PWM DC-DC Cntrllrs w/Built-In Strt-Up Regs Simp …

1 LM5020,LM5025,LM5030 PWM DC-DC Controllers with Built-In Start-Up Regulators Simplify Switching Power Supply DesignLiterature Number: SNVA568 Featured ProductsVol. III, Issue :Simplified Power Supply DesignModular Current Sharing ControllerThe LM5080 is a simple and cost-effective load-share controllerthat provides all of the functions required to balance the currentsdelivered from multiple power converters operated in parallel. TheLM5080 implements an average program method of active loadshare control which adjusts the output voltage of individual powerstages either up or down to deliver nearly equal currents to a common load. The average program method improves stability andreduces the output voltage tolerance when compared to othercommon load sharing methods. The LM5080 supports two commonapplications for load share controllers: external control in whichthe load share circuit balances currents between separate powermodules (bricks), and internal control where the load share circuitis integrated into the voltage regulation loop of each power con-verter module or Single-wire star link current share bus No precision external resistors necessary 3V to 15V bias voltage range Adaptable for high or low side current sensing Flexible architecture allows 4 modes of operation.

2 Negative remote sense adjustment Positive remote sense adjustment Trim or reference adjustment Feedback divider adjustmentThe LM5080 is available in MSOP-8 packaging and is ideal for use in consumer electronics, industrial test equipment, data communications systems, automotive power systems, distributed power systems, and battery-powered 7V to 75V Input, Buck RegulatorThe LM5005 high-voltage switching regulator features all of thefunctions necessary to implement an efficient high-voltage buckregulator using a minimum of external components. This easy-to-use regulator includes a 75V N-Channel buck switch with an out-put current capability of The regulator control method isbased upon current-mode control utilizing an emulated currentramp. Current-mode control provides inherent line feed-forward,cycle-by-cycle current limiting, and ease of loop use of an emulated control ramp reduces noise sensitivity ofthe pulse-width modulation circuit, allowing reliable control of verysmall duty cycles necessary in high input voltage applications.

3 The operating frequency is programmable from 50 kHz to 500 Integrated 75V power MOSFET supports load currents up to Adjustable output voltage from Unique, easy-to-use emulated peak current mode controltopology enables high frequency operation at VINup to 75V Programmable switching frequency with bi-directional synchronization capability simplifies system design Highly integrated, high-speed, full-feature PWM regulatorreduces overall solution sizeThe LM5005 is available in a power enhanced TSSOP-20 packagefeaturing an exposed die attach pad to aid thermal dissipation. It is ideal for use in consumer electronics, telecommunications,data communications systems, automotive power systems, anddistributed power 9/13/05 11:43 AM Page 1L. Haachitaba Mweene, , Senior Applications EngineerNational PWM DC-DC switch-ing power converters are basedon simple topologies, makingpractical power supplies out of themrequires the addition of various functions such as start-up bias, soft-start,switch driving, regulation, short circuitprotection, over-voltage protection,over-temperature protection, etc.

4 Today,most of these functions are usually implemented within a compact DC-DCPWM controller integrated circuit. However, the problem of starting theDC-DC converter in telecom and otherhigh-voltage applications ( , where theinput voltage exceeds about 15V) isoften not addressed. The controllerrequires a bias supply voltage to runfrom so that it can produce gate drivepulses and other required signals. But atturn-on the only voltage available is theinput voltage, which, if it is greater than15V, is typically too high to be used asthe bias and gate drive supply voltage. It is therefore necessary to lower theinput voltage to 15V or below to start-up the power supply. Once the supply isrunning, the output voltage or a voltageoff of a transformer or inductor windingcan be used to provide the bias supplyfor the most DC-DC controllers aredesigned without start-up circuitry, andthe power supply designer is expected toadd a separate start-up circuit and a biassupply to them (Figure 1a).

5 This improvesthe versatility of the PWM controllers,allowing them to be operated with a widerinput voltage range, but the extra start-upcircuitry they require increases in the com-plexity and size of the power supply. National has solved this problem for thedesigner in its LM50xx family of 8V to100V PWM controllers (which includesthe LM5020, LM5025, LM5030, andothers) by integrating a high-voltage start-up circuit within the IC (Figure 1b). Thisis achieved by fabricating the controllerusing a 100V process. The high-input voltage can then bedirectly applied to the VINpin of the con-troller, which is the input to an internallinear voltage regulator. This regulatorproduces a voltage VCCof about 8V thatis used to provide start-up power to thecontroller. The VCCvoltage of the linearregulator is made externally accessible atthe VCCpin, for several reason is that the VCCpin is theconnection point in the linear regulatorfor an external output capacitor thatkeeps the VCCvoltage reason is that VCCcan serve asthe power source for other low-voltageICs in the circuit such as op amps, logic,and gate VCCpin can also be used to reducethe power dissipation in the controllerand to increase the efficiency of thepower supply.

6 LM50xx controllers willoperate indefinitely off of the input volt-age and the internally generated VCCPWM DC-DC Controllers with Built-In Start-Up Regulators SimplifySwitching Power Supply DesignVINS tart-up circuitLow voltagePWM controllerDC-DC Power stageVOUTHigh voltage PWM controller with integrated start-up circuitDC-DC Power stageVINVOUTF igure 1: Two Ways of Implementing a Start-Up Circuit for a PWM ControllerFigure 1a: Power supply built around PWM controller with discrete start-up circuitFigure 1b: Power supply built around PWM controller with integrated start-up 9/13/05 11:43 AM Page 2ior Applications EngineerNational SemiconductorVisit the online Analog Edge technical journal and an archive of design ideas, application briefs, and other informative (as illustrated in Figure 2ain aflyback converter). But this leads toincreased power dissipation in the dissipation isPD=(VIN-VCC)ISwhere IS, the supply current of the controller is the sum of the controllerquiescent current, and IG, the frequency-dependent gate drive current.

7 ThisMOSFET current is given byIG=QFSfSwhere QGSis the total gate charge ofthe MOSFET at a gate voltage of VCCand fSis the switching be excessively large for the controller at high-input voltages, high-switching frequency, and when theIC is driving a large MOSFET thatrequires a significant gate-drive controllers are designed suchthat this power dissipation can be circumvented. In all switching powersupply topologies, it is easy to derive a bias voltage from a transformer orinductor winding once the power supplyhas started running. In LM50xx controllers, this voltage (once available),can be applied directly to the VCCpin toprovide power for the IC, and can alsobe used to power other parts of the system. In all LM50xx controllers, if thisapplied voltage is greater than the 8 Voutput of the internal regulator, the regulator shuts down, eliminating thepower dissipation just described.

8 Thiscan lead to an efficiency improvement of 1% or more if the biassupply is properly designed. (Figure 2b)Nevertheless, in lower power systemswith lower input voltages it is oftenadvantageous to dispense with the biassupply and to run the supply off of theinternal linear regulator. This simplifiesthe supply and reduces its cost and isoften used with LM50xx controllers foroutput power levels of up to about the input voltage lies between 8 and15V, the LM50xx controllers are particularly easy to power. The VINandVCC pins can simply be tied together andto the input voltage, which then directlypowers the controller. (Figure 2c). If thevoltage exceeds 15V the two pins cannotbe tied together, and the input voltagehas to be higher than about 12V in orderfor the controller to start summary, National Semiconductor shigh-voltage PWM controllers with inte-grated start-up regulators allow thepower supply designer to reduce circuitcomplexity, solution size, componentcosts, design time, and to increase circuitreliability.

9 SimplifyVIN = 12 to 100 VLM50xxVINVCCVOUTVIN = 12 to 100 VLM50xxVINVCCVOUTVIN = 8 to 15 VLM50xxVINVCCVOUTF igure 2: Three Ways of Powering the LM50xx Family of PWM ControllersFigure 2a: PWM IC running off a high input voltage in a low power systemFigure 2b: PWM IC running off a bias supply in a high-power systemFigure 2c: PWM IC running off a low input 9/13/05 11:43 AM Page 3 Featured National Semiconductor Corporation, 2005. National Semiconductor , , and LLP are registered trademarks and Analog Edge is a service mark of National Semiconductor other brand or product names are trademarks or registered trademarks of their respective Controller for Forward Converterswith Active-Clamp ResetThe LM5026 PWM controller contains all of the features necessaryto implement power converters utilizing the active clamp/resettechnique with current-mode control. With the active-clamp technique, higher efficiencies and greater power densities can berealized compared with conventional catch winding or RDC clamp/reset techniques.

10 The device can be configured to control either aP-Channel or N-Channel clamp switch. The main gate driver features a compound configuration, consisting of both MOS andBipolar devices, providing superior gate drive features include line under-voltage lockout, cycle-by-cycle current limit, PWM slope compensation, soft-start, 1 MHzcapable oscillator with synchronization input/output capability,precision reference, and thermal Wide range (8V to 100V) start-up bias regulator Two high-speed power MOSFET drivers: 3A main outputdriver and 1A clamp driver User-programmable maximum duty-cycle and UVLO hysteresis thresholds User-programmable gate driver overlap and dead-time Versatile dual-mode over-current protection with hiccupmode delay timerThe LM5026 is available in TSSOP-16 or thermally enhanced LLP-16 packaging and is ideal for use in telecommunicationspower systems, +42V automotive power systems, -48V distrib-uted power systems, industrial power supplies, and multi-outputpower s First 100V Dual Interleaved ActiveClamp Current-Mode ControllersThe LM5032 andLM5034 are flexi-ble controllers thatcan be configuredto control eithertwo independentlyregulated outputsor a single, high-current out-put from two primary power stages.