DEMO CIRCUIT 1570A-A/B QUICK STLT3663EDCB-3.3/5.0ART GUIDE LT3663EDCB-3.3/5.0 1.2A, 1MHz Step-Down Switching Regulator with Output Current Limit DESCRIPTION Demonstration Circuit 1570 is a 1.0MHz current mode mode, input overvoltage lockout and thermal shut- step-down switching regulator with programmable down. The LT3663EDCB-3.3/5.0 is available in an 8- output current limit. The current limit accurately con- lead (2mm 3mm) DFN surface mount package with trols the system power dissipation and reduces the exposed pad. size of the power path components. The wide operat- L, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy ing input voltage range of 7.5V to 36V (60V maxi- Drive, FilterCAD, Hot Swap, LinearView, Module, Micropower SwitcherCAD, Multimode mum) suits the LT3663-3.3/5.0 to a variety of input Dimming, No Latency , No Latency Delta-Sigma, No R , Operational Filter, PanelProtect, SENSE PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, sources, including unregulated 12V wall adapters, 24V UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. industrial supplies, and automotive power. The LT3663-3.3/5.0 includes a low current shutdown TABLE1 PERFORMANCE SUMMARY Specifications are at T = 25C A SYMBOL PARAMETER CONDITIONS MIN MAX UNITS VIN Input Voltage Range 7.5 to 36.0 V VOUTA Output Voltage Range Iout 0mA to ILIM 3.24 to 3.36 V VOUTB Output Voltage Range Iout 0mA to ILIM 4.9 to 5.1 V ILIM Current Limit Selectable 20% 0.6 to 1.2 A OPERATING PRINCIPLE Refer to the block diagram within the LT3663-3.3/5.0 put if it decreases, less current is delivered. An active data sheet for its operating principle. clamp (not shown) on the V node provides current C limit. The LT3663-3.3/5.0 is internally compensated The LT3663-3.3/5.0 is a constant frequency, current with a pole zero combination on the output of the gm mode step-down regulator. A switch cycle is initiated amplifier. when the 1MHz oscillator enables the RS flip flop, turn- An external capacitor and internal diode are used to ing on an internal power switch, Q1. An amplifier and generate a voltage at the BOOST pin that is higher than comparator monitor the current flowing between the the input supply. This allows the driver to fully saturate VIN and SW pins, turning the switch off when the cur- the internal bipolar NPN power switch for efficient op- rent reaches a level determined by the voltage at node eration. The switch driver operates from either VIN or V . The error amplifier measures the output voltage BOOST to ensure startup. C through an internal resistor divider tied to the VOUT pin and servos the V node. If the error amplifiers C output increases, more current is delivered to the out- 1 LT3663EDCB-3.3/5.0 An internal regulator provides power to the control cir- current sense resistor, RSENSE2, and compares it to a cuitry. This regulator includes an input under-voltage and voltage programmed by external resistor R1 on the ILIM overvoltage protection which disable switching action pin. A capacitor averages the inductor ripple current. If when VIN is out of range. When switching is disabled, the the averaged inductor current exceeds the programmed LT3663-3.3/5.0 can safely sustain input voltages up to value then the V voltage is pulled low, reducing the cur- C 60V. Note that while switching is disabled the output will rent in the regulator. The output current limit circuit al- start to discharge. lows for a lower current rated inductor and diode and provides better control of system power dissipation. Output current limiting is provided via the servo action of an amplifier. It compares the voltage across an inductor QUICK START PROCEDURE Using short twisted pair leads for any power con- voltage in Table 1. Verify LOAD1 current is be- nections, with all loads and power supplies off, refer tween 800mA and 1.12A. Reduce LOAD1 cur- to Figure 1 for the proper measurement and equip- rent to 120mA. ment setup. 7. Set JP2 to 0 and JP3 to 1. Increase LOAD1 Follow the procedure below: current until VOUT drops below the minimum voltage in Table 1. Verify LOAD1 current is be- 1. Jumper, PS and LOAD settings to start: tween 640mA and 960mA. Reduce LOAD1 JP1 = Run PS1 = OFF current to 120mA. JP2 = 1 LOAD1 = OFF JP3 = 1 8. Set JP2 to 0 and JP3 to 0. Increase LOAD1 current until VOUT drops below the minimum 2. Turn on PS1 and slowly increase voltage to voltage in Table 1. Verify LOAD1 current is be- 5.5V while monitoring the input current. If the tween 480mA and 720mA. Reduce LOAD1 current remains less than 50mA, increase PS1 current to 120mA. until the output turns on. Verify input voltage 9. Increase PS1 to 36V and verify voltage on UVLO of 6.5V to 7.5V. VOUT in Table 1. 3. Increase PS1 to 12V and set LOAD1 to 10. Increase PS1 to 40V and verify voltage on 120mA. Verify voltage on VOUT in Table 1. VOUT of <500mV. 4. Set LOAD1 to 1.0A. Verify voltage on VOUT in 11. Decrease PS1 to 30V and verify voltage on Table 1 and ripple voltage of <50mV. VOUT in Table 1. 5. Increase LOAD1 current until VOUT drops be- 12. Turn off PS1 and LOAD1. low the minimum voltage in Table 1. Verify LOAD1 current is between 0.96A and 1.44A. Reduce LOAD1 current to 120mA. 6. Set JP2 to 1 and JP3 to 0. Increase LOAD1 current until VOUT drops below the minimum 2