LINEAR Power Buck Regulator Safely Rides Through Automotive Load Dumps Design Note 475 handbook
Contents
1. DESIGN ECHNOLOGY NO T E S UPower Buck Regulator Safely Rides Through Automotive Load Dumps Design Note 475 Juan G Aranda Introduction When abuck regulator is used in applications where output short circuitand overload conditions occur the duty cycle required to keep the inductor current under control can be lower than the regulator s minimum duty cycle when operating at full speed Therefore to effectively protect a traditional regulator under such conditions its switching frequency must be reduced to a speed that can safely handle the maximum expected input voltage In some cases frequency foldback can help reduce the effective duty cycle by reducing the switching frequency as the output voltage falls out of regulation However this technique might not provide enough protection if the folded frequency is not low enough In the end the duty cycle problem imposes alimit onthe maximum switching frequency at which the regulator can be safely operated especially in automotive applications where the input supply can see positive voltages several times higher than the normal 12V operating voltage The LT 3682 is a new 1A buck regulator that overcomes the duty cycle limitation by monitoring the currentthrough the external catch diode and delaying the generation of new switch pulses if this current exceeds a defined value The LT3682 safely accommodates output shorts and overload condi2. 5VouT application with a 800kHz and 1 MHz programmed switching frequency respectively The resistive load at Vout is increased until the regulator hits its maximum current limit Figures 3 4 6 and 7 show the DA pin cur rent sense protection for input voltages of 12V and 36V for both applications In all cases the lowest value of the inductor current is pinned to about 1 1A which keeps its peak value well under control By delaying the generation of new switch pulses the switching frequency is effec tively reduced to satisfy the new duty cycle requirements introduced by the fault condition 1A DIV Vsw m 20V DIV f DN475 F03 1us DIV Figure 3 The 800kHz Application with Viy 12V Overload Condition Forces Voyz to Drop to About 3 2V ia E E EE ee ee ee A ee 1A DIV M DOWD FPP DN475 F04 1us DIV Figure 4 The 800kHz Application with Viy 36V Overload Condition Forces Voyy to Drop to About 3 5V Data Sheet Download www linearcom geil ata bight Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linear com Conclusion The LT3682 is a 1A monolithic buck switching regula tor that accepts input voltages from 3 6V to 36V and transients up to 60V It features an adjustable and synchronizable switching frequency from 250kHz to 2 2MHz It also has the ability to monitor the current flowing through the external catch di
3. ate the situation further Under extreme overloads the internal current limit is ultimately clamped to its maximum value jy and the output voltage falls out of regulation reducing the negative slope of the inductor 47 LI LTC LTM Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation All other trademarks are the property of their respective owners VIN V 6 9V TO 36V TRANSIENT DIA Vin gt 6 9V YA Vin gt 6 TO 60V 1A Viy gt 12V Cin 2 2UF Court 10uF f 800kHz Figure 2 A Typical 800kHz Application 02 10 475 current accordingly If during the switch off time the inductor current does not return to the same or smaller values it had at the end of the previous cycle its peak value will ratchet higher in every cycle and increase to unacceptably high levels Monitoring Current Via the DA Pin The LT3682 constantly monitors the inductor current during the switch off time by looking at the current flow ing through the external catch diode via the DA diode anode pin see Figure 1 and delays the generation of new switch pulses if this current does not fall below a defined threshold Ipa thus reducing the regulator s ef fective duty cycle Thus itis now possible to use the small footprint inductors found in high frequency applications without sacrificing robustness during anumber of fault conditions Figures 2 and 5 show how to configure the LT3682 in a
4. ode thus providing an extra level of protection against output fault conditions over the entire operating frequency range regardless of input voltage These features together with its typical 75uA no load quiescent current makes the LT3682 the right choice in high frequency automotive and battery powered applications VIN 9V TO 19 5V Vout TRANSIENT TOA TO 60V VIN BD i RUN SS BOOST CN Ve LT3682 2 2uF Ry PG DA c SYNC FB 10uF GND PGND Figure 5 A Typical 1 7MHz Application VouT _ 1A DIV ane all QOV DIV peter fas DN475 F06 500ns DIV Figure 6 The 1 7MHz Application with Viy 12V Overload Condition Forces Voyy to Drop to About 4 4V VOUT Poot _ TA DIV F DOVE ceded a r lea tec Meier toll DN475 F07 500ns DIV Figure 7 The 1 7MHz Application with Viy 36V Overload Condition Forces Voyy to Drop to About 4 4V For applications help call 408 432 1900 Ext 3513 dn475 LT TP 0210 116K PRINTED IN THE USA LY MYR LINEAR TECHNOLOGY CORPORATION 2010
5. tions up to its maximum adjustable operating frequency of 2 2MHz regardless of input volt age This added level of protection allows automotive systems designers to take advantage of the maximum SW INHIBIT DN475 F01 Figure 1 The LT3682 Monitors the Current Flowing Through the External Catch Diode at the DA Pin switching frequency of the regulator without concern for transients on the input supply The LT3682 accepts input voltages from 3 6V to 36V and transients up to 60V Additional features such as soft Start power good flag frequency foldback and thermal shutdown are all included in the thermally enhanced 12 lead 3mm x 3mm DFN package Minimum Switch On Time Under normal load conditions the internal switch current limit is regulated to meet the required peak inductor cur rent Dueto internal delays the power switch does notturn off immediately when the internal currentlimit is reached butinstead takes the minimum on time ton miny t0 do so This delay allows the inductor current to continue rising to values that depend on the current slope and the value Of toncmin During positive input voltage transients the Slope of the rising inductor current is greatly increased thus raising its peak value A well designed buck converter must skip switch pulses to maintain regulation if the new duty cycle requirements cannot be met due to minimum on time limitations Overload conditions during the input transients may ag grav
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LINEAR Power Buck Regulator Safely Rides Through Automotive Load Dumps Design Note 475 handbook