AND8425/D Detection Voltage Selection Guidelines for Application of NCP30X Family Series http://onsemi.com APPLICATION NOTE Device NCP30X Application Input Voltage Output Power Topology I/O Isolation Voltage Supervisory Rest IC N/A N/A N/A N/A Circuit Description VIN Output Stage Open Drain or Complementary Output Vref RESET Driver Block for Delay Capacitor * * For NCP302/303 Series GND CD Figure 1. General Block Diagram of NCP30X Family Series The Figure 1 shows the basic block diagram of NCP30X supervisory family series. It features a highly accurate undervoltage detector with hysteresis. Some parts also feature an externally programmable time delay generator by adding a delay capacitor at the CD pin. This combination of features prevents the system from erratic reset operation. © Semiconductor Components Industries, LLC, 2009 September, 2009 − Rev. 0 1 Publication Order Number: AND8425/D AND8425/D To guarantee the microprocessor (uP) operating normally, the power supply should be well monitored by using voltage monitor device such as NCP30X. In order to make sure that the uP RESET input is asserted when the power supply is not ready and, RESET pin is de−asserted for normal operation when power supply voltage reaches above the minimum operating range of VCC input. This document demonstrates the guideline how to select NCP30X detection voltage option based on the given system parameters. Vo VIN Vcc RST RST Power Supply NCP30X uP GND GND GND Figure 2. Typical System Configuration Using with Supervisory Device The configuration in the Figure 2 shows how the power supply connects to uP under the voltage monitoring by NCP30X device. To make sure that uP is in normal operation, typically the voltage % tolerance of the power supply must be tighter than that of the uP. For example, if the uP’s VCC voltage tolerance (it can be found in uP’s DC electrical specification at data sheet) is $5%, then the power supply % tolerance should be less than 5%, say, for example 3%. http://onsemi.com 2 AND8425/D NCP30X Detection Voltage Selection Criteria For selecting the NCP30X detection voltage option, basically there are three major factors to be considered: 1. VDET+_max = Maximum detection voltage (Vin rises) of the NCP30X. 2. Vin_min = Minimum voltage output of the power supply. 3. VCC_min = Minimum voltage input of the device (powered by supply Vin) that can normally operate. For the VDET+_max, it can be given by the following formula: V DET+_max + V DET−_max ) V HYS_max (eq. 1) Where: VDET−_max = Maximum detector threshold voltages VHYS_max = Maximum detector threshold hysteresis For the NCP30X family, for given VDET−_typ typical detection voltage which reflects on the part number at data sheet, the device’s threshold values are designed to the following targets (at 25°C). V DET−_min + V DET−_typ * 2% (eq. 2) V DET−_max + V DET−_typ * 2% (eq. 3) V HYS_typ + 5% of V DET−_typ (eq. 4) V HYS_min + V HYS_typ * 40% (eq. 5) V HYS_max + V HYS_typ * 40% (eq. 6) The below table shows how those information can be found in the data sheet: VDET-_min VDET-_typ VDET-_max VHYS_min VHYS_typ VHYS_max By simple mathematical re−combination of Equations 2 to 6, Equation 1 becomes: V DET+_max + V DET−_typ 1.09 So, VDET+_max can be easily figure out by just using a single variable VDET_typ. For having the value of VDET+_max, the NCP30X device detection voltage option must be chosen such that: V CC_min t V DET+_max t V in_min The physical meaning of VCC_min < VDET+_max is that it makes sure the reset from NCP30X is asserted (in RESET hold state) before Vin supply becomes higher than VCC_min for prevention from incorrect device (uP) initialization. For the VDET+_max < Vin_min, it makes sure the NCP30X is able to start up even though Vin is at the Vin_min. Theoretically speaking, by principle of two points averaging, the ideal detection voltage threshold value, VDET−_typ_ideal, can be given by the following formula: V DET−_typ_ideal + ǒV in_min ) V CC_minǓ ń (2 http://onsemi.com 3 1.09) AND8425/D Example of Detection Voltage Threshold Selection Calculation 1. Power supply output specification: 3.3 V $ 3% 2. Microprocessor core voltage specification: 3.3 V $ 5% Vo: 3.3V +/− 3% Vcc: 3.3V +/− 5% Vo VIN Vcc RST RST Power Supply NCP30X uP GND GND GND So, we have: Vin_min = 3.3 V * 0.97 = 3.201 V VCC_min = 3.3 V * 0.95 = 3.135 V Recall the formula V DET−_typ_ideal + ǒV in_min ) V CC_minǓ ń (2 1.09) So, the ideal detection voltage option = (3.201 + 3.135) / (2 * 1.09) = 2.9064 V Therefore, the device detection voltage option 2.9 V should be the right choice. That is to say, customer should select the part number NCP30xxSx29T1G. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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