Atmel AVR180, Application Note

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AVR180
3
• Alternative 3: High accuracy. High-quality semiconductor
ICs are used to build an accurate brown-out detector
with low-power consumption.
Design Hint: Supply Voltage Filtering
Use low impedance capacitors (low ESR and ESL) on the
V
CC
and multi-layer PCB with power planes to improve
transient rejection from the power supply.
Alternative 1: Low-Power Consumption
Characteristics
•
Very Low-Power Consumption, (Typ 0.5
µ
A@3V, 1
µ
A@5V)
•
Low-Cost
•
Large Hysteresis, Typ. 0.3 Volts
•
Fast Output Transitions
•
Accuracy
±
5-10%
•
High Component Count
•
Long Response Time on V
CC
Figure 1. Low-Power Consumption Brown-Out Detector
Figure 2. These Oscilloscope Plots Show How the Voltage on RESET Varies with V
CC
Introduction
The circuit in Figure 1 benefits from low-power consump-
tion, which makes it suitable for battery operated applica-
tions. Standard discrete components give a low cost
design.
The voltage transition on the RESET pin is very steep.
Combined with the large hysteresis, the accuracy is high.
On the other hand, the response time is slow, which makes
it unsuitable for rapidly varying supply voltages.
Theory of Operation
This detector has two stages, the detector and the ampli-
fier. In the detector stage, the threshold voltage is set by
the resistors R1 and R2 in relation to the critical voltage of
transistor T1. Under normal operation, this transistor is
conducting, When the supply voltage drops below the
threshold voltage, the transistor shuts off.
The output from this detector is lead to the input of the ultra
low power amplifier stage. Under normal operation, the low
voltage of the base of transistor T2 causes it to remain
shut, allowing resistor R5 to pull the RESET input high. The
amplifier stage also contains a hysteresis feedback loop
through transistor T3, shorting resistor R3 in the amplifier
when the RESET output is kept low.
V
CC
C3
R1
R2
R3
R4
R5
ISP
AVR
V
CC
100 - 500K
GND
OPTIONAL
RESET
SWITCH RESET
T2
C2
T1
C1
T3
<50K