELEASE VOLTAGE (+ VDET) DETECT VOLTAGE (- VDET) - + MINIMUM OPERATING VOLTAGE GROUND LEVEL VOUT 1 OUTPUT VOLTAGE VREF VSS 3 GROUND LEVEL Figure 1. TC54 AS A MICROPROCESSOR RESET GENERATOR VIN Following a power outage, microprocessors require a reset to get them back into their starting state. Commonly used simple Schmitt trigger/capacitor circuits are inadequate since they do not accurately monitor the power supply voltage tolerance. The TC54 series overcomes this problem with precise voltage detection. Figure 2 shows a reset circuit using a standard TC54 and external RC time constant, replacing bipolar integrated voltage detectors (see Table 1). VIN R VOUT Microprocessor Circuit TC54VN C VSS Figure 2. Microprocessor Reset Circuit using TC54 and External Delay AN2-2 8/22/97 1 USING THE TELCOM TC54 VOLTAGE DETECTOR AN-2 Table 1. TC54 Cross Reference Listing MFGR PART# TELCOM # DESCRIPTION TELCOM ATTRIBUTE Motorola Motorola Motorola Motorola Motorola Motorola T.I. T.I. MC33064P-5 MC34064P-5 MC34164P-3 MC34164P-5 MC33164P-3 MC33164P-5 TL7757ILP TL7757CLP TC54VN4502EZB TC54VN4502EZB TC54VN2702EZB TC54VN4302EZB TC54VN2702EZB TC54VN4302EZB TC54VN4502EZB TC54VN4502EZB 4.5V trip, -40 to +85C, TO-92 4.5V trip, 0 to +70C, TO-92 2.7V trip, 0 to +70C, TO-92 4.3V trip, 0 to +70C, TO-92 2.7V trip, -40 to +85C, TO-92 4.3V trip, -40 to +85C, TO-92 4.5V trip, -40 to +85C, TO-92 4.5V trip, 0 to +70C, TO-92 500 times less power 500 times less power 10 times less power 10 times less power 10 times less power 10 times less power 2000 times less power 2000 times less power MODIFYING THE TRIP POINT, -VDET Although the TC54 has a pre-programmed VDET, it is sometimes necessary to make adjustments during prototyping. This can be accomplished by connecting an external resistor divider to a TC54 which has a VDET lower than that of VSOURCE (Figure 3). To maintain detector accuracy, the bleeder current through the divider should be significantly higher than the 1A operating current required by the TC54. A reasonable value for this bleeder current is 100A (100 times the 1A required by the TC54). For example, if (-VDET) = 2V and the desired trip point is 2.5V, the value of R1 + R2 is 250K Ohms (2.5V/100A). The value of R1 + R2 can be rounded to the nearest standard value and plugged into the equation of Figure 3 to calculate values for R1 and R2. 1% tolerance resistors are recommended. OTHER APPLICATIONS VSOURCE VSOURCE R2 VIN VOUT VSS Figure 3. Modify trip point of TC54 using external resistor divider AN2-2 8/22/97 ) DET Where: VSOURCE = Voltage to be monitored (-VDET) = Threshold Voltage Setting of TC54 Note: In this example, VSOURCE must be greater than (-VDET) TC54 R1 ( R1R1+ R2 ) = (-V 2 USING THE TELCOM TC54 VOLTAGE DETECTOR AN-2 4.3V VCC 270 VIN VIN RL VOUT Pwr Sply MTP3055EL TC54VN VOUT TC54Vx POWER GOOD VSS VSS Figure 6. Power good monitor Figure 4. MOSFET low drive protection Low operating power and small physical size make the TC54 series ideal for many voltage detector applications, such as those shown below. Figure 4 shows a low-voltage gate drive protection circuit, which prevents overheating of the logic-level MOSFET due to insufficient gate voltage. When the input signal is below the threshold of the TC54VN, its output grounds the gate of the MOSFET. Figure 5 and 6 show the TC54 in conventional voltage monitoring applications. VIN TC54Vx VOUT BATLOW VSS Figure 5. Battery voltage monitor Sales Offices TelCom Semiconductor 1300 Terra Bella Avenue P.O. Box 7267 Mountain View, CA 94039-7267 TEL: 650-968-9241 FAX: 650-967-1590 E-Mail: liter@c2smtp.telcom-semi.com AN2-2 8/22/97 TelCom Semiconductor Austin Product Center 9101 Burnet Rd. Suite 214 Austin, TX 78758 TEL: 512-873-7100 FAX: 512-873-8236 3 TelCom Semiconductor H.K. Ltd. 10 Sam Chuk Street, Ground Floor San Po Kong, Kowloon Hong Kong TEL: 852-2324-0122 FAX: 852-2354-9957 Printed in the U.S.A.