AND8170/D Small, Simple, PWM Buck Controller Can Replace High Current LDOs Prepared by: Jim Hill ON Semiconductor Applications Engineer http://onsemi.com APPLICATION NOTE One example would be a 3.3 V rail from an existing 5.0 V rail. For a 1.0 A load, an LDO could be used as a post−regulator, but the best−case efficiency would be 3.3 V/5.0 V which equates to approximately 66%. Also, the LDO dissipates 1.7 W. Another example is a 2.5 V power rail derived from an existing 3.3 V rail. For a 1.0 A load, the best−case efficiency would be 2.5 V/3.3 V which equates to approximately 76%. The power dissipation is 0.8 W. For the 3.3 V example, a linear regulator would have to come in a TO−220 or D2PAK package (23 mm x 11 mm, and 4.8 mm high). If the circuit in question cannot dissipate power efficiently (for instance, enclosed systems with little or no airflow) the heat given off by an LDO can exceed the thermal budget of the system. Alternatively, the circuit in Figure 1 shows a simple, five pin (Thin SOT−23−5), buck controller. For the design that provides 2.5 V at 1.0 A from a 3.3 V input, the efficiency is 88% at 1.0 A. The output ripple voltage for this circuit is 30 mVp−p. This circuit also does not require external compensation and can be disabled to shut off the load with its chip enable (CE) pin. All of this makes this solution about as simple to implement as it’s rival, the LDO. Modern electronic systems require a host of different regulated voltages to power their various subsystems. The number of voltage levels needed on boards has risen as new generations of processors, memory, etc., have been introduced with lower voltage requirements. Also, the required voltage keeps decreasing so designers must include 5.0 V, 3.3 V and other voltage as low as 1.0 V. The trend in distributed power architectures is to design individual, nonisolated, Point−Of−Load (POL) converters to supply power to each individual load. A power supply usually exists for 5.0 V and 3.3 V, but generating additional lower voltages often requires additional post−regulators. Low dropout, linear regulators, or LDOs, are typically used for post−regulation because they are easily implemented and provide a relatively noise−free power source. However, for higher currents, such as 1.0 A and above, LDOs take up a great deal of space and can dissipate too much power and thus heat. For low dropout applications where you need another voltage rail, and you already have a 5.0 V or 3.3 V rail, a simple PWM buck converter provides a more efficient choice than a linear regulator. Vout = 3.3 V for NCP1550SN33 Vout = 2.5 V for NCP1550SN25 Vout = 1.8 V for NCP1550SN18 D1 L1 MBRM110L VIN U1 4 C1 33 F TR1 NTHS4101P 5 Vin = 5.0 V for NCP1550SN33 Vin = 3.3 V for NCP1550SN25 Vin = 3.3 V for NCP1550SN18 3.3 H for Vout = 3.3 V 5.6 H for Vout = 2.5 V 6.8 H for Vout = 1.8 V EXT CE 3 2 CE 1 NCP1550SN18 NCP1550SN25 NCP1550SN33 GND GND C2 68 F for Vout = 3.3 V 33 F for Vout = 2.5 V, 1.8 V VOUT GND Figure 1. Simple Buck Controller for Converting 1.8, 2.5, or 3.3 V from 3.3 or 5.0 V Semiconductor Components Industries, LLC, 2004 August, 2004 − Rev. 0 1 Publication Order Number: AND8170/D AND8170/D 100 90 80 Vin = 5.0 V for NCP1550SN33 Vin = 3.3 V for NCP1550SN25 Vin = 3.3 V for NCP1550SN18 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.0 Circuit from Figure 1 0.2 0.4 0.6 0.8 Circuit from Figure 1 1.0 Iout (A) Figure 2. NCP1550 Efficiency vs. Iout Figure 3. Output Ripple at Vin = 3.3 V, Vo = 2.5 V @ 1.0 A (C4 = 500 mA/div) Proper passive component selection helped raise the system’s overall efficiency. The MOSFET used (NTHS4101P) has a typical RDS(on) of around 20 m at the designed operating point and comes in a very small but thermally efficient ChipFET package. The Schottky used (MBRM110L) is a 10 V device which offers lower VF than most comparable 20 V devices. Figure 4 shows how the 10 V Schottky platform compares to other typical 20 V platforms. More information about this product family can be found in the application note AND8083/D, “Efficiency Improvements Using 10 V Schottky Diodes” from ON Semiconductor. ON Semiconductor The 10 V Schottky starts showing its benefits as the duty cycle of the circuit decreases and it conducts more current. For instance, since voltage levels continue to lower, a 1.8 V rail may be necessary. This solution still outperforms the LDO. For instance, if you wanted to supply 1.8 V at 1.0 A from the 3.3 V rail, an LDO would have a best−case efficiency of 1.8 V/3.3 V which equates to approximately 55% with a power dissipation of 1.5 W. The circuit from Figure 1 with the above conditions has an efficiency of 83% at 1.0 A. On another note, buck converters draw less current from input power sources than LDOs. An LDO’s input current is the same as its output current, but a buck converter’s input current is a function of the efficiency of the converter. For instance, for the 1.8 V, 1.0 A converter mentioned previously, which has an efficiency of 83%, Iin = (Vo*Io)/η*Vin. Therefore, Iin = 660 mA which equates to a 34% savings in input current from the LDO solution. For the 2.5 V and 3.3 V solutions, which have efficiencies at their given conditions of 88%, the input current equates to 860 mA and 750 mA respectively. This reduction in input current helps keep the power budget of the existing bus converters down which can equate to smaller size, better performance, and lower cost. Finally, the user can scale this design to higher currents up to 2.0 A by adjusting the transistor, diode, inductor, and capacitor accordingly. More details on the components required for 2.0 A operation can be found in the NCP1550 data sheet. This solution provides a good replacement for LDOs when one requires high currents, low dropout, and good thermal performance, i.e. efficiency, without adding much complexity. VF Improvement Industry Typical IF VF Figure 4. 10 V Schottky VF vs. Typical Competition http://onsemi.com 2 AND8170/D Table 1. Bill of Materials for Circuit from Figure 1 1.8 V Version Designator Qty U1 1 PFM/PWM Step−Down DC−DC Controller D1 1 TR1 Description Manufacturer Part Number Value Tolerance Footprint Manufacturer 1.8 Vout, 600 kHz NA Thin SOT−23−5 ON Semiconductor NCP1550SN18T1 Schottky Power Rectifier 1.0 A, 10 V NA POWERMITE ON Semiconductor MBRM110LT3 1 Power MOSFET 1.0 A, 20 V NA ChipFET ON Semiconductor NTHS4101PT1 C1, C2 2 Low Profile Tantalum Chip Capacitor 33 F, 10 V 10% 6032−28 Kemet T491C336K010AS L1 1 SMD Power Inductor 6.8 H, 1.36 A 20% 6.0 x 6.4 x 2.5 mm Sumida CDC5D236R8 Value Tolerance Footprint Manufacturer Manufacturer Part Number 2.5 Vout, 600 kHz NA Thin SOT−23−5 ON Semiconductor NCP1550SN25T1 2.5 V Version Designator Qty U1 1 PFM/PWM Step−Down DC−DC Controller D1 1 Schottky Power Rectifier 1.0 A, 10 V NA POWERMITE ON Semiconductor MBRM110LT3 TR1 1 Power MOSFET 1.0 A, 20 V NA ChipFET ON Semiconductor NTHS4101PT1 C1, C2 2 Low Profile Tantalum Chip Capacitor 33 F, 10 V 10% 6032−28 Kemet T491C336K010AS L1 1 SMD Power Inductor 5.6 H, 1.44 A 20% 6.0 x 6.4 x 2.5 mm Sumida CDC5D235R6 Value Tolerance Footprint Manufacturer Manufacturer Part Number 3.3 Vout, 600 kHz NA Thin SOT−23−5 ON Semiconductor NCP1550SN33T1 Description 3.3 V Version Designator Qty Description U1 1 PFM/PWM Step−Down DC−DC Controller D1 1 Schottky Power Rectifier 1.0 A, 10 V NA POWERMITE ON Semiconductor MBRM110LT3 TR1 1 Power MOSFET 1.0 A, 20 V NA ChipFET ON Semiconductor NTHS4101PT1 C1 1 Low Profile Tantalum Chip Capacitor 33 F, 10 V 10% 6032−28 Kemet T491C336K010AS C2 1 Low ESR Tantalum Chip Capacitor 68 F, 10 V 10% 7343−31 Kemet T494D686K010AS L1 1 SMD Power Inductor 3.3 H, 1.90 A 20% 6.0 x 6.4 x 2.5 mm Sumida CDC5D233R3 Figure 5. 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