SC283 POWER MANAGEMENT Features Dual Channel 2.5MHz, 1.8A Synchronous Step-Down Regulator Description The SC283 is a dual channel 1.8A synchronous stepdown regulator designed to operate with an input voltage range of 2.9 to 5.5 Volts. Each channel offers fifteen pre-determined output voltages via four control pins programmable from 0.8 to 3.3 Volts. The control pins allow for on-the-fly voltage changes, enabling system designers to implement dynamic power savings. The SC283 is also capable of adjusting the output voltage via an external resistor divider. VIN Range: 2.9 – 5.5V VOUT Selectable: 0.8 - 3.3V Up to 1.8A Output Current for Each Channel Ultra-Small Footprint, <1mm Height Solution 2.5MHz Switching Frequency Efficiency Up to 93% Low Output Noise Across Load Range Excellent Transient Response Start Up into Pre-Bias Output 100% Duty-Cycle Low Dropout Operation <1µA Shutdown Current Internal Soft Start Input Under-Voltage Lockout Output Over-Voltage, Current Limit Protection Over-Temperature Protection Adjustable Output Voltage 2mm x 3mm x 0.8mm thermally enhanced MLPQ-W18 package -40 to +85°C Temperature Range Pb-Free product. RoHS/WEEE and Halogen Free compliant The device operates with a fixed 2.5MHz oscillator frequency, allowing the use of small surface mount external components. Connecting CTL0 — CTL3 to logic low forces the device into shutdown mode reducing the supply current to less than 1µA. Connecting any of the control pins to logic high enables the converter and sets the output voltage according to Table 1. Other features include undervoltage lockout, soft-start to limit inrush current, and over-temperature protection. The SC283 is available in a thermally-enhanced, 2mm x 3mm x 0.8mm MLPQ-W18 package and has a rated temperature range of -40 to +85°C. Applications Desktop Computing Set-Top Box LCD TV Network Cards Printer Typical Application Circuit July 20, 2010 www.semtech.com SC283 Pin Configuration Ordering Information Device Package SC283WLTRT(2)(3) 2mm x 3mm x 0.8mm MLPQ-W18 SC283EVB(4) Evaluation Board Notes: (1) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (2) Available in tape and reel only. A reel contains 3,000 devices. (3) Pb-Free product. RoHS/WEEE and Halogen Free compliant. (4) Please specify the default VOUTA & VOUTB when ordering. Table 1 – Output Voltage Settings 2mm x 3mm x 0.8mm MLPQ-W18 θJA = 65°C/W (1) Marking Information Marking for 2mm x 3mm MLPQ-W 18 Lead Package: yw = Datecode (Reference Package Marking Design Guidelines, Appendix A) xxx = Semtech Lot number (Example: 901) © 2010 Semtech Corp. CTL3_ CTL2_ CTL1_ CTL0_ Output Voltage 0 0 0 0 Shutdown 0 0 0 1 0.80 0 0 1 0 1.00 0 0 1 1 1.025 0 1 0 0 1.05 0 1 0 1 1.20 0 1 1 0 1.25 0 1 1 1 1.30 1 0 0 0 1.50 1 0 0 1 1.80 1 0 1 0 2.20 1 0 1 1 2.50 1 1 0 0 2.60 1 1 0 1 2.80 1 1 1 0 3.00 1 1 1 1 3.30 www.semtech.com SC283 Recommended Operating Conditions Absolute Maximum Ratings VINA and VINB Supply Voltages ………………… -0.3 to 6.0V Supply Voltage VINA and VINB …………………… 2.9 to 5.5V LXA, LXB Voltage …. . Maximum Output Current for each channel ………… 1.8A -1 to VIN+1V, -3V (20ns Max), 6V Max VOUTA, VOUTB Voltage …………………… -0.3 to VIN+0.3V CTLxA/B pins Voltages -0.3 to VIN+0.3V ………………… Temperature Range …………………………… -40 to +85˚C Peak IR Reflow Temperature …………………………. 260°C ESD Protection Level(6) ……………………………… 3.5kV Thermal Information Thermal Resistance, Junction to Ambient(5) ………… 65 °C/W Maximum Junction Temperature …………………… +150°C Storage Temperature Range ………………… -65 to +150 °C Exceeding the absolute maximum ratings may result in permanent damage to the device and/or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. Notes: (5) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (6) Tested according to JEDEC standard JESD22-A114-B. Electrical Characteristics Unless specified: VINA= VINB= 5.0V, VOUTA= VOUTB=1.50V, CINA=CINB=10µF, COA=COB= 22µF, L= 2.2µH, -40°C≤ TJ≤ +125 °C. Unless otherwise noted typical values are TA= +25 °C. Parameter Conditions Min Typ Max Units Rising VINA, VINB 2.65 2.75 2.85 V Hysteresis 240 300 ΔVOUT Channel A & B; VIN= 2.9 – 5.5V; IOUT=0A -2.0 ILIMIT Channel A & B; Peak LX current 2.25 IQ Channel A & B; No load, Per channel 10 ISHDN CTL0-3= GND, Per channel 1 High Side Switch Resistance(8) RDSON_P Channel A & B; ILX= 100mA, TJ= 25 °C 95 Low Side Switch Resistance(8) RDSON_N Channel A & B; ILX= -100mA, TJ= 25 °C 65 Channel A & B; VIN= 5.5V; LX= 0V; CTL0-3= GND 1 Under-Voltage Lockout Output Voltage Tolerance(7) Current Limit Supply Current Shutdown Current LX Leakage Current(8) Load Regulation Symbol UVLO ILK(LX) Channel A & B; VIN= 5.5V; LX= 5.0V; CTL0-3= GND -10 3.0 mV +2.0 % 3.75 A mA 10 µA mΩ 10 -1 ΔVLOAD-REG Channel A & B; VIN= 5.0V; IOUT=1mA – 1.8A Oscillator Frequency fOSC Channel A & B Soft-Start Time tSS Channel A & B; IOUT= 1.8A 850 µs Average LX Current, VOUT=1.5V 240 mA Average LX Current, VOUT=3.3V 130 mA Foldback Holding Current CTLx Input Current(8) CTLx Input High Threshold © 2010 Semtech Corp. ICL_HOLD ±0.5 µA 2.125 ICTL_ Channel A & B; CTL0-3=VIN or GND -2.0 VCTLx_HI Channel A & B 1.2 2.500 % 2.875 2.0 MHz µA V www.semtech.com SC283 Electrical Characteristics (continued) Parameter Symbol Conditions VCTLx_LO Channel A & B VOUT Over Voltage Protection VOVP Channel A & B 115 % Thermal Shutdown Temperature TSD Channel A & B(9) 160 °C Channel A & B 10 °C CTLx Input Low Threshold Thermal Shutdown Hysteresis TSD_HYS (9) Min Typ Max Units 0.4 V Notes: (7) The “Output Voltage Tolerance” includes output voltage accuracy, voltage drift over temperature and the line regulation. (8) The negative current means the current flows into the pin and the positive current means the current flows out from the pin. (9) The thermal shutdown for both Channel A and B is independent from each other. © 2010 Semtech Corp. www.semtech.com SC283 Typical Characteristics Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, Unless otherwise noted, L= 2.2uH (TOKO: 1127AS-2R2M). Efficiency vs. Load Current Efficiency Total Loss (Per Channel) vs. Load Current Total Loss 100% 1000 VIN=5.0V;VOUT=3.3V TA=25°C 95% 90% 85% Loss (mW) Efficiency (%) VIN=5.0V;VOUT=3.3V 800 80% VIN=3.3V;VOUT=1.5V 75% 600 VIN=3.3V;VOUT=1.5V 400 70% VIN=5.0V;VOUT=1.5V 65% 200 TA=25°C VIN=5.0V;VOUT=1.5V 60% 0 0.0 0.3 0.6 0.9 1.2 Output Current (A) 1.5 1.8 0.0 0.9 1.2 Output Current (A) 1.5 1.8 500 1.0% TA=25°C 0.8% TA= 25°C 450 0.6% 400 0.2% Dropout Voltage (mV) VIN=3.3V;VOUT=1.5V 0.4% VIN=5.0V;VOUT=1.5V 0.0% -0.2% -0.4% -0.6% L= 1071AS-2R2M (DCR= 60m_max) 350 300 250 200 150 100 VIN=5.0V;VOUT=3.3V -0.8% L= 1127AS-2R2M (DCR=48m_max) 50 -1.0% 0 0.0 0.3 0.6 0.9 1.2 1.5 1.8 0.0 0.3 Output Current (A) 0.9 1.2 1.5 1.8 HysteresisVariation Variation UVLOUVLO Hysteresis 5% 0.8% 4% 0.6% 3% 0.4% 2% 0.2% 1% Variation 1.0% 0.0% -0.2% 0% -1% -0.4% -2% -0.6% -3% -0.8% 0.6 Output Current (A) Rising Threshold Variation UVLOUVLO Rising Threshold Variation Variation 0.6 DropoutDropout Voltage in 100% Duty Cycle Operation Voltage of 100% Duty Cycle Operation LoadLoad Regulation Regulation Load Regulation 0.3 -4% IOUT= 0A IOUT= 0A -5% -1.0% -40 -15 10 35 60 -40 85 © 2010 Semtech Corp. -15 10 35 60 85 Ambient Temperature (°C) Ambient Temperature (°C) www.semtech.com SC283 Typical Characteristics (continued) Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, Unless otherwise noted, L= 2.2uH (TOKO: 1127AS-2R2M). (P & N)vs. Variation Line RDS(ON)RDSON Variation Inputover Voltage RDSON (P & N) Variation Over Temperature RDS(ON) Variation vs. Temperature 30% 20% 25% 15% P-Channel 20% 10% 5% Variation 15% Variation VIN= 5.0V ILX= ±100mA 10% N-Channel 0% -5% 5% -10% 0% ILX= ±100mA TA= 25°C -5% N-Channel P-Channel -15% -20% -10% 2.5 3.0 3.5 4.0 4.5 5.0 -40 5.5 -15 5% 60 85 1.0% 4% 0.8% VOUT= 3.3V 3% 0.6% 2% 0.4% 1% 0.2% Variation Variation 35 Switching Frequency Variation vs. Temperature Switching Frequency Variation Switching Frequency Variation over Switching Frequency Variation vs. Line Input Voltage 0% -1% -2% -0.2% -0.6% IOUT= 0A TA= 25°C -4% 0.0% -0.4% VOUT= 1.5V -3% VIN= 5.0V IOUT= 0A -0.8% -5% -1.0% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 -40 -15 Input Voltage (V) 10 35 60 85 Ambient Temperature (°C) Line Regulation Line Regulation ove Line Line Regulation Temperature Line Regulationvs. over Temperature 1.0% 1.0% 0.8% 0.8% 0.6% 0.6% VOUT= 1.5V 0.4% 0.4% 0.2% Regulation Regulation 10 Ambient Temperature (°C) Input Voltage (V) 0.0% -0.2% -0.4% 0.2% 0.0% -0.2% -0.4% VOUT= 3.3V -0.6% -0.6% IOUT= 0A TA= 25°C -0.8% VOUT= 1.5V IOUT= 0A -0.8% -1.0% -1.0% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 -40 Input Voltage (V) © 2010 Semtech Corp. -15 10 35 60 85 Ambient Temperature (°C) www.semtech.com SC283 Typical Waveforms Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M). Output Voltage Ripple (VOUT=1.5V) Output Voltage Ripple (VOUT=1.5V) Output Voltage Ripple (VOUT=1.5V) Output Voltage Ripple (VOUT=1.5V) VOUT 10mV/div VOUT 10mV/div ILX 1A/div ILX 1A/div VLX 2V/div VLX 2V/div VIN=3.3V IOUT=1.8A VIN=5.0V IOUT=1.8A 500ns/div Output Voltage Ripple (VOUT=3.3V) 500ns/div Output Voltage Ripple (VOUT=3.3V) Output Voltage Ripple (VOUT=3.3V) Output Voltage Ripple (VOUT=3.3V) VOUT 10mV/div VOUT 10mV/div ILX 0.5A/div ILX 1A/div VLX 2V/div VLX 2V/div VIN=5.0V IOUT=0A VIN=5.0V IOUT=1.8A 500ns/div Transient Response (VOUT=1.5V) 500ns/div Transient Response (VOUT=3.3V) Transient Response (VOUT=1.5V; 0A to 1A to 0A) Transient Response (VOUT=3.3V; 0A to 1A to 0A) VOUT VOUT 100mV/div 100mV/div IOUT IOUT 1A/div 500mA/div VIN=5.0V IOUT=0A to 1A © 2010 Semtech Corp. VIN=5.0V IOUT=0A to 1A 50µs/div 50µs/div www.semtech.com SC283 Typical Waveforms (continued) Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M). Start Up (VOUT=1.5V) Start Up (VOUT=1.5V) Start Up (Enable)(VOUT=1.5V) Start Up (Enable)(VOUT=1.5V) VIN VIN 2V/div 2V/div VCTLx VCTLx 2V/div 2V/div VOUT VOUT 0.5V/div 0.5V/div VIN=5.0V ROUT=1k VIN=5.0V ROUT=0.83 (1.8A) 50µs/div Start Up (VOUT=1.5V), EN=VIN 200µs/div Start Up (VOUT=1.5V), EN=VIN Start Up (Power up VIN=VCTLx) (VOUT=1.5V) Start Up (Power up VIN=VCTLx) (VOUT=1.5V) VIN VIN 2V/div 2V/div VOUT VOUT 0.5V/div 0.5V/div VIN=5.0V ROUT=1k VIN=5.0V ROUT=0.83 (1.8A) 200µs/div Start Up (VOUT=3.3V) 200µs/div Start Up (VOUT=3.3V) Start Up (Enable)(VOUT=3.3V) Start Up (Enable)(VOUT=3.3V) VIN VIN 2V/div 2V/div VCTLx VCTLx 2V/div 2V/div VOUT VOUT 1V/div 1V/div VIN=5.0V ROUT=1k © 2010 Semtech Corp. VIN=5.0V ROUT=1.83 (1.8A) 100µs/div 200µs/div www.semtech.com SC283 Typical Waveforms (continued) Circuit Conditions: CIN= 10uF/6.3V; COUT= 22uF/6.3V, L= 2.2uH (TOKO: 1127AS-2R2M). Start Up (VOUT=3.3V), EN=VIN Start Up (Power up VIN=VCTLx) (VOUT=3.3V) Start Up (VOUT=3.3V), EN=VIN Start Up (Power up VIN=VCTLx) (VOUT=3.3V) VIN VIN 2V/div 2V/div VOUT VOUT 1.5V/div 1.5V/div VIN=5.0V ROUT=1k VIN=5.0V ROUT=1.83 (1.8A) 200µs/div Shutdown-Disable (1.5V) 200µs/div Shutdown-Disable (3.3V) Shutdown (Disable)(VOUT=1.5V) Shutdown (Disable)(VOUT=3.3V) VIN VIN 2V/div 2V/div VCTLx VCTLx 2V/div 2V/div VOUT VOUT 1V/div 1.5V/div VIN=5.0V ROUT=1.5 © 2010 Semtech Corp. VIN=5.0V ROUT=3.3 200µs/div 200µs/div www.semtech.com SC283 Pin Descriptions Pin # Pin Name 1 vinA Channel A. Input supply voltage for the converter power stage and internal circuitry. 2 LXA Switching node of Channel A - connect an inductor between this pin and the output capacitor. 3, 13, T1 GNDA Channel A. Ground connection for converter power stage and internal circuitry. 4, 12, T2 GNDB Channel B. Ground connection for converter power stage and internal circuitry. 5 CTL3B Channel B. Control bit 3 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 6 CTL2B Channel B. Control bit 2 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 7 CTL1B Channel B. Control bit 1 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 8 CTL0B Channel B. Control bit 0 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 9 VOUTB Output voltage sense pin of Channel B. 10 vinB Channel B. Input supply voltage for the converter power stage and internal circuitry. 11 LXB Switching node of Channel B - connect an inductor between this pin and the output capacitor. 14 CTL3A Channel A. Control bit 3 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 15 CTL2A Channel A. Control bit 2 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 16 CTL1A Channel A. Control bit 1 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 17 CTL0A Channel A. Control bit 0 - see Table 1 for decoding. This pin has a 1 MΩ internal pulldown resistor. This resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in undervoltage lockout. 18 VOUTA Output voltage sense pin of Channel A. © 2010 Semtech Corp. Pin Function 10 www.semtech.com SC283 Block Diagram Current Amp AVIN PVIN VINA Plimit Amp Comp Plimit Oscillator & Slope Generator Control Logic VOUT VOUTA LXA LX CTL0A CTL0 CTL1A CTL1 Voltage Select CTL2 CTL2A Error Amp PWM Comp 500mV Ref CTL3A CTL3 GNDA PGND AGND Current Amp AVIN PVIN VINB Plimit Amp Comp Plimit Oscillator & Slope Generator Control Logic VOUT VOUTB LXB LX CTL0B CTL0 CTL1B CTL1 CTL2 CTL2B CTL3B CTL3 Voltage Select Error Amp PWM Comp 500mV Ref GNDB PGND AGND © 2010 Semtech Corp. 11 www.semtech.com SC283 Applications Information Detailed Description enough in value for the current through the resistor chain to be at least 20µA in order to ignore the VOUT pin current. The SC283 is a two channel synchronous step-down converter. Both channels on this device are designed to operate in fixed-frequency PWM mode at 2.5MHz and provide the same current capacity of up to 1.8A. The switching frequency is chosen to minimize the size of the external inductor and capacitors while maintaining high efficiency. Both channels of SC283 are independent. RFB1 = (1) where VOSTD is the pre-determined output voltage via the CTL pins. Operation CFF is needed to maintain good transient response performance. The correct value of CFF can be found using Equation 2. During normal operation, the PMOS MOSFET is activated on each rising edge of the internal oscillator. The voltage feedback loop uses an internal feedback resistor divider. The period is set by the internal oscillator. The device has an internal synchronous NMOS rectifier and does not require a Schottky diode on the LX pin. The device operates as a buck converter in PWM mode with a fixed frequency of 2.5MHz. C FF [nF ] = 2.5 × (VOUT − 0.5)2 VOSTD ×( ) RFB1[kΩ] ⋅ (VOUT − VOSTD ) VOSTD − 0.5 (2) To simplify the design, it is recommended to program the desired output voltage from a standard 1.0V as shown in Figure 1 with the correct CFF calculated from Equation 2. For programming the output voltage from other standard voltages, RFB1, RFB2 and CFF need to be adjusted to meet Equations 1 and 2. Programmable Output Voltage Both channels on SC283 have fifteen pre-determined output voltage values which can be individually selected by programming the CTL input pins (see Table 1 — Output Voltage Settings). Each CTL pin has an active 1 MΩ internal pulldown resistor. The 1MΩ resistor is switched in circuit whenever the CTL input voltage is below the input threshold, or when the part is in undervoltage lockout. It is recommended to tie all high CTL pins together and use an external pull-up resistor to VIN if there is no enable signal, or if the enable input is an open drain/collector signal. The CTL pins may be driven by a microprocessor to allow dynamic voltage adjustment for systems that reduce the supply voltage when entering sleep states. Avoid all zeros being present on the CTL pins when changing programmable output voltages as this would disable the device. L_ VIN VOUT_ LX_ VIN CIN_ 10µF SC283 (Channel A or B) COUT_ RFB1 CFF VOUT_ RFB2 CTL0_ Enable CTL1_ CTL3_ RFB1 = (VOUT − 1) × RFB 2 for CTLx= 0010 (1.0V) CTL2_ GND Figure 1 — Typical Schematic for Adjusting the Output Voltage Up from an Output Voltage of 1.0V (CTLx=[0010]) Maximum Power Dissipation Each channel of SC283 has its own ΘJA of 65°C/W when only one channel is in operation. Since both channels are within same package, there is about 50% heat which will be transferred to the adjacent channel. The equivalent total thermal impedence will be higher when the neighboring channel is also in operation. To guarantee an operating junction temperature of less than 125°C, Figure 2 shows the maximum allowable power loss of each channel. The curve is based upon the junction temperature of either channel reaching a maximum of 125°C. Each channel of SC283 can support up to 1.8A load current. Figures 3a SC283 is also capable of regulating a different (higher) output voltage, which is not shown in the Table 1, via an external resistor divider. There will be a typical 2µA current flowing into the VOUT pin. The typical schematic for an adjustable output voltage option from the standard 1.0V with CTLx=[0010], is shown in Figure 1. RFB1 and RFB2 are used to adjust the desired output voltage. If the RFB2 current is such that the 2µA VOUT pin current can be ignored, then RFB1 can be found by Equation 1. RFB2 needs to be low © 2010 Semtech Corp. VOUT − VOSTD ⋅ RFB 2 VOSTD 12 www.semtech.com SC283 Applications Information (continued) SC283 Maximum Load Current for T J=125°C and 3b show the maximum allowable load current based upon the limit of maximum loss for VIN=3.3V and VIN=5.0V, respectively. The curves are drawn for high duty-cycle operation. If the operating duty-cycle is lower, the loss is SC283 load Maximum Loss for T J=125°C lower allowing higher current. 2.0 Load Current of Channel B (A) 1.8 1.6 TA= 25°C 1.4 1.2 TA= 55°C 1.6 1.4 1.2 1.0 TA= 60°C 0.8 0.6 VIN= 3.3V VOUT= 2.5V 0.2 1.0 TA= 85°C 0.4 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 TA= 85°C 0.2 0.0 0.8 0.0 Load Current of Channel A (A) 0.6 (a)Maximum VIN= 3.3V, =2.5V SC283 LoadV Current for T J=125°C OUT 0.4 Load Current of Channel B (A) 1.8 Loss of Channel A (W) Figure 2 — Maximum allowable loss for each channel for a maximum junction temperature of 125°C Protection Features The SC283 provides the following protection features: Current Limit Over-Voltage Protection Soft-Start Operation Thermal Shutdown • • • • TA= 70°C 1.6 1.4 1.2 1.0 0.8 TA= 85°C 0.6 0.4 VIN= 5.0V VOUT= 3.3V 0.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.0 0.0 Load Current of Channel A (A) (b) VIN= 5.0V, VOUT=3.3V Current Limit and Protection Figure 3 — Maximum allowable Load Current for each channel for a maximum junction temperature of 125°C The internal PMOS power device in the switching stage is protected by a current limit feature. If the inductor current is above the PMOS current limit for 16 consecutive cycles, the part enters foldback current limit mode and the output current is limited to the current limit holding current (ICL_HOLD) of a few hundred milliampere. Under this condition, the output voltage will be the product of ICL_HOLD and the load resistance. The current limit holding current will decrease when the output voltage increases. The load presented must fall below the current limit holding current for the part to exit foldback current limit mode. Figure 4 shows how the typical current limit holding current varies with output voltage. The SC283 is capable of sustaining an indefinite short circuit without damage and will resume normal operation when the fault is removed. The foldback current limit mode is disabled during soft-start. Current limit functionality is shown in Figure 6. © 2010 Semtech Corp. TA 47°C 0.4 1.6 1.4 1.2 1.0 0.8 0.6 1.8 0.4 0.0 0.2 2.0 0.0 0.2 0.2 Loss of Channel B (W) TA 42°C 1.8 Current Limit Holding Current over Vout Current Limit Holding Current (mA) 300 TA= 25°C 250 VIN= 3.6V VIN= 5.0V 200 150 100 VIN= 3.3V 50 0 1.0 1.5 2.0 2.5 3.0 3.5 Output Voltage (V) Figure 4 — Typical Current Limit Holding Current 13 www.semtech.com SC283 Applications Information (continued) vs. Output Voltage corner frequency of the output filter is shown in Equation 3. Over-Voltage Protection In the event of a 15% over-voltage on the output, the PWM drive is disabled leaving the LX pin floating. fC = Soft-Start The soft-start mode is activated after VIN reaches its UVLO and one or more CTL pins are set high to enable the part. A thermal shutdown event will also activate the soft start sequence. Soft-start mode controls the maximum current during startup thus limiting inrush current. The PMOS current limit is stepped through four soft start levels of approximately 20%, 25%, 40%, & 100%. Each step is maintained for 200μs following an internal reference start up duration of 50μs giving a total nominal startup period of 850μs. During startup, the chip operates by controlling the inductor current swings between 0A and current limit. If at any time VOUT reaches 86% of the target or at the end of the soft-start period, the SC283 will switch to PWM mode operation. Figure 5 shows the typical diagram of soft start operation. (3) Values outside this range may lead to instability, malfunction, or out-of-specification performance. In general, the inductance is chosen by making the inductor ripple current to be less than 30% of maximum load current. When choosing an inductor, it is important to consider the change in inductance with DC bias current. The inductor saturation current is specified as the current at which the inductance drops a specific percentage from the nominal value. This is approximately 30%. Except for short-circuit or other fault conditions, the peak current must always be less than the saturation current specified by the manufacturer. The peak current is the maximum load current plus one half of the inductor ripple current at the maximum input voltage. Load and/or line transients can cause the peak current to exceed this level for short durations. Maintaining the peak current below the inductor saturation specification keeps the inductor ripple current and the output voltage ripple at acceptable levels. Manufacturers often provide graphs of actual inductance and saturation characteristics versus applied inductor current. The saturation characteristics of the inductor can vary significantly with core temperature. Core and ambient temperatures should be considered when examining the core saturation characteristics. The SC283 is capable of starting up into a pre-biased output. When the output is precharged by another supply rail, the SC283 will not discharge the output during the soft start interval. Shut Down When all CTL pins of each channel are low, the channel will run in shutdown mode, drawing less than 1μA from the input power supply. The internal switches and bandgap voltage will be immediately turned off. When the inductance has been determined, the DC resistance (DCR) must be examined. The efficiency that can be achieved is dependent on the DCR of the inductor. The lower values give higher efficiency. The RMS DC current rating of the inductor is associated with losses in the copper windings and the resulting temperature rise of the inductor. This is usually specified as the current which produces a 40˚C temperature rise. Most copper windings are rated to accommodate this temperature rise above maximum ambient. Thermal Shutdown The device has a thermal shutdown feature to protect the SC283 if the junction temperature exceeds 160°C. During thermal shutdown, the on-chip power devices are disabled, tri-stating the LX output. When the temperature drops by 10°C, it will initiate a soft start cycle to resume normal operation. Inductor Selection The SC283 converter has internal loop compensation. The compensation is designed to work with an output filter corner frequency of less than 40kHz for a VIN of 5V and 50KHz for a VIN of 3.3V over any operating condition. The © 2010 Semtech Corp. 1 2π L ⋅ COUT Magnetic fields associated with the output inductor can interfere with nearby circuitry. This can be minimized by the use of low noise shielded inductors which use the 14 www.semtech.com SC283 Applications Information (continued) minimum gap possible to limit the distance that magnetic fields can radiate from the inductor. However shielded inductors typically have a higher DCR and are thus less efficient than a similarly sized non-shielded inductor. Final inductor selection depends on various design considerations such as efficiency, EMI, size, and cost. Table 2 lists the manufacturers of recommended inductor options. The saturation characteristics and DC current ratings are also shown. mined by the capacitance of the ceramic output capacitor. The ceramic capacitor supplies the load current initially until the loop responds. Within a few switching cycles the loop will respond and the inductor current will increase to match the required load. The output voltage droop during the period prior to the loop responding can be related to the choice of output capacitor by the relationship from Equation 4. COUT = Manufacturer Part Number L (μH) DCR Max (Ω) Rated Current (A) L at Rated Current (μH) Dimensions LxWxH (mm) TOKO 1071AS-2R2M 2.20±20% 0.060 1.80 1.54 2.8x3.0x1.5 TOKO 1071AS-1R0N 1.00±30% 0.040 2.70 0.70 2.8x3.0x1.5 TOKO 1127AS-2R2M 2.20±20% 0.048 2.50 1.54 3.5x3.7x1.8 Panasonic ELLVGG1R0N 1.00±23% 0.062 2.20 0.70 3.2x3.2x1.5 (4) The output capacitor RMS ripple current may be calculated from Equation 5. I COUT ( RMS ) = 1 VOUT ⋅ (VIN ( MAX ) − VOUT ) L ⋅ f OSC ⋅VIN 2 3 (5) Table 3 lists the manufacturers of recommended output capacitor options. Table 2 – Recommended Inductors COUT Selection The internal voltage loop compensation in the SC283 limits the minimum output capacitor value to 22µF if using a 2.2µH inductor or 44µF if using a 1µH inductor. This is due to its influence on the the loop crossover frequency, phase margin, and gain margin. Increasing the output capacitor above this minimum value will reduce the crossover frequency and provide greater phase margin. The total output capacitance should not exceed 50µF to avoid any start-up problems. For most typical applications it is recommended to use an output capacitance of 22µF to 44µF. When choosing the output capacitor’s capacitance, verify the voltage derating effect from the capacitor vendor’s data sheet. Type Rated Voltage (VDC) Value at 3.3V (μF) Dimensions LxWxH (mm) 10±10% X5R 6.3 4.74 2.0x1.25x1.25 (EIA:0805) Murata GRM219R60J106K 10±10% X5R 6.3 4.05 2.0x1.25x0.85 (EIA:0805) Murata GRM21BR60J226M 22±20% X5R 6.3 6.57 2.0x1.25x1.25 (EIA:0805) Murata GRM31CR60J476M 47±20% X5R 6.3 20.3 3.2x1.6x1.6 (EIA:1206) Manufacturer Part Nunber Value (μF) Murata GRM21BR60J106K Table 3 – Recommended Capacitors CIN Selection Capacitors with X7R or X5R ceramic dielectric are recommended for their low ESR and superior temperature and voltage characteristics. Y5V capacitors should not be used as their temperature coefficients make them unsuitable for this application. The SC283 source input current is a DC supply current with a triangular ripple imposed on it. To prevent large input voltage ripple, a low ESR ceramic capacitor is required. A minimum value of 10μF should be used. It is important to consider the DC voltage coefficient characteristics when determining the actual required value. It should be noted a 10µF, 6.3V, X5R ceramic capacitor with 5V DC applied may exhibit a capacitance as low as 4.5µF. The output voltage droop due to a load transient is deter© 2010 Semtech Corp. 3 ⋅ ∆I LOAD VDROOP ⋅ f OSC 15 www.semtech.com SC283 Applications Information (continued) To estimate the required input capacitor, determine the acceptable input ripple voltage and calculate the minimum value required for CIN from Equation 6. C IN VOUT 1 − VIN = ∆V − ESR ⋅ f OSC I OUT VOUT VIN (6) The input capacitor RMS ripple current varies with the input and output voltage. The maximum input capacitor RMS current is found from Equation 7. I CIN ( RMS ) = VOUT VIN VOUT 1 − VIN (7) The input voltage ripple and RMS current ripple are at a maximum when the input voltage is twice the output voltage or 50% duty cycle. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the PMOS switch. Low ESR/ESL X5R ceramic capacitors are recommended for this function. To minimise stray inductance ,the capacitor should be placed as closely as possible to the VIN and GND pins of the SC283. © 2010 Semtech Corp. 16 www.semtech.com SC283 Applications Information (continued) SC4633 Soft Start B Stage 1 A Stage 0 Stage 2 C G F Stage 3 H D Stage 5 Stage 4 E I Stage 6 Figure 5 — Typical Diagram of Soft Start Operation SC183C/SC283/SC4633 Over Current Protection J Stage 7 K Stage 8 Stage 6 M L Figure 6 — Typical Diagram of Current Limit Protection © 2010 Semtech Corp. 17 www.semtech.com SC283 Applications Information (continued) PCB Layout Considerations The layout diagram in Figure 7 shows a recommended top-layer PCB for the SC283 and supporting components. Figure 8 shows the bottom layer for this PCB. Fundamental layout rules must be followed since the layout is critical for achieving the performance specified in the Electrical Characteristics table. Poor layout can degrade the performance of the DC-DC converter and can contribute to EMI problems, ground bounce, and resistive voltage losses. Poor regulation and instability can result. The following guidelines are recommended when developing a PCB layout: LB VIN VOUTB COUTB CINB GND COUTA CTLxA GND 1. The input capacitor, CIN, should be placed as close to the VIN and GND pins as possible. This capacitor provides a low impedance loop for the pulsed currents present at the buck converter’s input. Use short wide traces to connect as closely to the IC as possible. This will minimize EMI and input voltage ripple by localizing the high frequency current pulses. 2. Keep the LX pin traces as short as possible to minimize pickup of high frequency switching edges to other parts of the circuit. COUT and L should be connected as close as possible between the LX and GND pins, with a direct return to the GND pin from COUT. 3. Route the output voltage feedback/sense path away from the inductor and LX node to minimize noise and magnetic interference. 4. Use a ground plane referenced to the SC283 GND pin. Use several vias to connect to the component side ground to further reduce noise and interference on sensitive circuit nodes. 5. If possible, minimize the resistance from the output and GND pin to the load. This will reduce the voltage drop on the ground plane and improve the load regulation. It will also improve the overall efficiency by reducing the copper losses on the output and ground planes. GND U1 CTLxB CINA VOUTA VIN LA GND Figure 7 — Recommended PCB Layout (Top Layer) VIN GND Figure 8 — Bottom Layer Detail © 2010 Semtech Corp. 18 www.semtech.com SC283 A D DIMENSIONS MILLIMETERS MIN NOM MAX B DIM 0.70 0.80 A 0.05 A1 0.00 (0.20) A2 b 0.15 0.20 0.25 D 1.90 2.00 2.10 D1 0.136 0.286 0.386 2.90 3.00 3.10 E E1 0.55 0.70 0.80 e 0.40 BSC L 0.375 0.425 0.475 18 N ND 2 7 NE aaa 0.08 bbb 0.10 Outline Drawing – 2x3 MLPQ-W18 A D DIM A aaa C A2 A1 C MIN NOM MAX 0.70 0.80 A 0.05 A1 0.00 (0.20) A2 b 0.15 0.20 0.25 D 1.90 2.00 2.10 A 0.386 D1 0.136 0.286 Eaaa2.90 C 3.00 3.10 E1 0.55 0.70 A2 0.80 e 0.40 BSC L 0.375 0.425 0.475 18 N D1 ND 2 7 NE aaa 0.08 bbb 0.10 E PIN 1 INDICATOR (LASER MARK) SEATING PLANE 1.700 0.850 2X E1 D1 LxN A1 C SEATING PLANE LxN E/2 2 1 N E/2 1.700 0.850 2X E1 E PIN 1 DIMENSIONS INDICATOR MILLIMETERS (LASER MARK) B bxN e/2 e bbb C A B D/2 2 NOTES: 1 N bxN e/2 e bbb C A B 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. D/2 Land Pattern – 2x3 MLPQ-W18 NOTES: © 2010 Semtech Corp. 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. 19 www.semtech.com SC283 © Semtech 2010 All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent or other industrial or intellectual property rights. Semtech assumes no responsibility or liability whatsoever for any failure or unexpected operation resulting from misuse, neglect improper installation, repair or improper handling or unusual physical or electrical stress including, but not limited to, exposure to parameters beyond the specified maximum ratings or operation outside the specified range. SEMTECH PRODUCTS ARE NOT DESIGNED, INTENDED, AUTHORIZED OR WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT APPLICATIONS, DEVICES OR SYSTEMS OR OTHER CRITICAL APPLICATIONS. INCLUSION OF SEMTECH PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE UNDERTAKEN SOLELY AT THE CUSTOMER’S OWN RISK. Should a customer purchase or use Semtech products for any such unauthorized application, the customer shall indemnify and hold Semtech and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs damages and attorney fees which could arise. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com © 2010 Semtech Corp. 20 www.semtech.com