Detection Voltage Selection Guidelines for Application of NCP30X Family Series

AND8425/D
Detection Voltage Selection
Guidelines for Application
of NCP30X Family Series
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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
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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%.
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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
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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.
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