TPS3431
SNVSB66A – JULY 2018 – REVISED OCTOBER 2021
TPS3431 Standard Programmable Watchdog Timer with Enable
1 Features
3 Description
•
The TPS3431 is a standard programmable watchdog
timer with an enable feature for a wide variety of
applications. The watchdog timeout features a 15%
accuracy, high-precision timing (–40°C to +125°C)
and 2.5% typical at 25°C. The watchdog timeout can
be programmed either by an external capacitor, or
by factory-programmed default delay settings. The
watchdog can be disabled via the Enable pin or the
SET logic pins to avoid undesired watchdog timeouts
during the development process.
•
•
•
•
•
•
•
•
Factory-programmed precision watchdog timers:
– ±2.5% Accurate watchdog timeout (WDT)
typical at 25°C
Watchdog disable feature
User-programmable watchdog timeout
Input voltage range: VDD = 1.8 V to 6.5 V
Low quiescent current: IDD = 10 µA (typical)
Active-low open-drain output
Enable input (EN) and enable output (ENOUT)
Available in a small 3-mm × 3-mm, 8-pin VSON
package
Junction operating temperature range:
–40°C to +125°C
The TPS3431 is available in a small 3.00-mm ×
3.00-mm, 8-pin VSON package.
Device Information(1)
2 Applications
PART NUMBER
Cellular module asset tracker
Energy storage power conversion system (PCS)
Electricity meter
Motion detector (PIR, uWave, etc.)
IP network camera
Surgical equipment
3.3 V
VDD
TPS3431
VDD
SET1
EN
NC
*1
CWD
Microcontroller
ENOUT
*2
WDO
WDI
GND
NMI (enable output)
NMI (watchdog fault)
GPIO
GND
Copyright © 2018, Texas Instruments Incorporated
A.
B.
EN can also be left floating and is internally pulled-up to VDD
ENOUT can also be left floating or tied to WDO
Standard Watchdog Timer Circuit
PACKAGE
TPS3431
(1)
BODY SIZE (NOM)
VSON (8)
3.00 mm × 3.00 mm
For all available packages, see the orderable addendum at
the end of the data sheet.
Normalized Window Watchdog Timeout (%)
•
•
•
•
•
•
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
-50
-25
0
25
50
Temperature (qC)
75
100
125
TPS3
Normalized Watchdog Timeout (tWD) Accuracy
(SET1 = 1, CWD = NC)
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
TPS3431
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Table of Contents
1 Features............................................................................1
2 Applications..................................................................... 1
3 Description.......................................................................1
4 Revision History.............................................................. 2
5 Pin Configuration and Functions...................................3
6 Specifications.................................................................. 4
6.1 Absolute Maximum Ratings........................................ 4
6.2 ESD Ratings............................................................... 4
6.3 Recommended Operating Conditions.........................4
6.4 Thermal Information....................................................5
6.5 Electrical Characteristics.............................................5
6.6 Timing Requirements.................................................. 6
6.7 Timing Diagrams ........................................................ 7
6.8 Typical Characteristics................................................ 8
7 Detailed Description........................................................9
7.1 Overview..................................................................... 9
7.2 Functional Block Diagram........................................... 9
7.3 Feature Description.....................................................9
7.4 Device Functional Modes..........................................11
8 Application and Implementation.................................. 13
8.1 Application Information............................................. 13
8.2 Typical Application.................................................... 15
8.3 Programmable Application........................................17
9 Power Supply Recommendations................................20
10 Layout...........................................................................21
10.1 Layout Guidelines................................................... 21
10.2 Layout Example...................................................... 21
11 Device and Documentation Support..........................22
11.1 Device Support........................................................22
11.2 Documentation Support.......................................... 22
11.3 Receiving Notification of Documentation Updates.. 22
11.4 Support Resources................................................. 22
11.5 Trademarks............................................................. 22
11.6 Electrostatic Discharge Caution.............................. 22
11.7 Glossary.................................................................. 22
12 Mechanical, Packaging, and Orderable
Information.................................................................... 22
4 Revision History
Changes from Revision * (July 2018) to Revision A (June 2021)
Page
• Removed "±15% Accurate Watchdog Timeout and Watchdog Reset Delays Over Temperature"..................... 1
• Updated the Applications section with web links................................................................................................ 1
• Removed "15% timing accuracy (–40°C to +125°C),"........................................................................................ 1
• Updated ESD Ratings.........................................................................................................................................4
• Updated ICWD min and max spec........................................................................................................................5
• Updated VCWD min and max spec...................................................................................................................... 5
• Added a footnote to for tINIT ............................................................................................................................... 6
• Updated tWDU min and max boundry values from 0.85 and 1.15 to 0.905 and 1.095 respectively...................14
• Updated tWDU min and max values for all capacitors........................................................................................14
• Updated the equations 3 and 4 with tWD min and max boundry values from 0.85 and 1.15 to 0.905 and 1.095
respectively.......................................................................................................................................................17
2
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5 Pin Configuration and Functions
VDD
ENOUT
CWD
WDO
EN
*1
WDI
SET1
GND
A.
B.
*2
EN can also be left floating and is internally pulled-up to VDD
ENOUT can also be left floating or tied to WDO
Figure 5-1. DRB Package: TPS3431
3-mm × 3-mm VSON-8
Top View
Table 5-1. Pin Functions
PIN
NAME
NO.
I/O
DESCRIPTION
VDD
1
I
Supply voltage pin. For noisy systems, connecting a 0.1-μF bypass capacitor is recommended.
CWD
2
I
Programmable watchdog timeout input. The watchdog timeout is set by connecting a capacitor between this pin and
ground. Connecting via a 10-kΩ resistor to VDD or leaving unconnected further enables the selection of the preset
watchdog timeouts; see the CWD Functionality section.
TheTPS3431 determines the watchdog timeout using Equation 1
Enable input pin. This pin is internally pulled up to VDD and must be logic high or left floating. When EN goes logic
low, ENOUT goes logic low and WDI is ignored and WDO remains logic high. When EN goes logic high, ENOUT goes
high (asserts) after the watchdog reset delay time (tRST). This pin can also be driven with an external push-button,
transistor, or microcontroller.
EN
3
I
GND
4
—
SET1
5
I
Logic input. Grounding the SET1 pin disables the watchdog timer. SET1 and CWD select the watchdog timeouts; see
the SET1 section.
Ground pin
WDI
6
I
Watchdog input. A falling edge must occur at WDI before the timeout (tWD) expires.
When the watchdog is not in use, the SET1 pin can be used to disable the watchdog. WDI is ignored when WDO is
low (asserted) and when the watchdog is disabled. If the watchdog is disabled, WDI cannot be left unconnected and
must be driven to either VDD or GND.
WDO
7
O
Watchdog open-drain active-low output. Connect WDO with a 1-kΩ to 100-kΩ resistor to the correct pull-up voltage
rail (VPU). WDO goes low (asserts) when a watchdog timeout occurs. When a watchdog timeout occurs, WDO goes
low (asserts) for the watchdog reset delay time (tRST). When EN goes low, WDO is in a high-impedance state and will
be pulled to logic high.
ENOUT
8
O
Enable open-drain active-high output. Connect ENOUT with a 1-kΩ to 100-kΩ resistor to the correct pull-up voltage
rail (VPU). When EN goes logic high, ENOUT goes high impedance and pulls logic high (asserts) due to the external
pull-up resistor after the watchdog reset delay time (tRST). When EN is forced logic low, ENOUT goes low after 200 ns
and remains logic low as long as EN is logic low.
—
Connect the thermal pad to a large-area ground plane. The thermal pad is internally connected to GND.
Thermal pad
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)(1)
MIN
MAX
UNIT
Supply voltage range
VDD
–0.3
7
V
Output voltage range
ENOUT, WDO
–0.3
7
V
SET1, WDI, EN
–0.3
7
CWD
–0.3
VDD + 0.3(3)
Voltage ranges
Output pin current
ENOUT, WDO
Input current (all pins)
Temperature
(1)
(2)
(3)
±20
mA
±20
mA
See Section
6.4
See Section
6.4
Operating junction, TJ (2)
–40
150
Operating free-air temperature, TA (2)
–40
150
Storage, Tstg
–65
150
Continuous total power dissipation
V
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress
ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under
Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device
reliability.
TJ = TA as a result of the low dissipated power in this device.
The absolute maximum rating is VDD + 0.3 V or 7.0 V, whichever is smaller.
6.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC
JS-001(1)
±4000
Charged-device model (CDM), per JEDEC
specification JESD22-C101(2)
±1000
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process. Manufacturing with
less than 500-V HBM is possible with the necessary precautions.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process. Manufacturing with
less than 250-V CDM is possible with the necessary precautions.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
MIN
VDD
Supply pin voltage
VSET1
SET1 pin voltage
MAX
UNIT
1.8
6.5
V
0
6.5
V
0.1(1)
1000(1)
nF
kΩ
CCWD
Watchdog timing capacitor
CWD
Pullup resistor to VDD
9
10
11
RPU
Pullup resistor, ENOUT and WDO
1
10
100
kΩ
IEN
EN pin current
10
mA
IWDO
Watchdog output current
10
mA
TJ
Junction Temperature
125
°C
(1)
4
NOM
–40
Using a CCWD capacitor of 0.1 nF or 1000 nF gives a tWDU(typ) of 62.74 ms or 77.45 seconds, respectively.
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6.4 Thermal Information
TPS3431
THERMAL
METRIC(1)
UNIT
DRB (VSON)
8 PINS
RθJA
Junction-to-ambient thermal resistance
50.7
RθJC(top)
Junction-to-case (top) thermal resistance
51.6
RθJB
Junction-to-board thermal resistance
25.8
ψJT
Junction-to-top characterization parameter
1.3
ψJB
Junction-to-board characterization parameter
25.8
RθJC(bot)
Junction-to-case (bottom) thermal resistance
7.1
(1)
°C/W
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
6.5 Electrical Characteristics
at 1.8 V ≤ VDD ≤ 6.5 V over the operating temperature range of –40°C ≤ TJ ≤ +125°C (unless otherwise noted); the
open-drain pullup resistors are 10 kΩ; typical values are at TJ = 25°C
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
GENERAL CHARACTERISTICS
VDD (2) (3)
Supply voltage
IDD
Supply current
VPOR (1)
Power-on reset voltage
1.8
10
VOL(MAX) = 0.25 V
6.5
V
19
µA
0.8
V
700
nA
WINDOW WATCHDOG FUNCTION
IEN
EN pin internal pullup current
VEN = 0V
ICWD
CWD pin charge current
CWD = 0.5 V
VCWD
CWD pin threshold voltage
VOL
ENOUT, WDO output low
VDD = 5 V, ISINK = 3 mA
ID
ENOUT, WDO output leakage current
VDD = 1.8 V, VWDO = 6.5 V
VIL
Low-level input voltage (EN, SET1)
VIH
High-level input voltage (EN, SET1)
VIL(WDI)
Low-level input voltage (WDI)
VIH(WDI)
High-level input voltage (WDI)
(1)
(2)
(3)
500
620
347
375
403
nA
1.196
1.21
1.224
V
0.4
V
1
µA
0.25
V
0.8
V
0.3 × VDD
0.8 × VDD
V
V
When VDD falls below VPOR, WDI and ENOUT is undefined.
When VDD falls below VDDMIN, WDI is ignored and ENOUT is driven low
During power-on, VDD must be a minimum 1.8 V for at least 300 µs before WDI is active and ENOUT is high impedance.
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6.6 Timing Requirements
MIN
TYP
MAX
UNIT
GENERAL
CWD pin evaluation period (1)
tINIT
EN, SET1 pin setup time
Startup delay(2)
381
µs
1
µs
300
µs
200
ns
DELAY FUNCTION
tEN_ENOUT
EN to ENOUT delay
tRST
Watchdog reset delay
170
200
230
ms
1360
1600
1840
ms
170
200
230
ms
WINDOW WATCHDOG FUNCTION
CWD = NC, SET1 = 1
tWD
Watchdog timeout
tWD-setup
tWD-del
(1)
(2)
6
CWD = 10 kΩ to VDD, SET1 = 1
CWD = NC, SET1 = 0
Watchdog disabled
CWD = 10 kΩ to VDD, SET1 = 0
Watchdog disabled
Setup time required for device to respond to changes on WDI after being enabled
150
µs
Minimum WDI pulse duration
50
ns
WDI to WDO delay
50
ns
Refer to Section 8.1.1.2.
During power-on, VDD must be a minimum 1.8 V for at least 300 µs before WDI is active and ENOUT is high impedance.
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6.7 Timing Diagrams
VDD (min)
VDD V
POR
tRST
t < tWD
WDI
t = tWD(1)
X
WDO
tRST
Figure 6-1. Timing Diagram
A.
See Figure 6-2 for WDI timing requirements.
WDI
Correct
Operation
WDO
WDI
Late Fault
WDO
Timing
Valid
Region
tWD(MIN)
tWD(TYP)
tWD(MAX)
= Tolerance Window
Figure 6-2. Watchdog Timing Diagram
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6.8 Typical Characteristics
16
0.7
12
0.6
Enable Threshold (V)
Supply Current (PA)
all typical characteristics curves are taken at 25°C with 1.8 V ≤ VDD ≤ 6.5 V (unless other wise noted)
8
-40qC
0qC
25qC
105qC
125qC
4
1
2
3
4
VDD (V)
5
6
0.4
0.3
-50
0
0
0.5
7
VIL
VIH
-25
0
25
50
Temperature ( °C)
75
100
125
VDD = 1.8 V
Figure 6-3. Supply Current vs VDD
Figure 6-4. EN Threshold vs Temperature
Normalized Window Watchdog Timeout (%)
CWD Charging Current (nA)
380
376
372
368
1.6 V
6.5 V
364
-50
-25
0
25
50
Temperature (qC)
75
100
125
Normalized Window Watchdog Timeout (%)
Figure 6-5. CWD Charging Current vs Temperature
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
-50
-25
0
25
50
Temperature (qC)
75
100
125
TPS3
Figure 6-6. Normalized Watchdog Timeout (tWD)
Accuracy (SET1 = 1, CWD = NC)
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
-50
-25
0
25
50
Temperature (qC)
75
100
125
TPS3
Figure 6-7. Normalized Watchdog Timeout (tWD) Accuracy (SET1 = 1, CWD = 10kΩ)
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7 Detailed Description
7.1 Overview
The TPS3431 is a standard programmable watchdog timer with enable/disable feature. This device includes a
precision watchdog timer that achieves 15% timing accuracy over the specified temperature range of –40°C to
+125°C.
7.2 Functional Block Diagram
ENOUT
VDD
Precision
Clock
WDO
VDD
CWD
State
Machine
Cap
Control
VDD
EN
WDI
SET1
GND
7.3 Feature Description
7.3.1 Enable Input (EN) and Enable Output (ENOUT)
The Enable (EN) input allows a processor or other logic circuits to initiate a single cycle watchdog reset by
momentarily bringing Enable low, or a permanent disable by keeping Enable low. After EN goes to a logic high
and VDD is above VDD (min), ENOUT and WDO go logic high after the watchdog reset delay time (tRST). If EN is
not controlled externally, then EN can either be connected to VDD or left floating because the EN pin is internally
pulled up to VDD. When EN is forced logic low, ENOUT goes low after a propagation delay of 200 ns and WDO
goes high impedance and pulls to logic high due to the external pull-up resistor. Because WDO and ENOUT are
both open-drain outputs, these outputs can be tied together to create an OR logic function so that if either output
pulls down to logic low, the other will also pull down logic low.
7.3.2 Watchdog Mode
This section provides information for the watchdog mode of operation.
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7.3.2.1 CWD
The CWD pin provides the user the functionality of both high-precision, factory-programmed watchdog timing
options and user-programmable watchdog timing. The TPS3431 features three options for setting the watchdog
timer: connecting a capacitor to the CWD pin, connecting a pull-up resistor to VDD, and leaving the CWD pin
unconnected. The configuration of the CWD pin is evaluated by the device every time VDD rises above VDD (min).
The pin evaluation is controlled by an internal state machine that determines which option is connected to the
CWD pin. The sequence of events typically takes 381 μs (tINIT) to determine if the CWD pin is left unconnected,
pulled-up through a resistor, or connected to a capacitor. If the CWD pin is being pulled up to VDD, a 10-kΩ
resistor is required.
7.3.2.2 Watchdog Input WDI
WDI is the watchdog timer input that controls the WDO output. The WDI input is triggered by the falling edge
of the input signal. To ensure proper functionality of the watchdog timer, always issue the WDI pulse before
tWD(min). If the pulse is issued in this region, then WDO remains unasserted. Otherwise, the device asserts WDO,
putting the WDO pin into a low-impedance state therefore WDO will be logic low.
The watchdog input (WDI) is a digital pin. To ensure there is no increase in IDD, drive the WDI pin to either VDD
or GND at all times. Putting the pin to an intermediate voltage can cause an increase in supply current (IDD)
because of the architecture of the digital logic gates. When EN is logic low, the watchdog is disabled and all
signals input to WDI are ignored. When EN is logic high, the device resumes normal operation and no longer
ignores the signal on WDI. If the watchdog is disabled, drive the WDI pin to either VDD or GND. Figure 7-1
shows the valid region for a WDI pulse to be issued to prevent WDO from being triggered and pulled low.
WDI
Correct
Operation
WDO
WDI
Late Fault
WDO
Timing
Valid
Region
tWD(MIN)
tWD(TYP)
tWD(MAX)
= Tolerance Window
Figure 7-1. Watchdog Timing Diagram
10
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7.3.2.3 Watchdog Output WDO
The TPS3431 features an active-low open-drain watchdog output that asserts when a pulse on WDI fails to
arrive within the watchdog timeout. When EN is logic high, the WDO signal maintains normal operation. When
the EN pin is logic low, the WDO pin goes to a high-impedance state and pulls logic high due to the external
pull-up resistor. Because WDO and ENOUT are both open-drain outputs, these outputs can be tied together to
create an OR logic function so that if either output pulls down to logic low, the other will also pull down logic low.
7.3.2.4 SET1
The SET1 pin can enable and disable the watchdog timer and should be used when disabling the watchdog
timer for longer than one watchdog reset cycle. If SET1 is set to GND, the watchdog timer is disabled and WDI
is ignored. If the watchdog timer is disabled, drive the WDI pin to either GND or VDD to ensure that there is
no increase in IDD. When SET1 is logic high, the watchdog operates normally. The SET1 pin can be changed
dynamically; however, if the watchdog is going from disabled to enabled there is a 150 µs setup time where the
watchdog does not respond to changes on WDI, as shown in Figure 7-2. Note: disabling using SET1 pin causes
a delay defined by the fixed 150-us setup time when enabling again.
VDD
EN
t RST
ENOUT
SET1
150 µs
Watchdog
Enabled/Disabled
Enabled
Disabled
when ENOUT or SET1 is logic low
Enabled
Figure 7-2. Enabling and Disabling the Watchdog
7.4 Device Functional Modes
Table 7-1 summarizes the functional modes of the TPS3431.
Table 7-1. Device Functional Modes
(1)
(2)
VDD
EN
ENOUT
WDI
WDO
VDD < VPOR
---
---
---
---
VPOR ≤ VDD < VDD(min)
---
Low
Ignored
High
VDD > VDD (min) (1)
High
High
tPULSE < tWD(min)
High
VDD > VDD (min) (1)
High
High
tPULSE > tWD(min)
(2)
Low
VDD > VDD (min) (1)
Low
Low
Ignored
High
(2)
VDD must be above VDD (min)for longer than 300 µs.
Where tpulse is the time between the falling edges on WDI.
7.4.1 VDD is Below VPOR ( VDD < VPOR)
When VDD is less than VPOR, WDO is undefined and can be either high or low. The state of WDO largely
depends on the load that the WDO pin is experiencing.
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7.4.2 Above Power-On-Reset, But Less Than VDD(min) (VPOR ≤ VDD < VDD(min))
When the voltage on VDD is less than VDD(min), and greater than or equal to VPOR, the WDO signal is asserted
(logic low). When EN is logic low, the watchdog output WDO is in a high-impedance state and logic low
regardless of the WDI signal that is input to the device.
7.4.3 Normal Operation (VDD ≥ VDD(min))
When VDD is greater than or equal to VDD(min) and EN is logic high, the WDO signal is determined by WDI. When
WDI is within the watchdog timeout, the internal MOSFET turns off and WDO is pulled high through external
pull-up resistor. When WDI is not within the watchdog timeout, the internal MOSFET turns on and WDO is pulled
to logic low. When EN is logic low, ENOUT goes to logic low and WDO goes to a high-impedance state and pulls
to logic high due to the external pull-up resistor.
12
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8 Application and Implementation
Note
Information in the following applications sections is not part of the TI component specification,
and TI does not warrant its accuracy or completeness. TI’s customers are responsible for
determining suitability of components for their purposes, as well as validating and testing their design
implementation to confirm system functionality.
8.1 Application Information
The following sections describe in detail proper device implementation, depending on the final application
requirements.
8.1.1 CWD Functionality
The TPS3431 features three options for setting the watchdog timeout: connecting a capacitor to the CWD pin,
connecting a pull-up resistor to VDD, and leaving the CWD pin unconnected. Figure 8-1 shows a schematic
drawing of all three options. If this pin is connected to VDD through a 10-kΩ pullup resistor or left unconnected
(high impedance), then the factory-programmed watchdog timeouts are enabled; see the Section 8.1.1.1 section.
Otherwise, the watchdog timeout can be adjusted by placing a capacitor from the CWD pin to ground.
TPS3431
VDD
TPS3431
VDD
VDD
VDD
10 k
375 nA
VDD
TPS3431
VDD
375 nA
375 nA
CWD
CWD
CWD
CCWD
Cap
Control
Cap
Control
Cap
Control
User Programmable
Capacitor to GND
CWD
Unconnected
10 NŸ 5HVLVWRU
to VDD
Figure 8-1. CWD Charging Circuit
8.1.1.1 Factory-Programmed Timing Options
If using the factory-programmed timing options (listed in Table 8-1), the CWD pin must either be unconnected
or pulled up to VDD through a 10-kΩ pull-up resistor. Using these options enables high-precision, 15% accurate
watchdog timing.
Table 8-1. Factory Programmed Watchdog Timing
INPUT
STANDARD WATCHDOG TIMEOUT WDT (tWD)
CWD
SET1
NC
0
NC
1
10 kΩ to VDD
0
10 kΩ to VDD
1
MIN
TYP
MAX
UNIT
Watchdog disabled
1360
1600
1840
ms
230
ms
Watchdog disabled
170
200
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8.1.1.2 CWD Adjustable Capacitor Watchdog Timeout
Adjustable capacitor timing is achievable by connecting a capacitor to the CWD pin. If a capacitor is connected
to CWD, then a 375-nA, constant-current source charges CCWD until VCWD = 1.21 V. Table 8-2 shows how to
calculate tWD using Equation 1 and the SET1 pin. The TPS3431 determines the watchdog timeout with the
formulas given in Equation 1, where CCWD is in nanofarads and tWD is in milliseconds.
tWD(ms) = 77.4 x CCWD(nF) + 55 (ms)
(1)
The TPS3431 is designed and tested using CCWD capacitors between 100 pF and 1 µF. Note that Equation 1
is for ideal capacitors and capacitor tolerances vary the actual device timing. For the most accurate timing, use
ceramic capacitors with COG dielectric material. If a CCWD capacitor is used, Equation 1 can be used to set
tWD for the watchdog timeout. Table 8-3 shows the minimum and maximum calculated tWD values using an ideal
capacitor.
Table 8-2. Programmable CWD Timing
WATCHDOG TIMEOUT WDT (tWD)(1)
INPUT
(1)
CWD
SET1
CCWD
0
CCWD
1
MIN
TYP
MAX
tWD Equation 1
tWD × 1.095
UNIT
Watchdog disabled
tWD × 0.905
ms
Calculated from Equation 1 using an ideal capacitor.
Table 8-3. tWD Values for Common Ideal Capacitor Values
CCWD
(1)
14
WATCHDOG TIMEOUT WDT (tWD)
UNIT
MIN(1)
TYP
MAX(1)
100 pF
56.77
62.74
68.7
ms
1 nF
119.82
132.4
144.98
ms
10 nF
750
829
908
ms
100 nF
7054
7795
8536
ms
1 μF
70096
77455
84814
ms
The minimum and maximum values are calculated using an ideal capacitor.
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8.2 Typical Application
TPS3431
VDD
ENOUT
SET1
EN
NC
*2
CWD
Microcontroller
NMI (enable output)
NMI (watchdog fault)
WDO
*1
VDD
100 NŸ
100 k
3.3 V
WDI
GPIO
GND
GND
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A.
B.
EN can also be left floating and is internally pulled-up to VDD
ENOUT can also be left floating or tied to WDO
Figure 8-2. Monitoring a Microcontroller with Standard Watchdog Timer
8.2.1 Design 1 Requirements
PARAMETER
DESIGN REQUIREMENT
DESIGN RESULT
Output logic voltage
3.3V Open-Drain
3.3V Open-Drain
Watchdog Timeout
Leave CWD disconnected: 1.6 seconds (typical)
tWD(min) = 1360 ms, tWD(TYP) = 1600 ms, tWD(max) =
1840 ms
Maximum device current
consumption
35 µA
33 µA when WDO is asserted
8.2.2 Detailed Design 1 Procedure
8.2.2.1 Calculating WDO Pullup Resistor Design 1
The TPS3431 uses an open-drain configuration for the WDO circuit, as shown in Figure 8-3. When the internal
MOSFET is off, the external pull-up resistor pulls the drain of the transistor to VDD and when the MOSFET
is turned on, the MOSFET attempts to pull the drain to ground, thus creating an effective resistor divider. The
resistors in this divider must be chosen to ensure that VOL is below the maximum value.
To choose the proper pull-up resistor, there are three key specifications to keep in mind: the pull-up voltage
(VPU), the recommended maximum WDO pin current (IWDO), and VOL.
The maximum VOL is 0.4 V, meaning that the effective resistor divider created must be able to bring the voltage
on the reset pin below 0.4 V with IWDO kept below 10 mA. For this example, with a VPU of 3.3 V, a resistor
must be chosen to keep IWDO below 35 μA because this value is the maximum consumption current allowed. To
ensure this specification is met, a pull-up resistor value of 100 kΩ was selected, which sinks a maximum of 33
μA when WDO is asserted.
VDD
WDO
WATCHDOG
CONTROL
Figure 8-3. WDO Open-Drain Configuration
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8.2.2.2 Setting the Watchdog Design 1
As illustrated in Figure 8-1 there are three options for setting the watchdog timer. The design specifications
in this application allow for a factory-programmed timing option by leaving CWD floating. To ensure proper
functionality, a falling edge must be issued before tWD(min) with is set for 1.36 seconds when CWD is not
connected. Figure 8-8 illustrates that a WDI signal with a period of 1 second keeps WDO from asserting.
Figure 8-4 shows WDO asserting when the WDI signal has a period longer than tWD(max) which is 1.84 seconds
when CWD is not connected. Figure 8-5 shows a watchdog fault caused by missing WDI pulse followed by
correct timing WDI pulses to deactivate WDO.
8.2.3 Application Curves Design 1
Unless otherwise stated, application curves were taken at TA = 25°C.
VDD
VDD
Ignore all pulses that occur before t RST
First falling edge must occur before WDU
No pulse on WDI before WDU(max) so WDO asserts
Second falling edge occurs after WDU(max) triggering a watchdog reset
Falling edge on WDI occurs within WDU
WDI
WDI
WDO
WDO
WDO remains unasserted while
WDI is within watchdog window
WDO resets for tRST
Figure 8-4. WDO Fault Caused by WDI Pulse
Arriving Too Late (After tWD(max))
16
Figure 8-5. WDO Fault Caused by missing WDI
Pulses Followed by Correct Timing WDI Pulses
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8.3 Programmable Application
EN
MR
RESET
CT
4.7 µF
GND
TPS3431
VDD
SENSE
100 k
100 k
TPS3890
VDD
100 k
1.8 V
ENOUT
SET1
WDI
GND
2.7 nF
NMI (enable output)
WDO
CWD
VCORE
Microcontroller
NMI (watchdog fault)
GPIO (WDI)
GND
GPIO (SET1)
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Figure 8-6. Monitoring the Supply Voltage and Watchdog Supervision of a Microcontroller
8.3.1 Design 2 Requirements
PARAMETER
DESIGN REQUIREMENT
DESIGN RESULT
Watchdog disable for initialization Watchdog must remain disabled for 5 seconds until
5.02 seconds (typ)
period
logic enables the watchdog timer
Programmable disable feature
Microcontroller controls SET1 on TPS3431 via a
GPIO
The Microcontroller can disable TPS3431 via SET1
and thus disable the watchdog for any reason.
Output logic voltage
1.8-V Open-Drain
1.8V Open-Drain
Monitored rail (TPS3890)
1.8 V with a 5% threshold and 1% accuracy
Worst-case VITN = 1.714 V – 4.7%
Watchdog timeout (TPS3431)
265 ms typical
tWD(min) = 213 ms, tWD(TYP) = 264 ms, tWD(max) =
319 ms
Maximum device current
consumption
50 µA
37 µA when WDO is asserted
8.3.2 Detailed Design 2 Procedure
8.3.2.1 Calculating WDO Pullup Resistor Design 2
The TPS3431 uses an open-drain configuration for the WDO circuit. When the internal MOSFET is off, the
external pull-up resistor pulls the drain of the transistor to VDD and when the MOSFET is turned on, the
MOSFET attempts to pull the drain to ground, thus creating an effective resistor divider. The resistors in this
divider must be chosen to ensure that VOL is below the maximum value. To choose the proper pull-up resistor,
there are three key specifications to keep in mind: the pull-up voltage (VPU), the recommended maximum WDO
pin current (IWDO), and VOL. The maximum VOL is 0.4 V, meaning that the effective resistor divider created
must be able to bring the voltage on the reset pin below 0.4 V with IWDO kept below 10 mA. For this example,
with a VPU of 1.8 V, a resistor must be chosen to keep IWDO below 50 μA because this value is the maximum
consumption current allowed. To ensure this specification is met, a pull-up resistor value of 100 kΩ was selected,
which sinks a maximum of 18 μA when WDO is asserted.
8.3.2.2 Setting the Watchdog Design 2
As illustrated in Figure 8-1 there are three options for setting the watchdog timer. The design specifications
in this application require the programmable timing option (external capacitor connected to CWD). When a
capacitor is connected to the CWD pin, the watchdog timer is governed by Equation 1. This equation estimation
is only valid for ideal capacitors and any temperature or voltage derating must be accounted for separately.
CCWD (nF) = (tWD(ms) – 55) / 77.4 = (265 – 55) / 77.4 = 2.71 nF
(2)
The nearest standard capacitor value is 2.7 nF. Selecting 2.7 nF for the CCWD capacitor gives the following
minimum and maximum timing parameters:
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tWD(MIN) = 0.905 x tWD(TYP) = 0.905 x (77.4 x 2.7 + 55) = 238.902 ms
(3)
tWD(MAX) = 1.095 x tWD(TYP) = 1.095x (77.4 x 2.7 + 55) = 289.058 ms
(4)
Capacitor tolerance also influences tWD(MIN) and tWD(MAX). Select a ceramic COG dielectric capacitor for high
accuracy. For 2.7 nF, COG capacitors are readily available with 5% tolerances. This selection results in a 5%
decrease in tWD(MIN) and a 5% increase in tWD(MAX), giving 213.16 ms and 318.75 ms, respectively. To ensure
proper functionality, a falling edge must be issued before tWD(min). Figure 8-8 illustrates that a WDI signal with a
period of 260 ms keeps WDO from asserting.
8.3.2.3 Watchdog Disabled During Initialization Period Design 2
The watchdog is often needed to be disabled during startup to allow for an initialization period. When the
initialization period is over, the watchdog timer is turned back on to allow the microcontroller to be monitored by
the TPS3431. To achieve this setup, EN on TPS3431 is controlled by TPS3890 supervisor. In this application,
the TPS3890 was chosen to monitor VDD as well, which means that the RESET on the TPS3890 stays low until
VDD rises above VITN. When VDD comes up, the delay time can be adjusted through the CT capacitor on the
TPS3890. With this approach, the RESET delay can be adjusted from a minimum of 25 μs to a maximum of
30 seconds. For this design, a typical delay of 5 seconds is needed before the watchdog timer is enabled. The
CT capacitor calculation (see the TPS3890 data sheet) yields an ideal capacitance of 4.67 μF, giving a closest
standard ceramic capacitor value of 4.7 μF. When connecting a 4.7 μF capacitor from CT to GND, the typical
delay time is 5 seconds. Figure 8-7 shows that when the watchdog is disabled, the WDO output remains high.
However when SET1 goes high and there is no WDI signal, WDO begins to assert. See the TPS3890 datasheet
for detailed information on the TPS3890. The ENOUT pin on the TPS3431 reflects the status of the EN pin
and can be connected to the microcontroller for monitoring or can be left floating if not being used. When the
TPS3431 is disabled, ENOUT is logic low and WDO is logic high so the user can also tie ENOUT to WDO to
force WDO to logic low when TPS3431 is disabled.
8.3.2.4 Programmable Disable Feature Design 2
The watchdog is often needed to be disabled during operation to prevent false watchdog faults. When the
watchdog is disabled, all pulses or lack of pulses on WDI are ignored and WDO is high impedance as shown in
Figure 8-9. When the watchdog is re-enabled, the watchdog timer is turned back on after a watchdog start-up
delay of 150 µs to allow the microcontroller to be monitored by the TPS3431. To achieve this setup, SET1
on TPS3431 is controlled by a GPIO on the microcontroller and must be logic high to enable to watchdog.
To disable the watchdog, the microcontroller sets the GPIO connected to SET1 to logic low. To re-enable the
watchdog, the microcontroller sets the GPIO connected to SET1 back to logic high. This configuration is useful
when another device or signal is already using the EN pin on TPS3431, and a programmable disable feature
with minimal delay upon enable is still required. When the watchdog is disabled using SET1 instead of EN,
ENOUT remains unaffected which is useful when needing to disable the watchdog but not causing another
device connected to ENOUT to be disabled.
8.3.3 Application Curves Design 2
Unless otherwise stated, application curves were taken at TA = 25°C.
VDD
2 V/div
VDD
2 V/div
6 seconds
SET1
2 V/div
265 ms
WDI
2 V/div
WDO
2 V/div
WDO
2 V/div
1 s/div
Figure 8-7. Startup Without a WDI Signal
18
Figure 8-8. Typical WDI Signal
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VDD
When watchdog is disabled, WDI is ignored
WDI
WDO is high-impedance and remains logic high due to pull-up resistor
WDO
Figure 8-9. Watchdog Disabled With Missing WDI Pulse
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9 Power Supply Recommendations
This device is designed to operate from an input supply with a voltage range between 1.8 V and 6.5 V. An input
supply capacitor is not required for this device; however, if the input supply is noisy, then good analog practice is
to place a 0.1-µF capacitor between the VDD pin and the GND pin.
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10 Layout
10.1 Layout Guidelines
•
•
•
Make sure that the connection to the VDD pin is low impedance. Good analog design practice is to place a
0.1-µF ceramic capacitor as near as possible to the VDD pin.
If a CCWD capacitor or pull-up resistor is used, place these components as close as possible to the CWD pin.
If the CWD pin is left unconnected, make sure to minimize the amount of parasitic capacitance on the pin.
Place the pull-up resistor on WDO as close to the pin as possible.
10.2 Layout Example
CVDD
Vin
RPU2
Vin
CCWD
VDD
1
8
ENOUT *2
CWD
2
7
WDO
EN *1
3
6
WDI
GND
4
5
SET1
GND Plane
Denotes a via
A.
B.
EN can also be left floating and is internally pulled-up to VDD
ENOUT can also be left floating or tied to WDO
Figure 10-1. TPS3431 Recommended Layout
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11 Device and Documentation Support
11.1 Device Support
11.2 Documentation Support
11.2.1 Related Documentation
For related documentation see the following:
• TPS3890 Low Quiescent Current, 1% Accurate Supervisor with Programmable Delay (SLVSD65)
• TPS3431EVM-780 Evaluation Module (SBVU033)
11.3 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. Click on
Subscribe to updates to register and receive a weekly digest of any product information that has changed. For
change details, review the revision history included in any revised document.
11.4 Support Resources
TI E2E™ support forums are an engineer's go-to source for fast, verified answers and design help — straight
from the experts. Search existing answers or ask your own question to get the quick design help you need.
Linked content is provided "AS IS" by the respective contributors. They do not constitute TI specifications and do
not necessarily reflect TI's views; see TI's Terms of Use.
11.5 Trademarks
TI E2E™ is a trademark of Texas Instruments.
All trademarks are the property of their respective owners.
11.6 Electrostatic Discharge Caution
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled
with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric changes could cause the device not to meet its published
specifications.
11.7 Glossary
TI Glossary
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
22
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PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
TPS3431SDRBR
ACTIVE
SON
DRB
8
3000
RoHS & Green
NIPDAU
Level-1-260C-UNLIM
-40 to 125
431DD
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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