cd00151404

AN2504
Application note
Using STMicroelectronics Voltage Detectors
1
Introduction
Voltage detectors are designed to monitor the system supply voltage and assert the output
signal, OUT, every time it goes below a defined voltage threshold.
The major advantages of these circuits are low current consumption and a precise
temperature-compensated voltage reference, VREF (see Figure 1). STMicroelectronics
voltage detectors are also laser programmed to the desired voltage threshold over the range
of 1.6V to 6.0V in 100mV steps and they have a good transient immunity (see Figure 5).
Figure 1.
N-Channel Open Drain Output Block Diagram
VCC
VOUT
R1
+
–
R2
VREF
VSS
R3
AI10482b
Note that voltage detector output state simply indicates if the supply voltage is above or
below a specific threshold, which can be used for early power fail warning.
However microprocessors' inputs require some minimum input pulse width to register the
change of signal on the logical input. In this case we recommend using reset circuits, which
guarantee the minimum pulse width known as reset time-out period. Reset circuits have
some other advantages compared to voltage detectors (e.g. specification across
temperature, better voltage threshold accuracy, and lower temperature coefficient). On the
other hand voltage detectors are usually able to operate at higher voltages, have lower
current consumption, greater hysteresis and usually a lower price.
The qualities mentioned above cause voltage detectors to be preferentially used in the
following applications:
• Battery Monitoring
• Power-Supply Monitoring • Portable Medical Devices
• Power Fail Detection
• PDAs
• Notebook Computers
• Back-up Supply Switching
• Portable/Battery-
• Cell Phones.
Powered Electronics
February 2007
Rev 1
1/14
www.st.com
Contents
AN2504 - Application note
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4
Transient Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5
Open Drain Output Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6
Modifying the Voltage Threshold, VTH– . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7
Simulation of the Reset Time-out Period . . . . . . . . . . . . . . . . . . . . . . . . 9
8
Output MOSFET protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9
2/14
8.1
External N-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2
External P-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
AN2504 - Application note
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
N-Channel Open Drain Output Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
STM1061N Active-Low, Open Drain typical Hardware Hookup . . . . . . . . . . . . . . . . . . . . . . 4
Voltage timing waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Maximum Transient Duration vs. Reset Threshold Overdrive . . . . . . . . . . . . . . . . . . . . . . . 6
Additional Transient Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Voltage Detectors with Wire OR Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Voltage Detector Monitors Different Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Modifying the Voltage Threshold, VTH– . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Hardware Hookup with RC element on the Voltage Detector Output . . . . . . . . . . . . . . . . . . 9
Voltage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Circuit with external N-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Circuit with external P-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3/14
Description
2
AN2504 - Application note
Description
STMicroelectronics offers e.g. the STM1061 voltage detector. One of its typical uses is
shown in the Figure 2.
The STM1061N open drain output voltage detector is monitoring the 3.6V battery. This
allows the voltage detector to detect a voltage drop early, assert the output signal (OUT) so
the MCU can start safeguard routines even before the regulated supply voltage for the MCU
starts to fall.
The voltage detector open drain output sinks current when the output is asserted. It is
necessary to connect a pull-up resistor from OUT to any supply voltage (see Figure 2). The
resistor value must be large enough to register a logic low and small enough to register a
logic high while all of the input current and leakage paths connected to the reset output line
are being supplied. A10kΩ pull-up is sufficient in most applications.
Figure 2.
STM1061N Active-Low, Open Drain typical Hardware Hookup
DC/DC
Converter
VIN
VOUT
Regulated 3V
VSS
R
VCC
3.6V
+
STM1061N
(Open Drain)
OUT
VCC
VSS
MCU
VOUT
Suspend
VSS
AI10484a
4/14
AN2504 - Application note
3
Operation
Operation
The voltage detector monitors VCC voltage input continuously (see Figure 1) and compares
it with the precision voltage reference, V REF. When VCC falls below a specified trip point
threshold, the output (OUT) is forced low and remains asserted as long as the VCC input
remains below VTH+, where VTH+ = VTH– + VHYST (hysteresis) see Figure 3.
Remember that a pull-up resistor on the voltage detector open-drain output is required for
proper functionality (see Figure 2).
Figure 3.
Voltage timing waveform
VCC
VTH+
VTH–
Release Voltage (VTH+ = VTH– + VHYST)
Detect Voltage
VCC (min)
VSS
VOUT
tPD
Detect Delay
VSS
tPR
Release Delay
AI10483
5/14
Transient Immunity
4
AN2504 - Application note
Transient Immunity
The STM1061 device is relatively immune to negative-going V CC transients (glitches). The
graph (see Figure 4) indicates the maximum pulse width a negative VCC transient can have
without asserting output signal, OUT. As the magnitude of the transient increases (further
below the threshold), the maximum allowable pulse width decreases. Any combination of
duration and overdrive which lies under the curve will NOT assert the output signal, OUT.
A 0.1µF bypass capacitor, Cb, mounted as close as possible to the VCC pin provides
additional transient immunity (see Figure 5).
Figure 4.
Maximum Transient Duration vs. Reset Threshold Overdrive
160
Transient Duration (µs)
140
VTH– = 3.4V
120
VTH– = 1.6V
100
80
60
40
20
0
1
10
100
1000
10000
Threshold Overdrive (mV)
Figure 5.
AI11123b
Additional Transient Immunity
VCC
Cb
R
VCC
STM1061N
OUT
Power Good
VSS
AI12905a
6/14
AN2504 - Application note
5
Open Drain Output Advantages
Open Drain Output Advantages
The advantages of open drain output are the ability to connect more open drain outputs in
parallel (wired OR connections, see Figure 6) as well as connect the output to a power
supply voltage different from VCC (see Figure 7).
The hook up on Figure 6 monitors 2 independent supply voltages (3V and 5V). Every time
either the first OR the second supply voltage goes below defined threshold, the input signal
of the MCU, Suspend, goes low.
Figure 6.
Voltage Detectors with Wire OR Connection
3V
VCC
R
VCC
MCU
STM1061N
OUT
Suspend
VSS
VSS
5V
VCC
STM1061N
OUT
VSS
AI12906
A voltage detector with an open drain output can also monitor supply voltages different from
the MCU supply voltage (see Figure 7). The logic high on the MCU Suspend input is
adequate to supply voltage of the MCU.
Figure 7.
Voltage Detector Monitors Different Supply Voltage
5V
3V
R
VCC
VCC
MCU
STM1061N
OUT
VSS
Suspend
VSS
AI12907
7/14
Modifying the Voltage Threshold, VTH–
6
AN2504 - Application note
Modifying the Voltage Threshold, VTH–
Although the STM1061 voltage thresholds, V TH–, are adjusted in fine steps (100mV), it is
sometimes necessary to make adjustments during prototyping. This can be achieved by
connecting of external divider (see Figure 8). This hook up can be used if the required
threshold of voltage detector is lower than the desired monitored voltage.
To maintain detector accuracy, the current flow through the divider should be significantly
higher than the 0.9µA operating current required by the STM1061. A 90µA bleeder current is
sufficient in most applications.
Figure 8.
Modifying the Voltage Threshold, VTH–
VSUPPLY
R2
STM1061N
VCC
R1
OUT
VSS
AI12910
Example:
Threshold of STM1061 detector:
VTH– = 1.6V
Desired threshold:
VSUPPLY = 1.8V
Recommended bleeder current:
I = 90µA
R1+R2 is then:
(R1+R2) = VSUPPLY/I = 1.8V/90µA = 20kΩ
The voltage on the divider is:
R1
V CC = --------------------V
= V TH –
R 1 + R 2 SUPPLY
That is why:
V TH –
1.6V
R 1 = ------------------------ ( R + R 2 ) = ------------20kΩ = 17.7kΩ
1.8V
V SUPPLY 1
The value of R2 resistor is then:
R 2 = 20kΩ – 17.7k Ω = 2.2kΩ
We will choose the nearest values of the resistors (e.g. from E24):
R1 = 18kΩ, R2 = 2.2kΩ.
8/14
AN2504 - Application note
7
Simulation of the Reset Time-out Period
Simulation of the Reset Time-out Period
It is possible to simulate reset time-out period with an RC element on the output of voltage
detector (see Figure 9).
Figure 9.
Hardware Hookup with RC element on the Voltage Detector Output
VCC
VCC
R
VCC
MCU
STM1061N
OUT
Suspend
C
VSS
VCAP
VSS
AI12911a
If the supply voltage, V CC, drops below the defined voltage threshold, VRST, the capacitor, C,
is discharged through the output transistor of the voltage detector (see Figure 10).
When the supply voltage, VCC, rises above the voltage threshold plus hysteresis,
VRST + VHYST, the output transistor disconnects the output from the ground and the
capacitor, C, is charged through the resistor, R.
Figure 10. Voltage Waveforms
V
VCC
VRST – VHYST
VRST
VCAP
VIH
Capacitor is charging
Capacitor is
discharging
t1
t2
t3
t
AI12912
9/14
Simulation of the Reset Time-out Period
AN2504 - Application note
The voltage on the STM1061N output resp. capacitor, VCAP, rises as capacitor is charged.
When it reaches input high voltage trip point of the MCU (time t3), the high state is detected.
If there is no RC element on the output, the OUT pin goes high at the time t2. That is why
simulated reset time-out period is t3-t2.
Use following formula for determining the correct capacitor value:
t
C = -------------------------------------------V CC
R ⋅ I n -------------------------V CC – V IH
where,
C = Capacitor of RC element in Farads,
R = resistor of RC element in Ohms,
t = desired time-out period (t3-t 2) in seconds,
VCC = supply voltage,
VIH = input voltage trip point for the MCU
ln = natural logarithm.
Example:
The STM1061N27WX6F is used to monitor a 3V supply voltage. The MCU detects input
high state at VIH =0.7·VCC = 0.7·3V = 2.1V. Desired time-out period is 140ms. An R = 100Ω
resistor is chosen.
The value of the capacitor is following:
–3
–6
140 ⋅ 10
t
C = ------------------------------------------ = --------------------------------------------------------- = 1.16 ⋅ 10 = 1.16µF
3
V CC
3
100 ⋅ 10 ⋅ I n ----------------R ⋅ I n -------------------------3 – 2.1
V CC – V IH
The closest higher capacitance value from the most common E12 series (i.e. C = 1.2µF) is
chosen. The time-out delay will be slightly greater by choosing a higher value of capacitor.
Remember that this circuitry should be used only for temporary purposes. In the case of real
need of time-out delay the usage of reset device is highly recommended as mentioned in
the Introduction.
10/14
AN2504 - Application note
8
Output MOSFET protection
Output MOSFET protection
For applications requiring higher current drive capabilities an external MOSFET might be
used, which protect the output of a voltage detector from overloading and destroying. It is
possible to use either N channel or P channel transistor (see below).
8.1
External N-channel transistor
If the load should be connected to the supply voltage during the regular operation (output,
OUT, is not asserted), an external N-channel transistor will be needed (see Figure 11).
When the supply voltage VCC drops below the threshold of the voltage detector, the output
signal, OUT, is asserted, the external N-channel MOSFET is switched off and disconnects
the load from the ground.
Figure 11. Circuit with external N-channel transistor
R
VCC
VCC
RLOAD
VCC
STM1061N
OUT
VSS
AI12913
11/14
Output MOSFET protection
8.2
AN2504 - Application note
External P-channel transistor
If the load should be connected to the supply voltage during the output, OUT, assertion, an
external P-channel transistor will be needed (see Figure 12). When the supply voltage V CC
drops below the threshold of the voltage detector, the output signal, OUT, is asserted, the
external P-channel MOSFET is switched on and connects the load to the supply voltage,
VCC.
Figure 12. Circuit with external P-channel transistor
R
VCC
VCC
VCC
STM1061N
OUT
RLOAD
VSS
AI12913a
12/14
AN2504 - Application note
9
Revision history
Revision history
Table 1.
Document revision history
Date
Revision
06-Feb-2007
1
Changes
Initial release.
13/14
AN2504 - Application note
W
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