Texas Instruments | Discrete Solution for TPS24711 to Achieve Insertion ON Delay and Fast Turn OFF | Application notes | Texas Instruments Discrete Solution for TPS24711 to Achieve Insertion ON Delay and Fast Turn OFF Application notes

Texas Instruments Discrete Solution for TPS24711 to Achieve Insertion ON Delay and Fast Turn OFF Application notes
Application Report
SLVAE29 – March 2019
Discrete solution for TPS24711 to achieve insertion ON
delay and fast turn OFF
Johnny Guo
Jack Chen
ABSTRACT
The TPS24711 is an easy-to-use, 2.5 V to 18 V, hot-swap controller that safely drives an external NMOS.
The programmable current limit, MOSFET power limit and fault time protect the supply load and MOSFET
at startup. While in some chassis Switch, Router and Server application, insertion turn ON delay and fast
turn OFF feature is required due to input voltage ringing in insertion and removal. This application report
introduces a performance-cost-optimization discrete solution with TPS24711 to achieve these features.
Simulation and experiment test result is also given.
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Contents
Introduction ...................................................................................................................
Application Problem in Insertion and Removal ..........................................................................
Insertion Turn ON delay and Fast Turn OFF Circuit ...................................................................
Experiment Test Result ....................................................................................................
Conclusion ...................................................................................................................
Reference ....................................................................................................................
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List of Figures
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Board VIN and Backplane Bus Voltage Ringing in Insertion........................................................... 2
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Board VIN and VOUT in Removal ......................................................................................... 3
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Simulation Circuit ............................................................................................................ 4
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Insertion Turn ON Delay Simulation Result .............................................................................. 5
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Fast Turn OFF Simulation Result.......................................................................................... 5
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Insertion Turn ON Delay Test Result
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Fast Turn OFF Test Result
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Trademarks
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1
Introduction
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Introduction
In chassis applications, Hotswap is a typical requirement for higher system reliability. The TPS24711 is an
easy-to-use, 2.5 V to 18 V, hot-swap controller that safely drives an external NMOS. The programmable
current limit, MOSFET power limit and fault time protect the supply load and MOSFET at startup.
Programmable power limiting ensures the external MOSFET operates inside its safe operating area (SOA)
at all times. This allows the use of smaller MOSFET while improving system reliability.
While in some chassis application like Campus Switch, Edge Router and Blade Server, input voltage
ringing brings risk to system working and MOSFET, which is a design consideration. Thus insertion turn
ON delay and fast turn OFF feature is required. TPS24711 device does not have this feature, so a low
cost discrete solution is designed and verified through simulation and bench test.
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Application Problem in Insertion and Removal
During board insertion and removal, the board VIN and backplane bus voltage ringing is a normal
phenomenon, and the ringing range, peak and valley depends on the backplane power stability, inserting
board input capacitance and parasitic inductance in VIN/GND on-board routing.
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Board Insertion
Figure 1 is a typical plug-in board VIN and bus voltage ringing in insertion.
Figure 1. Board VIN and Backplane Bus Voltage Ringing in Insertion
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When a board is inserted, the input capacitor is charged to bus voltage, pulling charge out of the bus
and causing an LC ringing on the VIN of inserting board. A turn ON delay is recommended for inserting
board.
While on the backplane bus, the ringing appears because the CIN of inserting board is charged from
existing board bus, causing bus ringing on backplane and existing board.
The VIN valley drops to a low value and influence the bus voltage of existing board in chassis system,
especially when the system power supply stability is not quite good, it may trigger VIN OFF threshold
of existing board.
In order to ensure inserting board to turn ON after VIN becomes smooth and stable after ringing, insertion
turn ON delay feature is required. Considering the backplane ringing valley voltage will influence other
existing board, Hotswap OFF threshold should be set to a proper value.
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Discrete solution for TPS24711 to achieve insertion ON delay and fast turn
OFF
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Application Problem in Insertion and Removal
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2.2
Board Removal
Figure 2 shows a plug-out board voltage ringing in removal.
Figure 2. Board VIN and VOUT in Removal
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When the board is pulled out, the VIN and VOUT of removing board starts to drop from bus voltage to
0 V.
At VIN side, mechanical contact between power connector and backplane will cause extra ringing.
At this time, if Hotswap controller is still working and MOSFET is still ON. This mechanical contact
caused ringing at VIN will bring high ΔV across MOSFET drain-to-source, which leads to a high peak
current flowing through MOSFET. So MOSFET fast turn OFF when VIN reaches OFF threshold is
required in system.
Insertion Turn ON delay and Fast Turn OFF Circuit
According to the application problem analysis, the design requirements are listed below.
• A particular insertion turn ON delay should be designed.
• VIN OFF voltage should not be set too high to avoid existing board shutdown in insertion.
• A fast turn OFF control should be designed to avoid MOSFET high current in removal.
Below Hotswap design specification is raised based on chassis system.
• Turn ON delay after insertion: >= 1 s
• Hotswap ON/OFF threshold: VIN = 6.8 V / 6.65 V
• Hotswapturn OFF delay when EN drops below threshold: < 5 ms
Aiming to achieve above features, a discrete solution connected to TPS24711 EN pin is designed as
shown in below Figure 3.
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Insertion Turn ON delay and Fast Turn OFF Circuit
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Figure 3. Simulation Circuit
The circuit 1 is a resistor divider and ceramic capacitor to achieve insertion turn ON delay function. When
board is plugged in, TPS24711 EN pin voltage is controlled by R2, R3 and C1. EN typical rising threshold
voltage is 1.3 V with 50 mV hysteresis.
When VIN drops to OFF threshold, the EN pin voltage may be still higher than EN falling threshold voltage
1.25 V due to slow discharge of existing big capacitor C1. The circuit 2 is designed for fast turn OFF after
VIN falls to OFF threshold. A PMOS and 10 Ω resistor is used for discharging EN C1 capacitor voltage.
And the discharging starting voltage is controlled by R1, R5 and 5.1 V Zener diode.
Figure 4 and Figure 5 shows the simulation result of Figure 3 circuit. 1.3 s turn ON delay and 2.56 ms turn
OFF time is achieved.
4
Discrete solution for TPS24711 to achieve insertion ON delay and fast turn
OFF
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Insertion Turn ON delay and Fast Turn OFF Circuit
Figure 4. Insertion Turn ON Delay Simulation Result
Figure 5. Fast Turn OFF Simulation Result
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Experiment Test Result
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Experiment Test Result
Based on discrete circuit design, parameter calculation and simulation result, below Figure 6 and Figure 7
is bench test result of turn ON delay and fast turn OFF.
From test waveform, the insertion turn ON delay is 1.31 s, the fast turn OFF delay after EN drops below
OFF threshold is 0.5 ms.
Figure 6. Insertion Turn ON Delay Test Result
Figure 7. Fast Turn OFF Test Result
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Discrete solution for TPS24711 to achieve insertion ON delay and fast turn
OFF
Copyright © 2019, Texas Instruments Incorporated
SLVAE29 – March 2019
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Conclusion
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5
Conclusion
Hotswap insertion turn ON delay and fast turn OFF feature is required in some chassis system. A low-cost
discrete solution is proposed to achieve this feature. Simulation and experiment result shows the solution
works well. Besides, thousands of board insertion and removal test shows that the reliability of this
discrete solution is high.
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Reference
1. TPS2471x 2.5-V to 18-V High-Efficiency Power-Limiting Hot-Swap Controller, TI Datasheet
2. Using the TPS24700EVM-001, TPS24701EVM002, TPS24710EVM-003 and the TPS24711EVM004,
TI User's Guide
3. Handling System Transients in Hot Swap Applications, TI Application Report
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