DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch General Description Features The AAT2138 is a high current synchronous step-down converter with an integrated current-limited load switch designed for high precision current control applications. By guarding against excessive input current, AAT2138 enables the system designer to maximize the output current from the step-down converter while protecting the input supply. It is designed for protection of USB ports from heavy load transient conditions commonly seen with high data rate modem applications. • Input Voltage Range: 2.7V to 5.5 V • Output Voltage Range: Down to 3.0V • Current-Limited Load Switch - ±10% High Precision - Programmable Current Limit Range: 500mA to 3A • High-Current Step-Down Converter - Efficiency up to 95% - 85mΩ High-Side; 50mΩ Low-Side FETs - 2.8MHz Switching Frequency - 100% Low Dropout Operation - No External Compensation Required - PFM/PWM or forced PWM Modes - 1MHz to 3MHz External Clock Support • Soft Start • Independent Enable Pins for Switch and Converter • Over-Temperature and Over Load Protection • -40°C to +85°C Temperature Range • Pb-Free, Low-Profile, 3mmx3mm 14-pin TDFN Package AAT2138 integrates a high precision programmable current limited P-channel MOSFET load switch to protect the input supply against large currents which may cause the supply to fall out of regulation. The current limit threshold is programmed up to 3A by an external resistor between RSET and ground. The AAT2138 is a 2.8MHz current mode step-down converter. It utilizes a tiny 1μH inductor and internal compensation to enable it to have an extremely small foot print. The regulator supports 100% duty cycle operation for dropout conditions. An external clock in the range of 1MHz to 3MHz can be fed into MODE as a synchronous signal for the step-down regulator. This feature offers the flexibility of fine tuning the switching frequency and optimizing performance in the application. Applications • • • • • • AAT2138 is available in a Pb-free, low profile, 14-pin 3mm x 3mm TDFN package. The product is rated over the -40°C to 85°C temperature range. Wireless Modem Data Cards Portable Hard Drives Portable Memory Card Readers Barcode Scanners Credit Card Readers General USB Powered Devices Typical Application OFF VIN 2.7V-5.5V SYSIN ENSYS RSET CIN 1μH REGIN CIN Bulk Capacitor CIN 0.1μF ON RSET 1μF 10μF R1 274k Ω AAT2138 PGND ON ENREG VOUT 3.4V/3A SW FB R2 59k Ω COUT 22μF OFF PWM MODE AGND PFM/PWM Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 1 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Pin Descriptions Pin # Symbol 1, 2 PGND 3, 4 REGIN 5 ENREG 6 RSET 7 AGND 8 ENSYS 9 FB 10 MODE 11, 12 SYSIN 13, 14 SW EP GND (EP) Function Power Ground. PGND is internally connected to the source of the low-side N-channel MOSFET. Input Power Supply for the Step-Down Regulator. Connect REGIN to the input power source. Bypass REGIN to PGND with a 4.7μF or greater ceramic capacitor. REGIN internally connects to the source of the load switch output and the step-down regulator’s high-side P-channel MOSFET (see the Functional Block Diagram). Step-Down Regulator Enable Input. A logic high enables the AAT2138 switching regulator. A logic low forces the AAT2138 into shutdown mode, placing the output into a high-impedance state and reducing the quiescent current to less than 1μA. Do NOT leave ENREG floating. Load Switch Current-Limit Adjustment. Connect a resistor between RSET and ground to set the input current limit threshold. Analog Ground. AGND is internally connected to the analog ground of the control circuitry. Load Switch Enable. Active Low Input. A logic low enables the AAT2138 load switch. A logic high disables the AAT2138 load switch. Do NOT leave ENSYS floating. Feedback Input / Output Voltage Sense. FB senses the output voltage for regulation control. For adjustable output versions, connect a resistive divider network from the output to FB to GND to set the output voltage accordingly. The FB regulation threshold is 0.6V. Regulator Operating Mode. Pull MODE high to force the regulator into forced PWM operation. Pull MODE low to allow automatic pulse-skipping under light-load operation. An external clock in the range of 1MHz to 3MHz can be connected into MODE as a synchronous clock signal for the step-down converter to bypass the internal oscillator. System Load Switch Input. SYSIN connects to the source of the P-channel load switch that limits the system input supply current. Bias Input Supply for the System and Step-Down Regulator. SYSIN supplies power to the analog and logic control circuitry of the AAT2138. Inductor Switching Node. SW is internally connected to the source of the high-side P-Channel MOSFET and the drain of the low-side N-channel MOSFET. Externally connected to the power inductor as shown in the Typical Application Circuit. Exposed Pad. Connect directly to the ground plane to reduce the thermal impedance. Pin Configuration TDFN33-14 (Top View) 2 PGND 1 14 SW PGND 2 13 SW REGIN 3 12 SYSIN REGIN 4 11 SYSIN ENREG 5 10 MODE RSET 6 9 FB AGND 7 8 ENSYS Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Absolute Maximum Ratings1 Symbol VIN VSW VFB VRSET VEN ISYSIN VGND TJ TA TLEAD Description SYSIN, REGIN to PGND SW to PGND FB to AGND RSET to AGND ENSYS, ENREG to AGND Load Switch Maximum RMS Current Capability PGND to AGND Operating Junction Temperature Range Operating Ambient Temperature Range Maximum Soldering Temperature (at leads, 10 sec) Value -0.3 to +6 -0.3 to (REGIN + 0.3) -0.3 to (SYSIN + 0.3) -0.3 to (VIN + 0.3) -0.3 to (VIN + 0.3) ±4.0 -0.3 to +0.3 -40 to 150 -40 to 85 300 Units V A V °C Thermal Characteristics Symbol ΘJA PD Description Maximum Thermal Resistance Maximum Power Dissipation2, 3 Value Units 60 1.67 °C/W W 1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. 2. Mounted on an FR4 board. 3. Derate 70mW/°C above 25°C. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 3 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Electrical Characteristics1 VSYSIN = 5.0V, ENREG = SYSIN, ENSYS = AGND = PGND, TA = -40°C to 85°C unless otherwise noted. Typical values are at TA = 25°C. Symbol VSYSIN Description Conditions System Input Voltage Range SYSIN, REGIN SYSIN Rising Hysteresis REGIN Rising2 Hysteresis SYSIN Rising Hysteresis VSYSIN = VREGIN = 3.6V Hysteresis = 15°C No Load Current; Not Switching ENREG = SW = GND, VUVLO(SYSIN) System Input Under-Voltage Lockout VUVLO(REGIN) Regulator Input Under-Voltage Lockout VDIS(SYSIN) Regulator Input Discharge Threshold RDISCHRG TSHDN IQ ISHDN System Input RDS(ON) ΔISYSILIM Discharge MOSFET On-Resistance Over-Temperature Shutdown Threshold No Load Supply Current Shutdown Current Load Switch Load Switch On-Resistance Current Limit Accuracy Short-Circuit Current-Limit Threshold Adjustable Current-Limit Range Current-Limit Response Time tSS(SYS) Load Switch Soft-Start Period VIH_ENSYS ENSYS Input Logic Threshold High ENSYS Input Logic Threshold Low VIL_ENSYS IENSYS ENSYS Input Current Step-Down Regulator VOUT Output Voltage Range IFB FB Leakage Current VSYSIN = 5.0V, TA = 25°C RSET = 6.4kΩ, VREGIN < VUVLO(REGIN), TA = 20°C to 85°C RSET = 6.4kΩ, VREGIN < VUVLO(REGIN), TA = 20°C to 85°C TA = 25°C Min 2.7 FB Regulation Threshold3 ΔVOUT/IOUT Load Regulation ΔVOUT/VIN Line Regulation ILIMPK RDS(ON)HI RDS(ON)LO fOSC tSS(REG) tOFF VIH_ENREG VIL_ENREG IENREG High-Side P-Channel MOSFET Current Limit High-Side P-Channel MOSFET OnResistance Low-Side N-Channel MOSFET OnResistance Internal Oscillator Frequency Step-Down Regulator Soft-Start Period4 Minimum Off-time ENREG Input Logic Threshold High ENREG Input Logic Threshold Low ENREG Input Current Max 5.5 2.7 2.7 0.2 3.15 0.15 8 150 90 1 85 -10 ±5 10 1.5 0.4 1.0 3.0 VFB = 0.65V 10mA Load, TA = +25°C No Load, TA = -40°C to +85°C IOUT = 0 to 2.5A, TA = 25°C VIN = 3.6V to 5.5V, VOUT = 3.3V, TA = 25°C, IOUT = 10mA 588 582 600 600 0.20 0.10 3.0 % mA 3 -1.0 μA mΩ 400 0.5 V Ω °C 5 1.5 VEN = 0V or < VIN + 0.3V Units 0.2 tILIM VFB Typ VIN 50 612 618 A μs ms V μA V nA mV %/A 0.30 %/V 4.0 A VREGIN = 3.6V 85 mΩ VREGIN = 3.6V 50 mΩ 2.8 1 50 MHz ms ns V V μA 2.4 COUT = 100μF 1.5 VEN = 0V or 5.5V < VIN + 0.3V -1.0 0.4 1.0 1. The AAT2138 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correlation with statistical process controls. 2. Also used for Load Switch Current-Limit Fold-back Threshold 3. The regulated feedback voltage is tested in an internal test mode that connects VFB to the output of the error amplifier. 4. This time period will keep the regulator start-up current around 300mA to 400mA. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics Step-Down Converter Efficiency Step-Down Converter Efficiency (VOUT = 3.65V) (VOUT = 3.4V) 90 90 80 80 Efficiency (%) 100 Efficiency (%) 100 70 60 50 VREGIN = 3.8V 40 VREGIN = 4.2V 30 VREGIN = 4.5V 20 VREGIN = 4.8V 10 VREGIN = 5.0V 70 60 50 40 VIN = 4.2V VIN = 4.5V VIN = 4.8V VIN = 5.0V VIN = 5.2V VIN = 5.5V 30 20 10 VREGIN = 5.5V 0 0 1 10 100 1000 10000 1 10 100 1000 Output Current (mA) Output Current (mA) Step-Down Converter Efficiency Step-Down Converter Load Regulation (VOUT = 3.8V) (VOUT = 3.4V) 100 1.2 VREGIN VREGIN VREGIN VREGIN VREGIN VREGIN 90 0.8 Output Error (%) 80 Efficiency (%) 10000 70 60 50 40 VREGIN VREGIN VREGIN VREGIN VREGIN VREGIN 30 20 10 = 4.2V = 4.5V = 4.8V = 5.0V = 5.2V = 5.5V 0.4 = 3.8V = 4.2V = 4.5V = 4.8V = 5.0V = 5.5V 0 -0.4 -0.8 0 -1.2 1 10 100 1000 0 10000 500 1000 Output Current (mA) 1500 2000 2500 3000 Output Current (mA) Step-Down Converter Line Regulation Step-Down Converter Load Regulation (VOUT = 3.4V) (VOUT = 3.8V) 1.2 1.2 0.8 0.8 Output Error (%) Output Error (%) 1 0.4 0 VREGIN = 4.2V VREGIN = 4.5V VREGIN = 4.8V VREGIN = 5.0V VREGIN = 5.2V VREGIN = 5.5V -0.4 -0.8 500 1000 0.4 0.2 0 -0.2 1mA 100mA 500mA 1000mA 1500mA 2000mA 2500mA -0.4 -0.6 -0.8 -1 -1.2 0 0.6 1500 2000 Output Current (mA) 2500 3000 -1.2 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 Input Voltage (V) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 5 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics Step-Down Converter Line Regulation Frequency vs Temperature (VOUT = 3.8V) 2.9 2.85 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 Frequency (MHz) Output Error (%) 1.2 1 0.8 0.6 1mA 100mA 500mA 1000mA 1500mA 2000mA 2500mA -1.2 4.2 4.4 4.6 4.8 5 5.2 2.8 2.75 2.7 2.65 2.6 -40 5.4 -15 Input Voltage (V) 35 60 Feedback Voltage vs Input Voltage 0.609 0.606 0.606 VFB (V) 0.609 0.603 VFB (V) 0.603 0.600 0.600 0.597 0.597 0.594 0.594 0.591 0.591 -40 -15 10 35 60 85 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 Step-Down Converter Enable Threshold vs Temperature Step-Down Converter Enable Threshold vs Input Voltage 1.2 1.1 1.1 1 1 VENREG (V) 1.2 0.9 0.8 0.9 0.8 VENH 0.7 4.2 4.4 4.6 4.8 5 Input Voltage (V) 6 5.2 5.4 VENH VENL 0.7 VENL 4 5.5 Input Voltage (V) Temperature (ºC) 0.6 3.8 85 Temperature(°C) Feedback Voltage vs Temperature VENREG (V) 10 5.6 0.6 -40 -15 10 35 Temperature (°C) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 60 85 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics No Load Quiescent Current vs Temperature No Load Quiescent Current vs Input Voltage (Open Loop, VSYSIN = 5V) (Open Loop) 120 Quiescent Current (μA) Quiescent Current (μA) 120 110 100 90 80 70 85°C 60 25°C 50 -40°C 40 3.1 3.5 3.9 4.3 4.7 5.1 110 100 90 80 70 60 50 -40 5.5 -15 35 60 No Load Quiescent Current vs Input Voltage No Load Quiescent Current vs Temperature (Close Loop, VOUT = 3.4V) (Close Loop, VSYSIN = 5V, VOUT = 3.4V) 16 Quiescent Current (mA) 18 15 12 9 6 85°C 25°C 3 -40°C 14 12 10 8 0 3.7 4 4.3 4.6 4.9 5.2 -40 5.5 -15 Input Voltage (V) 10 35 60 Step-Down Converter N-Channel RDS(ON) vs Input Voltage 130 80 85°C 85°C 70 112 25°C RDS(ON) (mΩ) 25°C -40°C 94 76 58 60 -40°C 50 40 30 40 3.1 85 Temperature (°C) Step-Down Converter P-Channel RDS(ON) vs Input Voltage RDS(ON) (mΩ) 85 Temperature (°C) Input Voltage (V) Quiescent Current (mA) 10 3.5 3.9 4.3 4.7 Input Voltage (V) 5.1 5.5 20 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Input Voltage (V) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 7 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics Soft Start Soft Start (VSYSIN = VREGIN = 5V, COUT = 22μF, L=1μH, VOUT = 3.4V, IOUT = 2.5A) (VSYSIN = VREGIN = 5V, COUT = 22μF,VOUT = 3.4V, L = 1μH, IOUT = 0A) VENREGIN 2V/div VENREGIN 2V/div 0V VOUT 2V/div 0V 0V VOUT 2V/div 0V IREGIN 0A 1A/div IREGIN 0A 50mA/div Time 1ms/div Time 1ms/div Output Ripple Output Ripple (VSYSIN = VREGIN = 5V, PFM Mode, L=1μH, COUT = 22μF, CREGIN = 10μF + 0.1μF,VOUT = 3.4V,IOUT = 10mA) (VSYSIN = VREGIN = 5V, PWM Mode, L = 1μH, COUT = 22μF, CREGIN = 10μF + 0.1μF, VOUT= 3.4V, IOUT = 10mA) VSW 2V/div VSW 2V/div 0V 0V VOUT(AC) 10mV/div 0V VOUT(AC) 0V 10mV/div 0A IIN 200mA/div 0A IIN 200mA/div Time 400ns/div Time 400ns/div Output Ripple Step-Down Converter Load Transient (VSYSIN = VREGIN = 5V, PWM Mode, L = 1μH, COUT = 22μF, CREGIN = 10μF + 0.1μF, VOUT = 3.4V, IOUT = 2A) (VSYSIN = VREGIN = 5V, VOUT = 3.8V , CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) VOUT(AC) 100mV/div VSW 2V/div 0V 1A 0V VOUT(AC) 0V 10mV/div ILOAD 500mA/div 0A 100mA IIN 1A/div 0A Time 400ns/div 8 Time 100μs/div Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics Step-Down Converter Load Transient Step-Down Converter Load Transient (VSYSIN = VREGIN = 5V,VOUT = 3.8V, CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) (VSYSIN = VREGIN = 5V, VOUT = 3.4V, CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) VOUT(AC) 200mV/div VOUT(AC) 100mV/div 0V 0V 1A 2.5A ILOAD 500mA/div 100mA 0A ILOAD 100mA 1A/div 0A Time 100μs/div Time 100μs/div Step-Down Converter Load Transient Step-Down Converter LineTransient (VSYSIN = VREGIN = 5V, VOUT = 3.4V, CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) (IOUT = 100mA, VOUT = 3.8V, COUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) VOUT(AC) 200mV/div 0V 5V VREGIN 1V/div 0V 4.2V 2.5A VOUT(AC) 0V 200mV/div ILOAD 1A/div 0A 100mA Time 100μs/div Time 100μs/div Step-Down Converter LineTransient (IOUT = 100mA, VOUT = 3.4V, COUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF) Load Switch Enable Threshold vs Input Voltage 1.2 1.1 5V 0V 4.2V VENSYS (V) VREGIN 1V/div VOUT(AC) 0V 200mV/div 1 0.9 0.8 VENH 0.7 VENL 0.6 3.1 Time 100μs/div 3.5 3.9 4.3 4.7 5.1 5.5 Input Voltage (V) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 9 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Typical Characteristics Load Switch RDS(ON) vs Input Voltage Load Switch Enable Threshold vs Temperature 1.3 140 85°C 25°C 120 RDS(ON) (mΩ) VENSYS (V) 1.2 1.1 1 0.9 -40°C 100 80 VENH 0.8 60 VENL 40 3.1 0.7 -40 -15 10 35 60 85 Load Switch Turn on VENSYS 2V/div 4 4.3 4.6 4.9 5.2 5.5 Load Switch Turn off (VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF, RL = 100Ω) (VOUT = 3.4V) VENSYS 2V/div 0V VREGIN 2V/div VOUT 2V/div 0V VOUT 2V/div VREGIN 2V/div 3.7 Input Voltage (V) Temperature (°C) (VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF, RL = 100Ω) (VOUT = 3.4V) 3.4 0V 0V 0V 0V IIN 50mA /div IIN 0A 500mA/div 0A Time 1ms/div Time 400μS/div Load Switch Turn On REGIN Discharge Function (VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF + 680μF, RL = 100Ω) (VOUT = 3.4V) (VSYSIN from 5V to 3V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF) VENSYS 2V/div VSYSIN 2V/div 0V VREGIN 5V/div 0V VOUT 2V/div 0V VREGIN 0V 2V/div IIN 500mA/div 0A 0V Time 2ms/div 10 Time 400μs/div Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Functional Block Diagram SYSIN 1.0V Ref. RSET Current Limit AGND Load Switch Control EN REGIN Discharge Control ENSYS FB VP VP Current Limit 600mV Reference Step-Down Converter EN ENREG SW MODE PGND MODE Functional Description The AAT2138 is a high performance, 2.8MHz, synchronous step-down converter with a high-precision, programmable current limited P-channel load switch up to 3A. The P-channel load switch is adopted to limit the system input current. The current limit value is programmed by external resistor between RSET and GND. Its fast transient response time makes it ideal to protect the system from input power surges. The AAT2138 employs internal error amplifier and compensation. It provides excellent transient response, load and line regulation. Its output voltage is programmed by an external resistor divider from 3.0V to converter input voltage. Soft start eliminates any output voltage overshoot when the enable or input voltage is applied. Dropout mode makes the converter increase the switch duty cycle to 100% and the output voltage tracks the input voltage minus the RDS(ON) drop of the P-channel high-side MOSFET of the converter. The AAT2138's input voltage range is 2.7V to 5.5V. Two independent enable pins control the load switch and step-down converter separately. The converter efficiency has been optimized for a 1μH inductor. trol scheme. The converter senses the current through the high-side P-channel MOSFET for current loop control as well as overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The input current of the step-down converter is limited by the load switch. The feedback amplifier compares the FB voltage against the 0.6V reference voltage. The error amplifier’s transconductance output is internally compensated, and programs the current-mode loop for the necessary peak switch current to force a constant output voltage over all load and line conditions. Enable/Soft Start AAT2138 has two independent enable pins: ENSYS and ENREG. When ENSYS is pulled high, the current limit load switch is turned off and REGIN drops to zero. When ENREG is pulled low, the step-down converter is forced into the low-power, no-switching, high impedance state. The total input current during shutdown is less than 1uA. Control Scheme When ENSYS is pulled low, the system will turn on the load switch in a soft start process. To avoid a big inrush current during the startup, the REGIN voltage will be charged from SYSIN with limited input current under 500mA. The startup time depends on the capacitance between REGIN and GND. The AAT2138 uses a peak current-mode step-down con- When ENREG is pulled high, the step-down converter is Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 11 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch active with soft start. When the REGIN voltage is above UVLO and reaches 95% of VSYSIN voltage, the reference voltage is charged from 0 to 0.6V slowly by an internal current source to avoid a large input inrush current. During the soft start, the output voltage increases with the reference voltage. The soft start time is about 1ms with a 100μF output capacitor. REGIN Discharge The voltage on the capacitor between REGIN and GND is discharged quickly when the power on SYSIN is removed quickly (unplugged adaptor). When ENSYS is pulled low and SYSIN is below 3.0V, AAT2138 automatically turns on the discharge MOSFET to discharge the REGIN capacitor storage. Discharge resistance is typical 8ohm and the specific discharge time depends on the capacitance between REGIN and GND. Application Information Load Switch Current Limit Setting The AAT2138's load switch current limit can be programmed by an external resistor RSET from RSET to GND. In most applications, the variation in ILIM must be taken into account when determining RSET. The ILIM variation is due to processing variations from part to part, as well as variations in the voltages at SYSIN and REGIN, plus the operating temperature. Together, these three factors add up to a ±5% tolerance (see load switch ILIM specification in Electrical Characteristics section). Table 1 gives 1% standard metal film resistor example values for PMOS current limit programming. ILIM (A) RSET (kΩ) 0.4 0.482 0.51 0.553 0.613 0.798 0.908 1.01 1.078 1.245 1.538 2.07 2.48 2.64 2.9 3 13.3 10 9.53 8.88 8.06 6.34 5.6 5.1 4.75 4.02 3.16 2.2 1.74 1.6 1.4 1.3 Input Under-Voltage Lockout Under-voltage lockout (UVLO) guarantees sufficient VREGIN bias and proper operation of all internal circuitry prior to activation. MODE Function When the MODE pin is pulled high, the part runs in forced PWM mode operation using the internal oscillator. When the MODE pin is pulled low, a light-load mode operation (PFM/PWM) is designed to reduce the dominant switching losses at low output voltage and lightload condition. The Step-Down converter can also be synchronized to an external clock signal fed into the MODE pin. In this case, the internal oscillator is bypassed. The frequency of the external clock must be in the range from 1MHz to 3MHz. Table 1: Examples of 1% Standard Resistor Value of RSET. 3.3 Protection Circuitry 3.0 The AAT2138 includes protection for overload and overtemperature conditions. The overload protection turns off the high-side switch when the inductor current exceeds the current-limit threshold (3A minimum). 2.4 The AAT2138 includes thermal protection that disables the regulator and the load switch when the die temperature reaches 150°C. The REGIN pin is maintained in a high-impendance state. It automatically restarts when the temperature drops by 15°C or more. ILIMIT (A) 2.7 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 RSET (kΩ) Figure 1: Load Switch Current Limit vs RSET. 12 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 14 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Dropout Voltage Dropout voltage is determined by RDS(ON) and the current passing through it. AAT2138 load switch typical RDS(ON) is 80mΩ for USB application. So, for a 750mA load switch current limit setting, the load switch dropout voltage can be calculated by: VSYSIN - VDROPOUT_SWITCH · η Next, calculate the maximum current CREGIN should provide: VDROPOUT_SWITCH = 750mA · 80mΩ = 60mV Step-Down Converter ICREGIN = IBUCKIN - ILIM Finally, derive the CSYSOUT at certain load on period TON Inductor Selection The step-down converter uses peak current-mode control with slope compensation to maintain stability for duty cycles greater than 50%. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. The inductor should be set equal to half the output voltage numeric value in μH. This guarantees that there is sufficient internal slope compensation. Manufacturer’s specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. Some inductors may meet the peak and average current ratings yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. VOUT · IOUT IBUCKIN = ICREGIN · TON CREGIN_MIN = ∆VREGIN Example: A 2A, 217Hz, 12.5% load pulse is applied on 3.8V VOUT in 5V VIN and 750mA load switch current limit, under the condition, VDROPOUT_SWITCH is 0.06V, VDROPOUT_BUCK is 0.17V. Therefore, considering the step-down converter at 2A, 3.8V, VOUT is 90%. ΔVREGIN = 5 - 3.8 - 0.06 - 0.17 = 0.970V 3.8 · 2 IBUCKIN = = 1.71A 5 - 0.06 · 0.9 ICREGIN = 1.71 - 0.75 = 0.96A tON is 576μs for a 217Hz, 12.5% duty cycle load pulse. CREGIN_MIN = 0.96 · 576 0.970 = 570μF CREGIN Selection CREGIN is not only the load switch output capacitor but also the step-down converter input capacitor. It is designed to provide the additional input current and maintain the SYSOUT voltage for the step-down converter when load switch limits the input current from SYSIN. If the input voltage of the step-down converter is lower than the VOUT plus the dropout voltage, the AAT2138 enters dropout mode. CREGIN minimum value can be calculated by the following steps: First, calculate the allowed maximum delta voltage on CREGIN to keep VOUT in regulation: ΔVREGIN = VIN - VOUT - VDROPOUT_SWITCH - VDROPOUT_BUCK Second, calculate the required input current at SYSOUT for the step-down converter: VOUT 500mV/div VREGIN 2V/div ISYSIN 1A/div IOUT 1A/div 0 0 0 800μs/div Figure 2: AAT2138 Operation Waveform 2A, 577μs Load Pulse (Applied) VIN = 5V, VOUT = 3.8V. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 13 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Considering 20% capacitance tolerance, the minimum capacitance should be 684μF. So select a 680μF tantalum capacitor as CREGIN, as well as an additional 10μF ceramic capacitor to closely filter the input voltage VREGIN of the step-down converter on the PCB board. Input Capacitor Selection Select a 1μF X7R or X5R ceramic capacitor for the system input. To estimate the required input capacitor size, determine the acceptable input ripple level and solve for CIN. The calculated value varies with input voltage and is a maximum when VIN is twice the output voltage. CIN = V VOUT · VIN 1 - VOUT IN VPP IOUT - ESR · fS for VIN = 2 · VOUT, VOUT · VIN 1- 1 VOUT = 4 VIN and CIN(MIN) = 1 VPP IOUT - ESR · 4 · fS The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2138. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function due to their low ESR and ESL. To minimize stray parasitic inductance, place the capacitor as close as possible to the IC. This keeps the high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C1) can be seen in the evaluation board layout in the Layout section of this datasheet (see Figure 4). A laboratory test set-up typically consists of two long wires running from the bench power supply to the eval- 14 uation board input voltage pins. The inductance of these wires, along with the low-ESR ceramic input capacitor, can create a high Q network that may affect converter performance. This problem often becomes apparent in the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain measurements can also result. Feedback Resistor Selection The output voltage on the AAT2138 is adjustable with external resistors R1 and R2. To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum suggested value for R2 is 59kΩ. Although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensitive to external noise and interference. The maximum value of R1 should be below 1Mohm to keep reference voltage normal and avoid noise coupling. The external resistor R1, combined with an external capacitor up to 22pF feed-forward capacitor (C4 in Figure 3), delivers enhanced transient response for extreme pulsed load applications and reduces ripple in light load conditions. The addition of the feed forward capacitor typically requires a larger output capacitor C31-C32 for stability. The external resistors set the output voltage according to the following equation: VOUT VOUT R1 = V - 1 · R2 = 0.6V - 1 · R2 REF Table 2 shows the standard 1% metal film resistor examples for different step-down ouput voltages VOUT (V) R2 = 59kΩ, R1 (kΩ) 3.3 3.4 3.6 3.8 267 274 294 316 Table 2: Resistor Selections for Different Output Voltage Settings. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Printed Circuit Board Layout Recommendations For best performance of the AAT2138, the following guidelines should be followed when designing the PCB layout: 1. Reliably solder the exposed pad (EP) to the GND plane. A GND pad below EP is strongly recommended. 2. Keep the power traces, including the GND trace, the SW trace and the SYSIN, REGIN short, direct and wide to allow large current flow. Keep the L connection to the SW pins as short as possible. Do not put any signal lines under the inductor. 3. Connect the input capacitors (C1 and C21) as close as possible between SYSIN and REGIN and GND to get good power filtering. 4. Connect the input capacitor C23 as close as possible between REGIN and AGND to get good power filtering and load regulation. 5. Keep the switching node, SW away from the sensitive FB node. 6. Separate the feedback trace from any power trace and connect as close as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. Place external feedback resistors as close as possible to the FB pin to minimize the length of the high impedance feedback trace. 7. Minimize the resistance of the trace from the load return to GND. This will help minimize any error in DC regulation due to differences in the potential of the internal signal ground and the power ground. REGIN SW 13 14 9 6 1 2 7 L1 VOUT 3.4V C32 Option C31 22μF 1uH R1 274K C21 10μF U1 AAT2138 C4 22pF RSET SW SW FB RSET PGND PGND AGND REGIN REGIN SYSIN SYSIN ENREG ENSYS MODE 3 4 11 12 5 8 10 C22 680μF C23 0.1μF GND SYSIN C1 1μF 2.7V-5.5V R2 59K GND GND J3 SYSIN J2 GND SYSIN J1 GND SYSIN GND Figure 3. AAT2138 Evaluation Board Schematic. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 15 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch (a) Top Layer (b) Bottom Layer Figure 4. AAT2138 Evaluation Board Layout Manufacturer Part Number Inductance (μH) Max DC Current (A) DCR (mΩ) Size (mm) LxWxH Type Samsung Murata CIG22H1R0MNE LQH44PN1R0NPO 1 1 3.3 2.95 83 30 2x2.5x1.2 4x4x1.65 Shielded Shielded Table 3: Recommended Inductor Selection Manufacturer Murata KEMET Part Number Value (μF) Voltage (V) Temperature Range Case Size GRM188R71H104KA93D GRM21BR71E105KA99L GRM21BR61C106KE15L GRM21BR60J226ME39 GRM0335C1H220GD01D T495D687M006ZTE150 0.1 1 10 22 22pF 680 50 25 16 6.3 50 6.3 X7R X7R X5R X5R COJ X5R 0603 0805 0805 0805 0603 7343 Table 4: Recommended Capacitor Selection. Component Part Number Description U1 AAT2138IWO-0.6 C1 C21 C23 C31 C4 C22 R1 R2 RSET L GRM21BR71E105KA99L GRM21BR61C106KE15L GRM21BR71E105KA99L GRM21BR60J226ME39 GRM0335C1H220GD01D T495D687M006ZTE150 RC0603FR-07274KL RC0603FR-0759KL RC0603FR-076K34L CIG22H1R0MNE Manufacturer High Current Step-Down Converter with Adjustable Current Limit Load Switch Cap Ceramic 1uF 0805 X7R 25V 10% Cap Ceramic 10uF 0805 X5R 16V 10% Cap Ceramic 0.1uF 0603 X7R 50V 10% Cap Ceramic 22uF 0805 X5R 6.3V 10% Cap Ceramic 22pF 0603 C0J 50V 10% Cap 680uF 7343 X5R 6.3V 20% Res 274KΩ 1/10W 1% 0603 SMD Res 59KΩ 1/10W 1% 0603 SMD Res 6.34KΩ 1/10W 1% 0603 SMD Power Inductor 1uH 3.3A 83mΩ SMD Skyworks Murata KEMET Yageo Samsung Table 5: Evaluation Board BOM List 16 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Ordering Information Output Voltage Package Marking1 Part Number (Tape and Reel) Adjustable ≥ 0.6V TDFN33-14 S6XYY AAT2138IWO-0.6-T12 Skyworks Green™ products are compliant with all applicable legislation and are halogen-free. For additional information, refer to Skyworks Definition of Green™, document number SQ04-0074. Package Information TDFN33-143 Detail "A" 3.000 ± 0.050 2.500 ± 0.050 Index Area 3.000 ± 0.050 1.650 ± 0.050 Top View Bottom View + 0.100 - 0.000 Pin 1 Indicator (Optional) 0.180 ± 0.050 Side View 0.400 BSC 0.000 0.203 REF 0.750 ± 0.050 0.425 ± 0.050 Detail "A" All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. 3. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 17 DATA SHEET AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch Copyright © 2012, 2013 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. 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Additional information, including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference. 18 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013