SC121EVB

SC121 Low Voltage Synchronous Boost Regulator POWER MANAGEMENT Features Input voltage — 0.7V to 4.5V Minimum start-up voltage — 0.85V Output voltage — fixed at 3.3V; adjustable from 1.8V to 5.0V Peak input current limit — 1.2A Output current at 3.3 VOUT — 80mA with VIN = 1.0V, 190mA with VIN = 1.5V Forced PWM operation at all loads Efficiency up to 94% Internal synchronous rectifier No forward conduction path during shutdown Switching frequency — 1.2MHz Soft-start startup current limiting Shutdown current — 0.1μA (typ) Ultra-thin 1.5 × 2.0 × 0.6 (mm) MLPD-UT-6 package Lead-free and halogen-free WEEE and RoHS compliant Description The SC121 is a high efficiency, low noise, synchronous step-up DC-DC converter that provides boosted voltage levels in low-voltage handheld applications. The wide input voltage range allows use in systems with single NiMH or alkaline battery cells as well as in systems with higher voltage battery supplies. It features an internal 1.2A switch and synchronous rectifier to achieve up to 94% efficiency and to eliminate the need for an external Schottky diode. The output voltage can be set to 3.3V with internal feedback, or to any voltage within the specified range using a standard resistor divider. The SC121 operates exclusively in Pulse Width Modulation (PWM) mode for low ripple and fixed-frequency switching. Output disconnect capability is included to reduce leakage current, improve efficiency, and eliminate external components sometimes needed to disconnect the load from the supply during shutdown. Low quiescent current is maintained with a high 1.2MHz operating frequency. Small external components and the space saving MLPD-UT-6, 1.5×2.0×0.6 (mm) package make this device an excellent choice for small handheld applications that require the longest possible battery life. Applications MP3 players Smart phones and cellular phones Palmtop computers and handheld instruments PCMCIA cards and memory cards Digital cordless phones Personal medical products Wireless VoIP phones Small motors Typical Application Circuit L1 IN Single Cell (1.2V) CIN EN GND LX OUT FB COUT 3.3V SC121 April 13, 2010 © 2010 Semtech Corporation 1 SC121 Pin Configuration — MLPD-UT Ordering Information Device SC121ULTRT(1)(2) LX GND IN 1 TOP VIEW 6 Package MLPD-UT-6 1.5×2 Evaluation Board OUT FB EN SC121EVB 2 5 3 T 4 Notes: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Lead-free packaging, only. Device is WEEE and RoHS compliant, and halogen-free. MLPD-UT; 1.5×2, 6 LEAD θJA = 84°C/W Marking Information — MLPD-UT 121 yw MLPD-UT; 1.5×2, 6 LEAD yw = date code 2 SC121 Absolute Maximum Ratings IN, OUT, LX, FB (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0 EN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN + 0.3) ESD Protection Level (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 (1) Recommended Operating Conditions Ambient Temperature Range (°C) . . . . . . . . . . . . -40 to +85 VIN ( V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.7 to 4.5 VOUT ( V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 to 5.0 Thermal Information Thermal Res. MLPD, Junction-Ambient(2) (°C/W) . . . . . . . 84 Maximum Junction Temperature (°C) . . . . . . . . . . . . . . . 150 Storage Temperature Range (°C) . . . . . . . . . . . -65 to +150 Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . +260 Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES: (1) Tested according to JEDEC standard JESD22-A114. (2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. Electrical Characteristics Unless otherwise noted VIN = 2.5V, CIN = COUT = 22μF, L1 = 4.7μH, TA = -40 to +85°C. Typical values are at TA = 25°C. Parameter Input Voltage Range Minimum Startup Voltage Shutdown Current Operating Supply Current(1) Internal Oscillator Frequency Maximum Duty Cycle Minimum Duty Cycle Output Voltage Adjustable Output Voltage Range Regulation Feedback Reference Voltage Accuracy (Internal or External Programming) FB Pin Input Current Startup Time Symbol VIN VIN-SU ISHDN IQ fOSC DMAX DMIN VOUT VOUT_RNG VReg-Ref IFB tSU Conditions Min 0.7 Typ Max 4.5 Units V V IOUT < 1mA, TA = 0°C to 85°C TA = 25°C, VEN = 0V IOUT = 0, VEN = VIN 0.85 0.1 3.5 1.2 90 20 1 μA mA MHz % % V VFB = 0V For VIN such that DMIN < D < DMAX 1.8 3.3 5.0 V -1.5 1.5 % VFB = 1.2V 1 0.1 μA ms 3 SC121 Electrical Characteristics (continued) Parameter P-Channel ON Resistance N-Channel ON Resistance N-Channel Current Limit P-Channel Startup Current Limit LX Leakage Current PMOS LX Leakage Current NMOS Logic Input High Logic Input Low Logic Input Current High Logic Input Current Low Symbol RDSP RDSN ILIM(N) ILIM(P)-SU ILXP ILXN VIH VIL IIH IIL Conditions VOUT = 3.3V VOUT = 3.3V VIN = 3.0V VIN > VOUT, VEN > VIH TA = 25°C, VLX = 0V TA = 25°C, VLX = 3.3V VIN = 3.0V VIN = 3.0V VEN = VIN = 3.0V VEN = 0V Min Typ 0.6 0.5 Max Units Ω Ω A mA 0.9 1.2 150 1 1 μA μA V 0.85 0.2 1 -0.2 V μA μA NOTES: (1) Quiescent operating current is drawn from OUT while in regulation. The quiescent operating current projected to IN is approximately IQ × ( VOUT/VIN). 4 SC121 Typical Characteristics — VOUT = 1.8V Efficiency vs. IOUT ( VOUT = 1.8V) 100 90 80 70 R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, TA = 25 C VIN = 1.6V ο Efficiency vs. IOUT ( VOUT = 1.8V) 100 90 80 70 R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, VIN = 1.2V TA = –40°C Efficiency (%) 60 50 40 30 20 10 0 0.1 0.2 0.5 1 2 5 10 VIN = 0.8V Efficiency (%) 60 50 40 30 20 10 TA = 85°C TA = 85°C VIN = 1.2V TA = –40°C TA = 25°C 20 50 100 200 0 0.1 0.2 0.5 1 2 5 10 20 50 100 200 IOUT (mA) IOUT (mA) Load Regulation (VOUT = 1.8V) 1.82 ο R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, TA = 25 C Load Regulation (VOUT = 1.8V) 1.82 R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, VIN = 1.2V 1.8 1.8 VOUT (V) VOUT (V) VIN = 1.6V TA = –40°C TA = 25°C 1.78 VIN = 0.8V VIN = 1.2V 1.78 TA = 85°C 1.76 0 50 100 150 200 250 1.76 0 50 100 150 200 250 IOUT (mA) IOUT (mA) Line Regulation — Low Load (VOUT = 1.8V) 1.82 R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 1.82 Line Regulation — High Load (VOUT = 1.8V) R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, IOUT = 50mA TA = 25°C 1.8 1.8 VOUT (V) VOUT (V) TA = –40°C TA = 85°C TA = 25°C TA = –40°C 1.78 1.78 TA = 85°C 1.76 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.76 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 VIN (V) VIN (V) 5 SC121 Typical Characteristics — VOUT = 1.8V (continued) Temperature Reg. — Low Load (VOUT = 1.8V) 1.82 R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 1.82 Temperature Reg. — High Load (VOUT = 1.8V) R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF, IOUT = 50mA VIN = 1.6V 1.8 VIN = 1.2V 1.8 VOUT (V) VOUT (V) VIN = 0.8V VIN = 1.2V 1.78 1.78 VIN = 0.8V 1.76 -50 -25 0 25 50 o 75 100 1.76 -50 -25 0 25 50 o 75 100 Junction Temperature ( C) Junction Temperature ( C) Max. IOUT vs. VIN ( VOUT = 1.8V) 350 300 TA = –40°C R1 = 499kΩ , R2 = 1MΩ , L = 4.7μH, CFB = 22pF 250 IOUT (mA) TA = 25°C 200 150 TA = 85°C 100 50 0 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 VIN (V) 6 SC121 Typical Characteristics — VOUT = 3.3V Efficiency vs. IOUT ( VOUT = 3.3V) FB grounded, L = 4.7μH, TA = 25 C 100 90 80 70 VIN = 2.95V ο Efficiency vs. IOUT ( VOUT = 3.3V) FB grounded, L = 4.7μH, VIN = 2V 100 90 80 70 TA = 85°C TA = –40°C Efficiency (%) Efficiency (%) 60 50 40 30 20 10 0 0.1 0.2 0.5 1 2 5 10 VIN = 1.0V VIN = 2.0V 60 50 40 30 20 10 TA = 25°C 20 50 100 200 500 0 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 IOUT (mA) IOUT (mA) Load Regulation (VOUT = 3.3V) FB grounded, L = 4.7μH, TA = 25 C 3.34 3.32 VIN = 2.95V ο Load Regulation (VOUT = 3.3V) FB grounded, L = 4.7μH, VIN = 2V 3.34 3.32 TA = 25°C 3.3 3.3 VOUT (V) 3.28 3.26 VIN = 2.0V VOUT (V) 3.28 3.26 3.24 3.24 VIN = 1.0V TA = 85°C TA = –40°C 3.22 3.2 0 3.22 3.2 0 50 100 150 200 250 300 350 400 450 500 50 100 150 200 250 300 350 400 450 500 IOUT (mA) IOUT (mA) Line Regulation — Low Load (VOUT = 3.3V) FB grounded, L = 4.7μH, IOUT = 1mA 3.34 3.32 TA = –40°C Line Regulation — High Load (VOUT = 3.3V) FB grounded, L = 4.7μH, IOUT = 90mA 3.34 3.32 3.3 TA = –40°C 3.3 VOUT (V) 3.28 3.26 3.24 TA = 25°C VOUT (V) TA = 85°C 3.28 TA = 85°C 3.26 3.24 TA = 25°C 3.22 3.2 0.6 3.22 3.2 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 VIN (V) VIN (V) 7 SC121 Typical Characteristics — VOUT = 3.3V (continued) FB grounded, L = 4.7μH, IOUT = 1mA 3.34 3.32 3.3 VIN = 2.0V VIN = 2.95V Temperature Reg. — Low Load (VOUT = 3.3V) 3.34 3.32 3.3 FB grounded, L = 4.7μH, IOUT = 90mA Temperature Reg. — High Load (VOUT = 3.3V) VIN = 2.95V VOUT (V) 3.28 VIN = 1.0V VOUT (V) 3.28 3.26 VIN = 2.0V 3.26 3.24 3.22 3.2 -50 VIN = 1.0V 3.24 3.22 3.2 -50 -25 0 25 50 o 75 100 -25 0 25 50 o 75 100 Junction Temperature ( C) Junction Temperature ( C) Max. IOUT vs. VIN ( VOUT = 3.3V) FB grounded, L = 4.7μH 500 450 400 350 TA = –40°C TA = 25°C IOUT (mA) 300 250 200 150 100 50 0 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 TA = 85°C VIN (V) 8 SC121 Typical Characteristics — VOUT = 4.0V Efficiency vs. IOUT ( VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, TA = 25 C 100 90 80 70 VIN = 3.6V ο Efficiency vs. IOUT ( VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, VIN = 2.4V 100 90 80 70 TA = –40°C TA = 85°C Efficiency (%) Efficiency (%) 60 50 40 30 20 10 0 0.1 0.2 0.5 1 2 5 10 20 VIN = 1.2V 60 50 40 30 20 10 TA = 25°C VIN = 2.4V 50 100 200 500 0 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 IOUT (mA) IOUT (mA) Load Regulation (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, TA = 25 C 4.1 4.1 ο Load Regulation (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, VIN = 2.4V 4.05 VIN = 3.6V 4.05 VOUT (V) VOUT (V) 4 4 TA = 25°C 3.95 VIN = 1.2V VIN = 2.4V 3.95 TA = 85°C 3.9 3.9 TA = –40°C 3.85 0 50 100 150 200 250 300 350 400 450 500 550 3.85 0 50 100 150 200 250 300 350 400 450 500 550 IOUT (mA) IOUT (mA) Line Regulation — Low Load (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 4.1 4.1 Line Regulation — High Load (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, IOUT = 110mA 4.05 TA = 85°C 4.05 VOUT (V) VOUT (V) 4 4 TA = –40°C TA = –40°C TA = 85°C 3.95 3.95 3.9 TA = 25°C 3.9 TA = 25°C 3.85 3.85 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 VIN (V) VIN (V) 9 SC121 Typical Characteristics — VOUT = 4.0V (continued) Temperature Reg. — Low Load (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 4.1 VIN = 3.6V Temperature Reg. — High Load (VOUT = 4.0V) R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF, IOUT = 110mA 4.1 4.05 4.05 VIN = 3.6V VIN = 2.4V VOUT (V) VIN = 1.2V VOUT (V) 4 VIN = 2.4V 4 VIN = 1.2V 3.95 3.95 3.9 3.9 3.85 -50 -25 0 25 50 o 75 100 3.85 -50 -25 0 25 50 o 75 100 Junction Temperature ( C) Junction Temperature ( C) Max. IOUT vs. VIN ( VOUT = 4.0V) 500 450 400 350 TA = –40°C R1 = 976kΩ , R2 = 412kΩ , L = 4.7μH, CFB = 22pF IOUT (mA) 300 250 200 150 100 50 0 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 TA = 85°C TA = 25°C VIN (V) 10 SC121 Typical Characteristics — VOUT = 5.0V Efficiency vs. IOUT ( VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, TA = 25 C 100 90 80 70 VIN = 4.2V ο Efficiency vs. IOUT ( VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, VIN = 3.6V 100 90 80 70 TA = –40°C TA = 85°C Efficiency (%) Efficiency (%) 60 50 40 30 20 10 0 0.1 0.2 0.5 60 50 40 30 20 10 TA = 25°C VIN = 1.2V VIN = 2.2V VIN = 3.2V 1 2 5 10 20 50 100 200 500 0 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 IOUT (mA) IOUT (mA) Load Regulation (VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, TA = 25 C 5.05 5.05 ο Load Regulation (VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, VIN = 3.6V 5 VIN = 4.2V 5 TA = –40°C VOUT (V) VOUT (V) 4.95 4.95 4.9 VIN = 3.2V VIN = 1.2V VIN = 2.2V 4.9 TA = 85°C TA = 25°C 4.85 4.85 4.8 0 50 100 150 200 250 300 350 400 450 500 550 4.8 0 50 100 150 200 250 300 350 400 450 500 550 IOUT (mA) IOUT (mA) Line Regulation — Low Load (VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 5.05 5.05 Line Regulation — High Load (VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, IOUT = 85mA 5 TA = –40°C 5 TA = –40°C VOUT (V) TA = 85°C TA = 25°C VOUT (V) 4.95 4.95 TA = 85°C 4.9 4.9 4.85 4.85 TA = 25°C 4.8 0.5 1 1.5 2 2.5 3 3.5 4 4.5 4.8 0.5 1 1.5 2 2.5 3 3.5 4 4.5 VIN (V) VIN (V) 11 SC121 Typical Characteristics — VOUT = 5.0V (continued) Temperature Reg. — Low Load (VOUT = 5.0V) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, IOUT = 1mA 5.05 VIN = 4.2V R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF, IOUT = 85mA 5.05 VIN = 4.2V VIN = 3.2V Temperature Reg. — High Load (VOUT = 5.0V) 5 5 VOUT (V) VOUT (V) 4.95 VIN = 3.2V VIN = 2.2V VIN = 1.2V 4.95 VIN = 2.2V 4.9 4.9 VIN = 1.2V 4.85 4.85 4.8 -50 -25 0 25 50 o 75 100 4.8 -50 -25 0 25 50 o 75 100 Junction Temperature ( C) Junction Temperature ( C) Max. IOUT vs. VIN ( VOUT = 5.0V) 500 450 400 350 TA = 25°C TA = –40°C R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF IOUT (mA) 300 250 200 150 100 50 0 0.5 TA = 85°C 1 1.5 2 2.5 3 3.5 4 4.5 VIN (V) 12 SC121 Typical Characteristics (continued) PWM Operation VOUT = 3.3V, VIN = 1.5V, IOUT = 50mA VOUT ripple (10mV/div) IOUT = 40mA to 140mA (50mA/div) IL (100mA/div) VOUT (100mV/div) VLX (5V/div) Time = (400ns/div) Time = (100μs/div) AC Coupled Load Transient VOUT = 3.3V, VIN = 1.5V, TA =25°C Startup Max Load Current vs. VIN (Any VOUT) 100 R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF 160 140 80 Startup Min Load Res. vs. VIN (Any VOUT) R1 = 931kΩ , R2 = 294kΩ , L = 4.7μH, CFB = 22pF TA = –40°C Equivalent R (Ω ) LOAD 120 100 80 60 40 TA = 85°C TA = 25°C IOUT (mA) 60 TA = 85°C TA = –40°C 40 TA = 25°C 20 20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 VIN (V) VIN (V) Min. Start-up Voltage vs. Temperature (Any VOUT) 0.9 VOUT = 3.3V, IOUT = 1mA 0.85 Startup Voltage (V) 0.8 0.75 0.7 0.65 0.6 -40 -20 0 20 40 Temperature (°C) 60 80 100 13 SC121 Pin Descriptions MLPD Pin # 1 2 3 4 5 Pin Name LX GND IN EN FB Pin Function Switching node — connect an inductor from the input supply to this pin. Signal and power ground. Battery or supply input — requires an external 10μF bypass capacitor (capacitance evaluated while under VIN bias) for normal operation. Enable digital control input — active high. Feedback input — connect to GND for preset 3.3V output. A voltage divider is connected from OUT to GND to adjust output from 1.8V to 5.0V. Output voltage pin — requires an external 10μF bypass capacitor (capacitance evaluated while under VOUT bias) for normal operation. Thermal Pad is for heat sinking purposes — connect to ground plane using multiple vias — not connected internally. 6 OUT Thermal Pad T 14 SC121 Block Diagram IN VOUT Comp. + + 1.7 V + OUT EN Start-up Oscillator PLIM Amp. Oscillator and Slope Generator Slope Comp. Gate Drive and Logic Control Bulk Bias LX PWM Comp. + - PWM Control + NLIM Amplifier + Current Amplifier FB Output Voltage Selection Logic + Error Amp. + VREF - 1.2 V GND 15 SC121 Applications Information Detailed Description The SC121 is a synchronous step-up fixed frequency Pulse Width Modulated (PWM) DC-DC converter utilizing a 1.2MHz fixed frequency current mode architecture. It is designed to provide output voltages in the range 1.8V to 5.0V from an input voltage as low as 0.7V, with a (output unloaded) start up input voltage of 0.85V. Quiescent current consumption is typically 3.5mA, entirely into the OUT pin during boost regulation. (See footnote 1 of the Electrical Characteristics table.) The regulator control circuitry is shown in the Block Diagram. It is comprised of a programmable feedback controller, an internal 1.2MHz oscillator, an nchannel Field Effect Transistor (FET) between the LX and GND pins, and a p-channel FET between the LX and OUT pins. The current flowing through both FETs is monitored and limited as required for startup and PWM operation. An external inductor must be connected between the IN pin and the LX pin. The values of the resistors in the voltage divider network are chosen to satisfy the equation R1 R2 VOUT 1.191 1 V A large value of R2, ideally 590kΩ or larger, is preferred for stability for VIN within approximately 400mV of VOUT. For lower VIN, lower resistor values can be used. The values of R1 and R2 can be as large as desired to achieve low quiescent current. CFB = 22pF is recommended to improve transient response. The Enable Pin The EN pin is a high impedance logical input that can be used to enable or disable the SC121 under processor control. VEN < 0.2V will disable regulation, set the LX pin in a high-impedance state (turn off both FET switches), and turn on an active discharge device to discharge the output capacitor via the OUT pin. Synchronous rectifier (p-channel FET) bulk switching prevents pass-through conduction from LX to OUT while disabled. VEN > 0.85V will enable the output. The startup sequence from the EN pin is identical to the startup sequence from the application of input power. Output Voltage Selection The SC121 output voltage can be programmed to an internally preset value or it can be programmed with external resistors. The output is internally programmed to 3.3V when the FB pin is connected to GND. Any output voltage in the range 1.8V to 5.0V can be programmed with a resistor voltage divider between OUT and the FB pin as shown in Figure 1. L1 IN EN CIN GND LX OUT R1 FB COUT CFB VOUT SC121 R2 Figure 1 — Output Voltage Feedback Circuit 16 SC121 Applications Information (continued) PWM Operation The PWM cycle runs at a fixed frequency (fosc = 1.2MHz), with a variable duty cycle (D). PWM operation continually draws current from the input supply, except for low output loads in which current flows periodically from, and back into, the input. During the on-state of the PWM cycle, the n-channel FET is turned on, grounding the inductor at the LX pin. This causes the current flowing from the input supply through the inductor to ground to ramp up. During the off-state, the n-channel FET is turned off and the p-channel FET (synchronous rectifier) is turned on. This causes the inductor current to flow from the input supply through the inductor into the output capacitor and load, boosting the output voltage above the input voltage. The cycle then repeats to re-energize the inductor. Ideally, the steady state (constant load) duty cycle is determined by D = 1 – (VIN/VOUT ), but must be greater in practice to overcome dissipative losses. The SC121 PWM controller constrains the value of D such that 0.20 < D < 0.90 (approximately). The average inductor current during the off-state multiplied by (1-D) is equal to the average load current. The inductor current is alternately ramping up (on-state) and down (off-state) at a rate and amplitude determined by the inductance value, the input voltage, and the on-time (TON = D×T, T = 1/fOSC). Therefore, the instantaneous inductor current will be alternately larger and smaller than the average. If the average output current is sufficiently small, the minimum inductor current can ramp down to zero during the off-state. Discontinuous mode operation (where both FETs turn off as the inductor current reaches zero) is not supported in the SC121, since this would result in a finite positive minimum current from input to output, which would cause an uncontrolled rise in output voltage in this case. Instead, the inductor current will reverse for the remainder of the off-state, flowing from the output capacitor into the OUT pin, through the p-channel FET to the LX pin, and through the inductor to the input capacitor. Negative inductor current ripple allows regulation even with zero output load. The energy returned to the input capacitor is not wasted, but dissipative conduction losses will inevitably occur. The minimum on-time limitation imposes a minimum boost ratio, so if VIN is too close to VOUT ( VIN > VOUT – 400mV, approximately), VOUT will rise above the programmed value for a sufficiently small output load. A higher output load requires a higher duty cycle to overcome dissipative losses, such that regulation at programmed VOUT will eventually be restored. But this regulation-restoration load rises rapidly with VIN, so this phenomenon can be beneficially exploited in only rare circumstances. If operation with high VIN and low load is required, please consider using the SC120, a pin compatible dual mode (PWM/ PSAVE) boost converter. The SC120 will support zero load in PSAVE mode for VIN up to VOUT + 150mV. Regulator Startup, Short Circuit Protection, and Current Limits The SC121 permits power up at input voltages from 0.85V to 4.5V. Soft-start startup current limiting of the internal switching n-channel and p-channel FET power devices protects them from damage in the event of a short between OUT and GND. As the output voltage rises, progressively less-restrictive current limits are applied. This protection unavoidably prevents startup into an excessive load. Upon enable, the p-channel FET between the LX and OUT pins turns on with its current limited to approximately 150mA, the short-circuit output current. When V OUT approaches VIN (but is still below 1.7V), the n-channel current limit is set to 350mA (the p-channel limit is disabled), the internal oscillator turns on (approximately 200kHz), and a fixed 75% duty cycle PWM operation begins. (See the section PWM Operation.) When the output voltage exceeds 1.7V, fixed frequency PWM operation begins, with the duty cycle determined by an nchannel FET peak current limit of 350mA. When this n-channel FET startup current limit is exceeded, the onstate ends immediately and the off-state begins. This determines the duty cycle on a cycle-by-cycle basis. When VOUT is within 2% of the programmed regulation voltage, the n-channel FET current limit is raised to 1.2A, and normal voltage regulation PWM control begins. Once normal voltage regulation PWM control is initiated, the output becomes independent of VIN and output regulation can be maintained for VIN as low as 0.7V, subject to the maximum duty cycle and peak current limits. The 17 SC121 Applications Information (continued) duty cycle must remain between 20% and 90% for the device to operate within specification. Note that startup with a regulated active load is not the same as startup with a resistive load. The resistive load output current increases proportionately as the output voltage rises until it reaches programmed VOUT/RLOAD, while a regulated active load presents a constant load as the output voltage rises from 0V to programmed VOUT. Note also that if the load applied to the output exceeds an applicable VOUT–dependent startup current limit or duty cycle limit, the criterion to advance to the next startup stage may not be achieved. In this situation startup may pause at a reduced output voltage until the load is reduced further. Once an overload has occurred, the load must be decreased to permit recovery. The conditions required for overload recovery are identical to those required for successful initial startup. Component Selection The SC121 provides optimum performance when a 4.7μH inductor is used with a 10μF output capacitor. Different component values can be used to modify input current or output voltage ripple, improve transient response, or to reduce component size or cost. Inductor Selection The inductance value primarily affects the amplitude of inductor peak-to-peak current ripple (Δ IL). Reducing inductance increases ΔIL and raises the inductor peak current, IL-max = IL-avg + ΔIL/2, where IL-avg is the inductor current averaged over a full on/off cycle. IL-max is subject to the n-channel FET current limit ILIM(N), therefore reducing the inductance may lower the output overload current threshold. Increasing Δ I L a lso lowers the inductor minimum current, IL-min = IL-avg – ΔIL/2, thus raising the load current threshold below which inductor negative–peak current becomes zero. Equating input power to output power and noting that input current is equal to inductor current, average the inductor current over a full PWM switching cycle to obtain IL 1 VOUT IOUT VIN Output Overload and Recovery The PWM steady state duty cycle is determined by D = 1 – (VIN/VOUT ), but must be somewhat greater in practice to overcome dissipative losses. As the output load increases, the dissipative losses also increase. The PWM controller must increase the duty cycle to compensate. Eventually, one of two overload conditions will occur, determined by VIN, VOUT, and the overall dissipative losses due to the output load current. Either the maximum duty cycle of 90% will be reached or the n-channel FET 1.2A (nominal) peak current limit will be reached, which effectively limits the duty cycle to a lower value. Above that load, the output voltage will decrease rapidly and in reverse order the startup current limits will be invoked as the output voltage falls through its various voltage thresholds. How far the output voltage drops depends on the load voltage vs. current characteristic. A reduction in input voltage, such as a discharging battery, will lower the load current at which overload occurs. Lower input voltage increases the duty cycle required to produce a given output voltage. And lower input voltage also increases the input current to maintain the input power, which increases dissipative losses and further increases the required duty cycle. Therefore an increase in load current or a decrease in input voltage can result in output overload. Please refer to the Max. IOUT vs. VIN Typical Characteristics plots for the condition that best matches the application. avg where η is efficiency. Neglecting the n-channel FET RDS-ON and the inductor DCR, for duty cycle D, and with T = 1/fosc, IL on 1 L DT 0 VIN dt VIN D T L This is the change in IL during the on-state. During the off-state, again neglecting the p-channel FET RDS-ON and the inductor DCR, IL 1 L T DT off VIN VOUT dt VIN VOUT L T 1D 18 SC121 Applications Information (continued) Note that this is a negative quantity, since VOUT > VIN and 0 < D < 1. For a constant load in steady-state, the inductor current must satisfy ΔIL-on + ΔIL-off = 0. Substituting the two expressions and solving for D, obtain D = 1 – VIN/VOUT. Using this expression, and the positive valued expression ΔIL = ΔIL-on for current ripple amplitude, obtain expanded expression for IL-max and IL-min. IL max,min Manufacturer/ Part # Murata LQM31PN4R7M00 Coilcraft XFL2006-472 Value (μH) 4.7 4.7 DCR (Ω) 0.3 0.7 Rated Current (mA) 700 500 Tolerance (%) 20 20 Dimensions LxWxH (mm) 3.2 x 1.6 x 0.95 2 x 2 x 0.6 VOUT IOUT VIN VIN 2 L VOUT T VOUT VIN From this result, obtain an alternative expression for ΔIL. IL IL max IL min T L VIN VOUT VOUT VIN The inductor selection should consider the n-channel FET current limit for the expected range of input voltage and output load current. The largest IL-avg will occur at the expected smallest VIN and largest IOUT. Determine the largest expected ΔIL. Then for the largest expected IL-avg, ensure that the n-channel FET current limit is not exceed. That is, for the minimum n-channel FET current limit, worst case inductor tolerance, highest expected output current, and lowest expected VIN, ensure that IL-max = IL-avg + ΔIL/2 < ILIM(N). Many of these equations include the parameter η, efficiency. Efficiency varies with VIN, IOUT, and temperature. Estimate η using the plots provided in this datasheet, or from experimental data, at the operating condition of interest. Any chosen inductor should have low DCR compared to the R DS-ON o f the FET switches to maintain efficiency, though for DCR