DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit General Description Features The AAT1138 is a 2A synchronous step-down converter with an integrated current-limiting load switch designed for precise input current control applications. By guarding against excessive input current, the AAT1138 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 5V USB ports and 3.3V supplies from heavy load transient conditions commonly seen with high data rate modem applications. • VIN Range: 2.5V to 5.5 V • VOUT Range: 0.6V to VPVIN • System Current Limited Load Switch ▪ Programmable Over-Current Threshold ▪ < 1μs Fast Transient Response to Short Current ▪ 145mΩ Typical RDS(ON) • Step-Down Converter ▪ Up to 2A Output Current ▪ 95% Peak Efficiency ▪ 1.2MHz Switching Frequency ▪ 135mΩ Low RDS(ON) Internal Switches ▪ 100% Duty Cycle Low Dropout Operation ▪ Soft Start • Under-Voltage Lockout • Independent Enable Pins • <2μA Shutdown Current • Over-Temperature and Current Limit Protection • Low Profile 16-pin 3mm x 4mm TDFN Package • -40°C to 85°C Temperature Range The AAT1138 integrates a 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. Current limit threshold is programmed by an external resistor from SET to ground. It can be adopted to control loads of the input supply that requires up to 1.4A. The AAT1138’s internal step-down converter is a 2A, 1.2MHz constant frequency current mode PWM stepdown converter. The step-down converter can run at 100% duty cycle for low dropout operation. The output voltage can be regulated as low as 0.6V. The AAT1138 is available in a Pb-free, low profile, 16-pin 3mm x 4mm TDFN package. The product is rated over the -40°C to 85°C temperature range. Applications • • • • • Cellular Phones MP3 Players PDAs and Handheld Computers Portable Media Players USB Devices Typical Application SYSIN VIN: 2.5V - 5.5V SYSOUT AAT1138 CIN 1μF TDFN34-16 ENSYS PVIN CSYSOUT Ceramic Cap 22μF + Large Tantalum Cap SVIN ENBUCK L 2.2μH SW R2 267kΩ SET RSET 11.3kΩ COUTB 22μF VOUT: 3.3V/ Up to 2A load pulse FB R1 59.0kΩ PGND GND Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 1 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Pin Descriptions Pin # Symbol 1, 6, 15 2, 3 4, 5 7 8 GND SW PGND SYSIN ENSYS 9 SET 10, 11 SYSOUT 12 ENBUCK 13 PVIN 14 SVIN 16 FB EP Function Analog ground pin. Switching node pin. Connect the output inductor to this pin. Power ground pin for the step-down converter. System input power. P-channel MOSFET source. Connect a 1μF capacitor from SYSIN to GND. Enable input for system power load switch. Active Low. Input current limit set input. A resistor from SET to ground sets the current limit for the input load switch. System output power. P-channel MOSFET drain. Enable input for step-down converter. Active high. Connect to PVIN when enabling the stepdown converter. Do not leave ENBUCK floating. Power supply input pin for step-down converter. Must be closely decoupled to PGND with a 22μF or greater ceramic capacitor. Analog supply input pin. Provides bias for internal circuitry. Connect to PVIN. Feedback pin for step-down converter. Connect FB to the center point of the external resistor divider. The feedback threshold voltage is 0.6V. Exposed pad. Must be connected to bare copper ground plane. Pin Configuration TDFN34-16 (Top View) GND SW SW PGND PGND GND SYSIN ENSYS 2 1 16 2 15 3 14 4 5 EP 13 12 6 11 7 10 8 9 FB GND SVIN PVIN ENBUCK SYSOUT SYSOUT SET Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Absolute Maximum Ratings1 TA = 25°C unless otherwise noted. Symbol Description VIN VGND VSW, VFB VSET, VOUT VEN IMAX,SYS TJ TLEAD SYSIN, PVIN, SVIN to GND PGND, GND SW, FB to GND SET, SYSOUT to GND ENSYS, ENBUCK to GND Maximum Continuous Current for SYSOUT Load Switch Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) Value Units -0.3 to 6.0 -0.3 to 6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 2 -40 to 150 300 V V V V V A C °C Value Units 68.86 1.45 °C/W W Thermal Characteristics Symbol ΘJA PD Description Maximum Thermal Resistance Maximum Power Dissipation2, 3 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. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board. 3. Derate 20mW/°C above 25°C. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 3 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Electrical Characteristics1 VPVIN = VSVIN = 3.6V, TA = -40°C to 85°C unless otherwise noted. Typical values are at TA = 25°C. Symbol Description VSYSIN Input Voltage TSD Over-Temperature Threshold Conditions Input DC Supply Current IFB Feedback Input Bias Current VFB_ACC Feedback Voltage Accuracy2 VFB_TOL Feedback Voltage Tolerance ∆VLINEREG/∆VIN ∆VLOADREG/∆IOUT ILIM RDS(ON)H RDS(ON)L FOSC TS VIL VIH ILEAK Load Switch IQ IQ(OFF) ISD(OFF) VUVLO RDS(ON) TCRDS ILIM ILIM(MIN) VENSYS\(L) VENSYS\(H) IENSYS\(SINK) TRESP TOFF TON Line Regulation Load Regulation Current Limit High Side Switch on Resistance Low Side Switch on Resistance Oscillator Frequency Startup Time ENBUCK Threshold Low ENBUCK Threshold High ENBUCK Leakage Current Quiescent Current Off Supply Current Off Switch Current Under-Voltage Lockout On Resistance On Resistance Temperature Coefficient Current Limit Minimum Current Limit ENSYS Input Low Voltage ENSYS Input High Voltage ENSYS Input Leakage Current Limit Response Time Turn-Off Time Turn-On Time Typ 2.5 VSYSIN = 5V, TJ Increasing VSYSIN = 5V, TJ Decreasing Step-Down Converter VOUT Output Voltage Range IQ Min 5.5 135 105 0.6 Active Mode: VFB = 0.5V Shutdown Mode: VEN = 0V, VPVIN = 5.5V VFB = 0.65V ILOAD = 10mA, TA = 25°C No load, -40°C ≤ TA VPVIN = 2.5V to 5.5V, 10mA to 2.0A Load VPVIN = 2.5V to 5.5V, 10mA Load, TA = 25°C ILOAD= 0A to 2A 300 0.1 0.588 0.582 -3.0 0.60 0.60 0.10 2.5 VPVIN = 3.6V VPVIN = 3.6V VFB = 0.6V Enable to output regulation Max 0.96 0.2 3.5 135 95 1.2 1.3 VEN = 5.5V ENSYS = GND, No Load ENSYS = 5V ENSYS = 5V, VSYSIN = 5V, VSYSOUT = 0V Rising Edge, 1% Hysteresis VSYSIN = 5.0V, TA = 25°C VSYSIN = 4.5V, TA = 25°C VSYSIN = 3.0V, TA = 25°C RSET = 6.8kΩ VSYSIN = 2.7V to 5.5V VSYSIN = 2.7V to < 4.2V VSYSIN ≥ 4.2V to 5.0V VENSYS = 5.5V VSYSIN = 5V VSYSIN = 5V, RL = 10Ω VSYSIN = 5V, RL = 10Ω 0.75 0.01 1.8 145 150 190 2800 1 130 C VPVIN 500 1 30 0.612 0.618 +3.0 V μA μA nA 0.20 %/V 200 150 1.44 1.0 9 V o 0.3 1.5 -1.0 Units 25 1 1 2.4 180 V % %/A A mΩ mΩ MHz ms V V μA μA μA μA V mΩ 230 1.25 0.8 2.0 2.4 ppm/°C A mA V V 0.01 0.4 4 24 1 12 200 μA μs μs μs 1. The AAT1138 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. The regulated feedback voltage is tested in an internal test mode that connects VFB to the output of the error amplifier. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Step-Down Converter Efficiency Step-Down Converter Efficiency (VOUT = 1.8V; L = 22μH; COUT = CSYSOUT = 22μF) (VOUT = 3.3V; L = 22μH; COUT = CSYSOUT = 22μF) 100 100 90 90 80 80 70 60 50 VSYSOUT = 2.5V VSYSOUT = 3.0V VSYSOUT = 3.6V VSYSOUT = 4.2V VSYSOUT = 5.0V VSYSOUT = 5.5V 40 30 20 10 0 0.1 1 10 100 1000 Efficiency (%) Efficiency (%) Typical Characteristics 70 60 VSYSOUT = 3.6V VSYSOUT = 3.7V VSYSOUT = 4.2V VSYSOUT = 4.5V VSYSOUT = 5.0V VSYSOUT = 5.5V 50 40 30 20 10 0 0.1 10000 1 Output Current (mA) 100 1000 10000 Output Current (mA) Step-Down Converter Load Regulation Step-Down Converter Load Regulation (VOUT = 1.8V; L = 22μH; COUT = CSYSOUT = 22μF) (VOUT = 3.3V; L = 22μH; COUT = CSYSOUT = 22μF) 1.2 VSYSOUT = 2.5V VSYSOUT = 3.0V VSYSOUT = 3.6V VSYSOUT = 4.2V VSYSOUT = 5.0V VSYSOUT = 5.5V 0.8 0.4 0.0 -0.4 -0.8 Output Error (%) 1.2 Output Error (%) 10 VSYSOUT = 3.7V VSYSOUT = 4.2V VSYSOUT = 4.5V VSYSOUT = 5.0V VSYSOUT = 5.5V 0.8 0.4 0.0 -0.4 -0.8 -1.2 -1.2 0.1 400 800 1200 1600 2000 Output Current (mA) 0 400 800 1200 1600 2000 Output Current (mA) Step-Down Converter Line Regulation vs. Input Voltage (VOUT = 1.8V) Output Error (%) 0.4 0.3 0.2 0.1 0.0 ILOAD = 1mA ILOAD = 0.5A ILOAD = 1A ILOAD = 1.5A ILOAD = 2A -0.1 -0.2 -0.3 2.5 3.1 3.7 4.3 4.9 5.5 Input Voltage (V) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 5 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Step-Down Converter Feedback Voltage vs. Temperature Step-Down Converter Feedback Voltage vs. Input Voltage (VSYSOUT = 3.6V) 0.606 0.609 0.604 0.602 0.603 VFB (V) VFB (V) 0.606 0.600 0.600 0.597 0.598 0.594 0.596 0.591 -40 -15 10 35 60 0.594 2.5 85 2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 Input Voltage (V) Temperature (°C) Step-Down Converter VENH and VENL vs. Temperature Step-Down Converter VENH and VENL vs. Input Voltage (VSYSOUT = 3.6V) 0.50 VENH VENL 0.48 VENH and VENL (V) VENH and VENL (V) 0.52 0.44 0.40 0.36 0.32 -40 0.47 0.44 0.41 0.38 VENH VENL 0.35 -15 10 35 60 85 2.5 2.8 3.1 Frequency vs. Temperature 4.3 4.6 4.9 5.2 5.5 1.0 1.24 0.8 IENBUCK (μA) Frequency (MHz) 4.0 Step-Down Converter EN Leakage vs. Temperature 1.26 1.22 1.20 1.18 0.6 0.4 0.2 1.16 -20 0 20 40 Temperature (°C) 6 3.7 Input Voltage (V) Temperature (°C) 1.14 -40 3.4 60 80 0.0 -40 -15 10 35 Temperature (°C) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 60 85 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Step-Down Converter P-Channel RDS(ON) vs. Input Voltage Step-Down Converter N-Channel RDS(ON) vs. Input Voltage 150 200 T = -40°C T = 25°C T = 85°C 160 140 120 110 90 70 100 80 2.5 T = -40°C T = 25°C T = 85°C 130 RDS(ON)L (mΩ) RDS(ON)H (mΩ) 180 2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 50 2.5 2.8 Input Voltage (V) 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 Input Voltage (V) Step-Down Converter Output Ripple Step-Down Converter Output Ripple (VSYSOUT = 5.0V, VOUT = 1.8V, ILOAD = 2A; CSYSOUT = COUT = 22μF, L = 2.2μH) (VSYSOUT = 3.6V, VOUT = 1.8V, ILOAD = 10mA; CSYSOUT = COUT = 22μF, L = 2.2μH) VSW (2V/div) VSW (2V/div) 0 0 VOUT 0 (20mV/div) VOUT 0 (10mV/div) IINDUCTOR (1A/div) IINDUCTOR (100mA/div) 0 0 Time (800ns/div) Time (2μs/div) Step-Down Converter Output Ripple Step-Down Converter Output Ripple (VSYSOUT = 5.0V, VOUT = 3.3V, ILOAD = 2A; CSYSOUT = COUT = 22μF, L = 2.2μH) (VSYSOUT = 5.0V, VOUT = 3.3V, ILOAD = 10mA; CSYSOUT = COUT = 22μF, L = 2.2μH) VSW (2V/div) VSW (2V/div) 0 0 VOUT 0 (20mV/div) VOUT 0 (10mV/div) IINDUCTOR (1A/div) IINDUCTOR (100mA/div) 0 0 Time (800ns/div) Time (2μs/div) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 7 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Step-Down Converter Output Ripple Step-Down Converter Output Ripple (VSYSOUT = 5.0V, VOUT = 3.8V, ILOAD = 2A; CSYSOUT = COUT = 22μF, L = 2.2μH) (VSYSOUT = 5.0V, VOUT = 3.8V, ILOAD = 10mA; CSYSOUT = COUT = 22μF, L = 2.2μH) VSW (2V/div) VSW (2V/div) 0 0 VOUT 0 (20mV/div) VOUT (10mV/div) IINDUCTOR (1A/div) 0 IINDUCTOR (100mA/div) 0 0 Time (800ns/div) VOUT (200mV/div) Step-Down Converter Load Transient Step-Down Converter Load Transient (10mA to 2A; VSYSOUT = 3.6V, VOUT = 1.8V; COUT = 22μF, CFF = 100pF, L = 2.2μH) (10mA to 2A; VSYSOUT = 5.0V, VOUT = 3.3V; COUT = 22μF, CFF = 100pF, L = 2.2μH) VOUT (200mV/div) 0 ILOAD (1A/div) Time (2μs/div) 0 ILOAD (1A/div) 10mA 10mA Time (40μs/div) Time (40μs/div) Step-Down Converter Line Transient Step-Down Converter Line Transient (VSYSOUT = 3.6V to 4.2V; VOUT = 1.8V; ILOAD = 2A; COUT = 22μF, CFF = 100pF, L = 2.2μH) (VSYSOUT = 4.5V to 5.0V; VOUT = 3.3V; ILOAD = 2A; COUT = 22μF, CFF = 100pF, L = 2.2μH) VSYSOUT (2V/div) VSYSOUT (2V/div) 0 VOUT (50mV/div) 0 VOUT (50mV/div) 0 Time (100μs/div) 8 0 Time (100μs/div) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Step-Down Converter Start-Up Sequence Step-Down Converter Start-Up Sequence (VSYSOUT = 3.6V; VOUT = 1.8V; ILOAD = 2A, CSYSOUT = COUT = 22μF, L = 2.2μH) (VSYSOUT = 5.0V; VOUT = 3.3V; ILOAD = 2A, CSYSOUT = COUT = 22μF, L = 2.2μH) EN (2V/div) 0 EN (2V/div) VOUT 0 (2V/div) VOUT 0 (2V/div) ISYSOUT 0 (500mA/div) ISYSOUT (500mA/div) 0 0 Time (400μs/div) Time (400μs/div) Quiescent Current vs. Input Voltage Quiescent Current vs. Temperature (VIN = 5.0V) 4000 500 VOUT = 1.8V VOUT = 3.3V VOUT = 3.8V 3200 470 IQ (μA) IQ (μA) 440 2400 1600 380 800 0 2.5 410 VOUT = 1.8V VOUT = 3.3V VOUT = 3.8V 350 2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 Input Voltage (V) 320 -40 -15 10 35 60 85 Temperature (°C) Shutdown Current vs. Temperature 0.10 ISD (μA) 0.08 0.06 0.04 0.02 0.00 -40 -15 10 35 60 85 Temperature (°C) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 9 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Switch Turn On Switch Turn Off (VIN = 5V; RL = 10Ω; CSYSOUT = 22μF) (VIN = 5V; RL = 10Ω; CSYSOUT = 22μF) EN (2V/div) EN (2V/div) 0 0 VOUT (2V/div) VOUT (2V/div) 0 0 Time (4μs/div) Time (4μs/div) Switch Short Circuit Response Switch RSET vs. ILIM (VIN = 5V; RLOAD = 0.3Ω) (VIN = 5.0V; VIN - VOUT = 0.5V) 100 VIN (2V/div) RSET (kΩ) VOUT 0 (5V/div) 0 10 IOUT (2A/div) 0 1 100 1000 10000 ILIM (mA) Time (1μs/div) Switch Current Limit vs. Temperature Switch RDS(ON) vs. Input Voltage (VIN = 5.0V; RSET = 11.3KΩ) 0.22 700 690 RDS(ON) (Ω) ILIM (mA) 0.20 680 670 0.16 660 650 -40 -15 10 35 Temperature (°C) 10 0.18 60 85 0.14 2.5 2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 Input Voltage (V) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 5.2 5.5 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Typical Characteristics Switch VEN(H) and VEN(L) vs. Input Voltage Switch VENH and VENL vs. Temperature 2.4 2.5 2.1 2.3 VENH and VENL (V) VEN(H) and VEN(L) (Ω) (VIN = 5.0V; RSET = 11.3KΩ) 1.8 1.5 1.2 VEN(H) VEN(L) 0.9 2.5 2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 Input Voltage (V) VENH VENL 2.1 1.9 1.7 1.5 -40 -15 10 35 60 85 Temperature (°C) Switch EN Leakage vs. Temperature 0.20 ISYSEN (μA) 0.16 0.12 0.08 0.04 0.00 -40 -20 0 20 40 60 80 Temperature (°C) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 11 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Functional Block Diagram SYSOUT 1.2V Ref. SET Current Limit GND VP Load Switch Control SVIN EN PVIN ENSYS Current Limit FB 600mV Reference ENBUCK Functional Description The AAT1138 is a high performance 2A 1.2MHz synchronous step-down converter with a programmable current limited P-channel load switch. It is designed with the goal of high performance with precise input current control under up to 2A load pulse on the step-down converter output. The P-channel load switch is adopted to limit the system input current. The current limit value is programmed by external resistor between SET and GND. Its fast transient response time make it protect the system input power ideally. The 2A step-down converter 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 0.6V to converter input voltage. Soft start eliminates any output voltage overshoot when the enable or input volt- 12 VP Step-Down Converter SW EN PGND age 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 AAT1138's input voltage range is 2.5V to 5.5V. Two independent enable pins control the load switch and step-down converter separately. The converter efficiency has been optimized for all load conditions, ranging from no load to 2A. Step-Down Converter Control Loop The internal DC-DC converter of the AAT1138 is a peak current mode synchronous step-down converter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as short circuit and overload protection. A slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The peak current mode loop Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit appears as a voltage-programmed current source in parallel with the output capacitor. The output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output voltage for all load and line conditions. Internal loop compensation terminates the transconductance voltage error amplifier output. For fixed voltage versions, the error amplifier reference voltage is internally set to program the converter output voltage. For the adjustable output, the error amplifier reference is fixed at 0.6V. Enable/Soft Start Current Limit When the step-down converter input voltage VPVIN is close to the value of the output voltage, the main switch is allowed to remain on for more than one switching cycle and increases the duty cycle until it reaches 100%. The duty cycle D of a step-down converter is defined as: The AAT1138 includes two kinds of current limit. One is input current limit by the load switch; the other is inductor current limit by the high-side MOSFET current sense loop of the step-down converter. For overload conditions, the input current is limited by RSET and the peak inductor current is limited to 3.5A. To minimize power dissipation and stresses under current limit and short-circuit conditions, step-down converter switching is terminated after entering current limit for a series of pulses. The termination lasts for seven consecutive clock cycles after a current limit has been sensed during a series of four consecutive clock cycles. Over-Temperature Protection Thermal protection completely disables load switch and step-down converter switching when internal dissipation becomes excessive. The junction over-temperature threshold is 125°C with 10°C of hysteresis. Once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers. AAT1138 has two independent enable pins: ENSYS and ENBUCK. When ENSYS is pulled high, the current limit load switch is turned off and SYSOUT drops to zero. When ENBUCK is pulled low, the step-down converter is forced into the low-power, no-switching state. Soft start of the step-down converter limits the current surge seen at the input and eliminates output voltage overshoot. Dropout Operation D = TON · fOSC · 100% = VOUT · 100% VIN Where TON is the main switch on time and fOSC is the oscillator frequency 1.2MHz. The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low input supply voltage, the RDS(ON) of the P-channel MOSFET increases, and the efficiency of the converter decreases. Caution must be exercised to ensure the heat dissipated does not exceed the maximum junction temperature of the IC. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 13 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Application Information Setting the Load Switch Current Limit The AAT1138's load switch current limit can be programmed by an external resistor RSET from SET 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 SYSOUT, plus the operating temperature. Together, these three factors add up to a ±25% tolerance (see load switch ILIM specification in Electrical Characteristics section). Figure 1 illustrates a cold device with a statistically higher current limit and a hot device with a statistically lower current limit, both with RSET equal to 10.3kΩ. A 10.3kΩ RSET resistor sets the typical current limit to 0.665A. This figure shows that the actual current limit will be at least 0.5A and no greater than 0.83A. 1.2 1.0 At -40°C ISYSOUT (A) 0.83A 0.8 0.6 0.5A 0.4 At 85°C 0.2 0.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VSYSOUT (V V) Figure 1: Load Switch Current Limit (RSET = 10.3kΩ). Though the relationship between typical ILIM and RSET is not linear throughout the current limit setting range, there is constant coefficient between them within a small enough current limit range for the system designer to select a suitable RSET value. Table 1 shows the current limit range using a standard 1% metal film resistor. To determine RSET, start with the maximum allowable input current from SYSIN as minimum ILIM and multiply by 1.33 to derive the typical ILIM value. Next, refer to Table 1 to find the constant coefficient ILIM range which includes the calculated ILIM value and get the constant coefficient value c. Then calculate the RSET by the following formula: RSET = RSET_Max - (ILIM - ILIM_Range_Low ) · c RSET_Max is the maximum standard RSET resistance at the certain constant coefficient current limit range. ILIM is the calculated typical current limit. ILIM_Range_Low is the low terminal of the current limit range. c is the coefficient of the current limit range. Example: Select RSET for 500mA current limit. The typical current limit is ILIM = 500 · 1.33 = 665mA. The constant coefficient current range is 600mA to 700mA and c = 25000. Therefore: RSET = 13kΩ - (665mA - 600mA) · 25000 = 11.375kΩ Select a standard 11.3kΩ resistor for RSET. Considering the ±25% tolerance, the current limit will be greater than 500mA but less than 831mA. ILIM Typ. (mA) Constant Coefficient c of RSET / ILIM 1% Standard RSET (kΩ) 200 250 300 350 400 450 500 550 600 700 800 900 1000 1100 1200 1300 1400 186000 120000 56000 50000 36000 32000 30000 34000 25000 16300 13700 6900 7700 5500 5000 3500 40.2 30.9 24.9 22.1 19.6 17.8 16.2 14.7 13.0 10.5 8.87 7.50 6.81 6.04 5.49 4.99 4.64 Table 1: Current Limit RSET Values. Dropout Voltage Dropout voltage is determined by RDS(ON) and the current passing through it. AAT1138 load switch maximum RDS(ON) is 180mΩ for USB applicationa and the step-down converter high side RDS(ON) is maximum 200mΩ. So for a 500mA load switch current limit setting (the load switch always limits below 831mA as described above), the load switch maximum dropout voltage can be calculated by: VDropout_Switch = 831mA · 180mΩ = 0.15V 14 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit To 2A step-down converter, the maximum dropout voltage is: VDropout_Buck = 2A · 200mΩ = 0.4V Operation in Heavy Load Pulse When a heavy load pulse is applied to the output of AAT1138 as typical application shows, the input current is limited to the value of ILIM determined by RSET. At this time, CSYSOUT has the important role of providing enough current and voltage to the step-down converter input to bring it into regulation. The duty cycle of the heavy load pulse should not exceed the maximum value which allows sufficient time to charge CSYSOUT from VIN and balance the capacitor charging and discharging to make the operation normally. Figure 2 shows the operation waveform at 5V VIN and 3.8V VOUT with a 2A load pulse applied when RSET = 11.5kΩ, CIN = 1μF, CSYSOUT = 22μF ceramic capacitor + 4x330μF tantalum capacitor, and COUTB = 22μF. VIN (100mV/div) VSYSOUT (AC Coupled) (1V/div) VOUT (200mV/div) Capacitor Selection CSYSOUT Selection CSYSOUT 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 (VPVIN and VSVIN) is lower than the VOUT plus the dropout voltage, the AAT1138 enters dropout mode. CSYSOUT minimum value can be calculated by the following steps: First, calculate the allowed maximum delta voltage on CSYSOUT to keep Vout in regulation: ∆VSYSOUT = VIN - VOUT - VDropout_Switch - VDropout_Buck Second, calculate the required input current at SYSOUT for the step-down converter: 5V IBUCKIN = 3.8V IIN (500mA/div) 0A IOUT (2A/div) 0A VOUT · IOUT (VIN - VDropout_Switch) · η Next, calculate the maximum current CSYSOUT should provide: ICSYSOUT = IBUCKIN - ILIM Finally, derive the CSYSOUT at certain load on period TON. CSYSOUT_Min = Time (1ms/div) Figure 2: AAT1138 Operation Waveform when 2A 217Hz 12.5% Load Pulse is Applied. ICSYSOUT · TON ΔVSYSOUT Example: A 2A 217Hz 12.5% load pulse is applied on 3.8V VOUT in 5V VIN and 500mA load switch current limit. Under the condition, VDropout_Switch is 0.15V. VDropout_Buck is 0.4V. Therefore: ∆VSYSOUT = 5 - 3.8 - 0.15 - 0.4 = 0.65V Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 15 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Considering the step-down converter at 2A 3.8V VOUT is 90%. IBUCKIN = 3.8 · 2 = 1.74A (5 - 0.15) · 90% ICSYSOUT = 1.74 - 0.5 = 1.24A In many practical designs, to get the required ESR, a capacitor with much more capacitance than is needed must be selected. For both continuous and discontinuous inductor current mode operation, the ESR of the COUT needed to limit the ripple to ΔVO, V peak-to-peak is: ESR ≤ TON is 576μs for a 217Hz 12.5% duty cycle load pulse. CSYSOUT_Min = 1.24A · 576μs = 1099μF 0.65V Considering 20% capacitance tolerance, the minimum capacitance should be 1319μF. So select 4x330μF tantalum capacitor as CSYSOUT, as well as an additional 22μF ceramic capacitor to closely filter the input voltage VSYSOUT of the step-down converter on the PCB board. COUTB Selection The value of output capacitance is generally selected to limit output voltage ripple to the level required by the specification. Since the ripple current in the output inductor is usually determined by L, VOUT and VIN, the series impedance of the capacitor primarily determines the output voltage ripple. The three elements of the capacitor that contribute to its impedance (and output voltage ripple) are equivalent series resistance (ESR), equivalent series inductance (ESL), and capacitance (C). The formula below gives the general output voltage ripple calculation: ∆VOUT ≤ VOUT · (VIN - VOUT) 1 VIN · fOSC · L · ESR + 8 · fOSC · COUT The output voltage droop due to a load transient is dominated by the capacitance of the output capacitor. During a step increase in load current, the output capacitor alone supplies the load current until the loop responds. Within three switching cycles, the loop responds and the inductor current increases to match the load current demand. The relationship of the output voltage droop during the three switching cycles to the output capacitance can be estimated by: COUT = 16 3 · ΔILOAD VDROOP · FOSC ΔVO ΔIL ESL can be a problem by causing ringing in the low megahertz region but can be controlled by choosing low ESL capacitors, limiting lead length (PCB and capacitor), and replacing one large device with several smaller ones connected in parallel. In conclusion, in order to meet the requirement of low output voltage ripple and regulation loop stability, ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR and high ripple current ratings. A 22μF ceramic capacitor can satisfy most applications. Inductor Selection For most designs, the AAT1138 operates with inductor values of 2.0μH to 6.8μH. Inductors with low inductance values are physically smaller but generate higher inductor current ripple leading to higher output voltage ripple. Refer to the “Capacitor Selection” section of this datasheet for the output ripple calculation. The inductor ripple current can be derived from the following equation: ∆IL ≤ VOUT · (VIN - VOUT) VIN · fOSC · L Large value inductors lower ripple current and small value inductors result in high ripple currents. Choose inductor ripple current approximately 30% of the maximum load current 2A, or ∆IL = 600mA 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. The DC current rating of the inductor should be at least equal to the maximum load current plus half the inductor ripple current to prevent core saturation (2A + 300mA). Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Manufacturer Part Number Value Voltage Tolerance Temp. Co. ESR (mΩ) Case Murata GRM21BR60J226ME39 TAJD337M006R TPSD337M006R0150 TAJD477M006R TPSD477M006R0150 TAJD687M006R TPSD687M006R0100 T491D337M006AT T495D337M006ATE100 T491D477M006AT T495D477M006ATE150 T491D687M006ZT T495D687M006ZTE150 22μF 330μF 330μF 470μF 470μF 680μF 680μF 330μF 330μF 470μF 470μF 680μF 680μF 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 20% 20% 20% 20% 20% 20% 20% 20% 20% 20% 20% 20% 20% X5R X5R X5R X5R X5R X5R X5R X5R X5R X5R X5R X5R X5R 26 400 150 400 150 500 100 400 100 400 150 500 150 0805 7343 7343 7343 7343 7343 7343 7343 7343 7343 7343 7343 7343 AVX KEMET Table 2: Surface Mount Capacitors. Manufacturer Part Number Inductance (μH) Saturation Current (A) DCR Typ. (mΩ) Size (mm) LxWxH Type CDRH5D16 CDRH5D16 CDRH8D28 SD53 SD53 SD53 2.2 3.3 4.7 2.0 3.3 4.7 3.0 2.6 3.4 3.3 2.6 2.1 28.7 35.6 19 23 29 39 5.8x5.8x1.8 5.8x5.8x1.8 8.3x8.3x3.0 5.2x5.2x3.0 5.2x5.2x3.0 5.2x5.2x3.0 Shielded Shielded Shielded Shielded Shielded Shielded Sumida Coiltronics Table 3: Surface Mount Inductors. Adjustable Output Resistor Selection For applications requiring an adjustable output voltage, the 0.6V version can be externally programmed. Resistors R1 and R2 of Figure 3 program the output to regulate at a voltage higher than 0.6V. 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 and the R1+R2 should be less than 1.5MΩ. The external resistor sets the output voltage according to the following equation: R1 VOUT = 0.6 · 1 + R2 R1 = VOUT 0.6 -1 · R2 Table 4 summarizes the standard 1% metal film resistor values for various output voltages with R2 set to 59kΩ. VOUT (V) R2 = 59kΩ R1 (kΩ) 1.0 1.2 1.5 1.8 2.0 2.5 3.3 3.6 3.8 4.2 39.2 59.0 88.7 118 137 187 267 294 316 357 Table 4: Resistor Selections for Different Output Voltage Settings. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 17 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Layout Guidance 4. When laying out the PC board, the following layout guidelines should be followed to ensure proper operation of the AAT1138: 5. 1. 2. 3. The exposed pad (EP) must be reliably soldered to the GND plane. A GND pad below EP is strongly recommended. The power traces, including the GND trace, the SW trace and the SYSIN, SYSOUT trace should be kept short, direct and wide to allow large current flow. The L1 connection to the SW pins should be as short as possible. Do not put any signal lines under the inductor. The input capacitor (C1 and C21) should connect as closely as possible to SYSIN and SYSOUT and GND to get good power filtering. 6. Keep the switching node, SW away from the sensitive FB node. The feedback trace should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. External feedback resistors should be placed as closely as possible to the FB pin to minimize the length of the high impedance feedback trace. The resistance of the trace from the load return to GND should be kept to a minimum. This will help to minimize any error in DC regulation due to differences in the potential of the internal signal ground and the power ground. R2 59k C4 100pF R1 316k SYSOUT SW L1 2.2μH VOUT U1 1 2 3 C32 330μF C31 22μF 4 5 6 7 VIN 8 C1 1μF C21 22μF AAT1138 GND FB SW GND SW SVIN PVIN PGND PGND ENBUCK EP SYSOUT GND SYSIN SYSOUT ENSYS SET C22 C23 330μF 330μF C24 330μF 16 15 GND 14 13 12 11 ENBUCK 10 9 RSET 11.5k ENSYS Figure 3: AAT1138 Evaluation Board Schematic. 18 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 C25 330μF DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Figure 4: AAT1138 Evaluation Board Layout Top Layer. Figure 5: AAT1138 Evaluation Board Layout Bottom Layer. Designation Part Number Description Manufacturer U1 C1 C4 C21, C31 C22, C23, C24, C25, C32 L1 R1 R2 RSET AAT1138IRN-0.6-T1 GRM21BR71E105K GRM1885C1H101J GRM21BR60J226M TPSY337M006R0150 CDRH5D16-2R2 RC0603FR-07316KL RC0603FR-0759KL RC0603FR-0711K5L 2A Buck with Current Limit CAP Ceramic 1μF 0805 X7R 25V 10% CAP Ceramic 100pF 0603 COG 50V 5% CAP Ceramic 22μF 0805 X5R 6.3V 20% Cap Tan 330μF Y case 6.3V 20% Power Inductor 2.2μH 3.0A SMD RES 316KΩ 1/10W 1% 0603 SMD RES 59KΩ 1/10W 1% 0603 SMD RES 11.5KΩ 1/10W 1% 0603 SMD Skyworks Murata AVX Sumida Yageo Table 5: AAT1138 Evaluation Board Bill of Materials (BOM). Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 19 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit Ordering Information Output Voltage Package Marking1 Part Number (Tape and Reel)2 Adjustable ≥ 0.6V TDFN34-16 7BXYY AAT1138IRN-0.6-T1 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 TDFN34-163 3.000 ± 0.050 0.450 ± 0.050 1.600 ± 0.050 Detail "A" 0.230 ± 0.050 0.450 ± 0.050 3.300 ± 0.050 4.000 ± 0.050 Index Area Top View Bottom View 0.750 ± 0.050 Detail "A" 0.000 + 0.100 -0.000 0.203 REF Side View 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. 20 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 DATA SHEET AAT1138 2A Step-Down Converter with Adjustable Input Current Limit 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. 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Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013 21