LM5069_10

Output Voltage Clamping Using the LM5069 Hot Swap Controller Output Voltage Clamping Using the LM5069 Hot Swap Controller The Issue One of the many benefits of using the LM5069 Hot Swap Controller, besides inrush current limiting and fault monitoring, is that the controller supplies a voltage to the load that is between defined limits. This feature prevents the load from receiving a voltage less than what it is rated for (which could result in erratic behavior), and prevents it from receiving a voltage higher than what it is rated for, which could result in overheating and/or damage. The voltage limits to the load are set by the UVLO (Under-Voltage Lock Out) and OVLO (Over- National Semiconductor Application Note 2040 Dennis Morgan March 15, 2010 Voltage Lock Out) thresholds, which are set with external resistors (R1-R3 in Figure 1). While the OVLO function can be used to keep excessive voltages at VIN from reaching the load at VOUT, a potential drawback to this method of over-voltage protection is that the voltage at VOUT is shut off for the duration of the over-voltage condition. This can (and likely will) result in a shutdown of the load circuitry, followed by a restart – an event which may interrupt the normal operation of other associated circuitry. 30119201 FIGURE 1. LM5069 Basic Application Circuit AN-2040 © 2010 National Semiconductor Corporation 301192 www.national.com AN-2040 The Solution The solution presented here is to limit the voltage at VOUT to a maximum value, rather than shut it off, when an over-volt- age condition appears at VIN. This is accomplished by adding a zener diode at the GATE pin of the LM5069, as shown in Figure 2. 30119202 FIGURE 2. Zener Clamp Added to the Gate In normal operation (when the over-voltage condition is not present) the GATE pin of the LM5069 is approximately 12V above the voltage at VOUT. As the voltage at VIN increases above the normal operating range, Q1’s gate voltage is clamped by Z1. As VIN continues to increase, the voltage at VOUT is clamped when Q1’s gate-to-source voltage (VGS) reduces to a level where Q1 limits the current to the load. Any additional increase in VIN is absorbed by Q1 as there is no additional increase in the voltage at VOUT. The choice of zener voltage for Z1 depends not only on the voltage at which VOUT is to be clamped, but also on the specific MOSFET chosen for Q1 – more specifically, its VGS characteristics. Typically Z1 should have a zener voltage approximately 2V to 5V above the desired clamping voltage at VOUT. Using the Transfer Characteristics information in the MOSFET’s datasheet can provide an initial value for the difference between Z1’s voltage and the desired clamping voltage at VOUT. The final selection for Z1 should be determined experimentally. www.national.com 2 AN-2040 Test Results Two LM5069 circuits were tested with different value zener diodes for Z1. In both tests the pertinent external components were: - RS = 20 mohms - RPWR = 12.5 kohms - Power Limit = 5 Watts - Q1 = Vishay SUM40N15-38 A) In the first test, a 20V zener diode was used for Z1. The circuit’s load resistance was 40 ohms, resistive. The result, shown in Figure 3, is that the GATE pin was clamped at approximately 20V for VIN >11V, and the output voltage was clamped at between 15.6V and 17V as VIN was increased from 16V to 27V. In this test circuit, increasing VIN above 27V activated the power limiting feature of the LM5069, and Q1 was shut off after the fault timeout period. 30119208 FIGURE 3. Z1 = 20V Transient testing was performed by quickly increasing VIN from 10V to 20V for 20 milliseconds, and then returning VIN to 10V. The scope photo in Figure 4 shows the results: 30119209 Trace 1: VOUT (10V/div) Trace 2: GATE pin (10/V/div) Trace 3: TIMER pin (2V/div) Trace 4: VIN (10V/div) FIGURE 4. Transient Testing with Z1 = 20V 3 www.national.com AN-2040 Referring to Figure 4, initially VIN and VOUT are at 10V, and the GATE pin is at approximately 17.6V. The voltage at the TIMER pin is at zero since the load current is below the current limit threshold. When VIN increases quickly from 10V to 20V, there is a momentary current surge through R S and Q1 to the capacitor at VOUT (CL). The current surge amplitude reached the Circuit Breaker limit of the LM5069, which triggered the strong pull-down at the GATE pin.* This is why the GATE pin voltage quickly dropped almost 10V when VIN increased, and the voltage at VOUT dropped a small amount as the load current was momentarily reduced. Immediately after that, the circuit breaker function is shut off (due to the reduced current level), and the current limit feature of the LM5069 then limits the current through RS and Q1. During this time of current limiting (approximately 3 ms in Figure 4) the voltage at the TIMER pin increases since current limiting is a fault condition. As the voltage at VOUT increases the voltage at the GATE pin increases with it until it reaches the clamp voltage set by Z1 (19.91V in Figure 4). At this point the voltage at VOUT is clamped at 15.55V, and remains at that level for the remaining time that VIN is at 20V. During the time that VOUT is clamped, the load current (0.389A) is below the current limit threshold, and the power dissipated in Q1 (1.73W) is below the power limit threshold. The TIMER pin voltage is decreasing back towards zero volts. When the voltage at VIN drops back to 10V, the voltage levels at the GATE pin and at VOUT reduce back to their original levels. *The current surge amplitude was verified with a current probe in series with RS. B) In the second test, a 56V zener diode was used for Z1. The circuit’s load resistance was 140 ohms, resistive. The result, shown in Figure 5, is that the GATE pin was clamped at approximately 56V for VIN >44V, and the output voltage was clamped at between 51V and 51.5V as VIN was increased from 52V to 62V. In this test circuit, increasing VIN above 62V activated the power limiting feature of the LM5069, and Q1 was shut off after the fault timeout period. 30119210 FIGURE 5. Z1 = 56V Transient testing was performed by quickly increasing VIN from 48V to 60V for 20 milliseconds, and then returning VIN to 48V. The load was 240 ohms, resistive, for this test. The scope photo in Figure 6 shows the results: www.national.com 4 AN-2040 30119211 Trace 1: VOUT (20V/div) Trace 2: GATE pin (20V/div) Trace 3: TIMER pin (1V/div) Trace 4: VIN (20V/div) FIGURE 6. Transient Testing with Z1 = 56V Referring to Figure 6, initially VIN and VOUT are at 48V. The GATE pin, which would normally be at 60V if Z1 were not present, is at approximately 55.8V due to Z1. But the gate-tosource voltage is sufficient to fully enhance Q1’s gate so that VOUT is not clamped, and is equal to VIN. The voltage at the TIMER pin is at zero since the load current is below the current limit threshold. When VIN increases quickly from 48V to 60V, there is a momentary current surge through R S and Q1 to the capacitor at VOUT (CL). The current surge amplitude reached the Circuit Breaker limit of the LM5069, which triggered the strong pull-down at the GATE pin.* This is why the GATE pin voltage quickly dropped 10V when VIN increased, and the voltage at VOUT dropped a small amount as the load current was momentarily reduced. Immediately after that, the circuit breaker function is shut off (due to the reduced current level), and the current limit feature of the LM5069 then limits the current through RS and Q1. During this time of current limiting (approximately 2 ms in Figure 6) the voltage at the TIMER pin increases since current limiting is a fault condition. As the voltage at VOUT increases the voltage at the GATE pin increases with it until it reaches the clamp voltage set by Z1 (55.8V in Figure 6). At this point the voltage at VOUT is clamped at 51.1V, and remains at that level for the remaining time that VIN is at 60V. During the time that VOUT is clamped, the load current (0.213A) is below the current limit threshold, and the power dissipated in Q1 (1.9W) is below the power limit threshold. The TIMER pin voltage is decreasing back towards zero volts. When the voltage at VIN reduces back to 48V, the voltage at VOUT reduces back to 48V. The GATE pin voltage remains at 55.8V since VIN is above 44V. *The current surge amplitude was verified with a current probe in series with RS. Transient Testing and Fault Timeout In the above two transient tests, the simulated transients were repetitive with VIN held at its higher level for 20 ms, and at the lower level for 80 ms. The LM5069 did not produce a fault timeout since the TIMER pin voltage, which increased during the brief current limiting period, was able to reduce to zero before the next transient arrived. However, a fault timeout, with the accompanying shutdown of Q1, would occur if any of the following test changes were made: - The upper voltage level at VIN was increased, or - The load current was increased by reducing the load resistance, or - The time interval between transients was decreased. If any of these changes were made, the TIMER pin voltage would not be able to decrease to zero during each transient cycle, causing the pin’s voltage to repetitively increase with the arrival of each transient. When the voltage at the TIMER pin reached 4 volts, Q1 was shut off. In an actual application, if transients are known to be very frequent, the TIMER pin should be monitored on a scope during testing of this proposed solution to see if the voltage stays near zero, or if it drifts up a significant amount with the arrival of each transient. If the TIMER pin voltage reaches 4V at any time Q1 is shut off. The conditions required to cause a fault timeout and a shutdown of Q1 are different for each application. 5 www.national.com AN-2040 Power Dissipation in Q1 Implementing this voltage clamp requires re-evaluating the possible power dissipation in Q1. During an over-voltage condition where VOUT is clamped, the power dissipated by Q1 is due to the voltage difference across Q1 (VIN – VOUT), and the load current. The different scenarios which can result are: - If the power dissipation in Q1 is less than the maximum power limit allowed by the LM5069 (set by RPWR and RS), and/ or the duration of the over-voltage condition is known to be less than the fault timeout set by CT, a fault timeout does not occur. The appropriate limit line in the MOSFET’s SOA chart can be used to determine the maximum power limit setting. - In the case where the duration of the over-voltage condition is extended (possibly lasting several seconds) rather than brief, and the power dissipation in Q1 is less than the maximum power limit allowed by the LM5069, a fault timeout does not occur. However, in this case Q1 can dissipate significant power for the extended time. The DC limit line of the MOSFET’s SOA chart must be checked, and the heat sink provided for Q1 must be reviewed. - If the power dissipation in Q1 reaches the maximum power limit allowed by the LM5069, the fault timer is activated. If the duration of the over-voltage condition is less than the LM5069’s fault timeout period (set by CT), the circuit returns to normal operation (VOUT = VIN) when the over-voltage condition subsides. But if the duration of the over-voltage condition is longer than the fault timeout period, Q1 is shut off at the end of the fault timeout period. Diode Selection As mentioned above, the selection of a diode for Z1 should be determined experimentally. The voltage at which VOUT is clamped during an over-voltage condition depends not only on Z1 and its tolerances, but also on Q1’s VGS characteristics, the load current, and Q1’s junction temperature. As for the power rating required for Z1, the current which flows through Z1 is the current supplied from the GATE pin of the LM5069, which is nominally 16 µA. This current flows through Z1 only when it is actively clamping the LM5069’s GATE pin. Change the OVLO Threshold When implementing this output voltage clamp, the OVLO threshold must either be disabled, or set higher than the maximum voltage expected at VIN during the transient or overvoltage condition. Otherwise, Q1 is shut off anytime the voltage at VIN exceeds the OVLO threshold. To disable the OVLO function connect the OVLO pin to Ground. To change the OVLO threshold, see the LM5069 datasheet for the procedure to calculate new values for the external resistors. PGD Output When VOUT is clamped during an over-voltage condition, a voltage difference exists across Q1 (VIN - VOUT). If that voltage difference exceeds 2.5V the PGD output switches low. When the over-voltage condition subsides, and the voltage across Q1 decreases below 1.25V, Q1 switches high. www.national.com 6 AN-2040 Notes 7 www.national.com Output Voltage Clamping Using the LM5069 Hot Swap Controller Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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