DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter General Description Features The AAT2158 SwitchReg is a 1.5A step-down converter with an input voltage range of 2.4V to 5.5V and an adjustable output voltage from 0.6V to VIN. The 1.4MHz switching frequency enables the use of small external components. The small footprint and high efficiency make the AAT2158 an ideal choice for portable applications. • • • • • • • • • • • • • • The AAT2158 delivers 1.5A maximum output current while consuming only 42μA of no-load quiescent current. Ultra-low RDS(ON) integrated MOSFETs and 100% duty cycle operation make the AAT2158 an ideal choice for high output voltage, high current applications which require a low dropout threshold. The AAT2158 provides excellent transient response and high output accuracy across the operating range. No external compensation components are required. 1.5A Maximum Output Current Input Voltage: 2.4V to 5.5V Output Voltage: 0.6V to VIN Up to 95% Efficiency Low Noise Light Load Mode 42μA No Load Quiescent Current No External Compensation Required 1.4MHz Switching Frequency 100% Duty Cycle Low-Dropout Operation Internal Soft Start Over-Temperature and Current Limit Protection <1μA Shutdown Current 16-Pin 3x3mm QFN Package Temperature Range: -40°C to +85°C Applications The AAT2158 maintains high efficiency throughout the load range. The AAT2158’s unique architecture produces reduced ripple and spectral noise. Over-temperature and short-circuit protection safeguard the AAT2158 and system components from damage. • • • • • • • The AAT2158 is available in a Pb-free, space-saving 16-pin 3x3mm QFN package. The product is rated over an operating temperature range of -40°C to +85°C. Cellular Phones Digital Cameras Hard Disk Drives MP3 Players PDAs and Handheld Computers Portable Media Players USB Devices Typical Application U1 AAT2158 3.3V 12 11 10 C1 10µF 7 9 6 8 5 VP FB VP LX VP LX EN LX VCC N/C N/C PGND N/C PGND SGND PGND 2.5V 4 R3 187k 15 14 13 L1 2.2µH 16 3 2 R4 59k C3 22µF 1 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 1 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Pin Descriptions Pin # Symbol 1, 2, 3 PGND 4 FB 5 6, 8, 16 SGND N/C 7 EN 9 10, 11, 12 VCC VP 13, 14, 15 LX EP Function Main power ground return pin. Connect to the output and input capacitor return. (See board layout rules.) Feedback input pin. For an adjustable output, connect an external resistive divider to this pin. For fixed output voltage versions, FB is the output pin of the converter. Signal ground. Connect the return of all small signal components to this pin. (See board layout rules.) Not internally connected. Enable input pin. A logic high enables the converter; a logic low forces the AAT2158 into shutdown mode reducing the supply current to less than 1μA. The pin should not be left floating. Bias supply. Supplies power for the internal circuitry. Connect to input power. Input supply voltage for the converter power stage. Must be closely decoupled to PGND. Connect inductor to these pins. Switching node internally connected to the drain of both high- and low-side MOSFETs. Exposed paddle (bottom); connect to PGND directly beneath package. Pin Configuration QFN33-16 (Top View) LX LX LX N/C 13 14 15 16 PGND PGND PGND FB 1 12 2 11 3 10 4 9 VP VP VP VCC 8 7 6 5 N/C EN N/C SGND 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Absolute Maximum Ratings1 Symbol VCC, VP VLX VFB VEN TJ Description VCC, VP to GND LX to GND FB to GND EN to GND Operating Junction Temperature Range Value Units 6 -0.3 to VP + 0.3 -0.3 to VCC + 0.3 -0.3 to -6 -40 to150 V V V V °C Value Units 50 4.2 2.0 °C/W °C/W W Value Units -40 to 85 °C Thermal Characteristics Symbol JA JC PD Description Maximum Thermal Resistance Maximum Thermal Resistance Maximum Power Dissipation (TA = 25°C)2, 3 Recommended Operating Conditions Symbol TA Description Ambient Temperature Range 1. Stresses above those listed in Absolute Maximum Ratings may cause 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 a demo board (FR4, in still air). Exposed pad must be mounted to PCB. 3. Derate 20mW/°C above 25°C. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 3 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Electrical Characteristics1 VIN = 3.6V; TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C. Symbol Description VIN VOUT Input Voltage Output Voltage Range VUVLO UVLO Threshold VOUT IQ ISHDN ILIM RDS(ON)H RDS(ON)L VLOADREG VLINEREG/VIN IFB FOSC TS TSD THYS Output Voltage Tolerance Quiescent Current Shutdown Current Current Limit High Side Switch On-Resistance Low Side Switch On-Resistance Load Regulation Line Regulation Feedback Threshold Voltage Accuracy (Adjustable Version) FB Leakage Current Internal Oscillator Frequency Start-Up Time Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis VIL VIH IEN Enable Threshold Low Enable Threshold High Enable Leakage Current VFB Conditions Min Typ 2.4 0.6 VIN Rising Hysteresis VIN Falling IOUT = 0A to 1.5A, VIN = 2.4V to 5.5V No Load VEN = GND Max Units 5.5 VIN 2.4 3.0 90 1.0 V V V mV V % μA μA A % %/V 0.609 V 0.2 1.68 μA MHz μs °C °C 0.6 V V μA 250 1.8 -3.0 42 1.8 0.120 0.085 0.5 0.2 ILOAD = 0A to 1.5A VIN = 2.4V to 5.5V No Load, TA = 25°C 0.591 0.60 1.12 1.4 150 140 15 VOUT = 1.0V From Enable to Output Regulation EN VIN = VEN = 5.5V 1.4 -1.0 1.0 1. The AAT2158 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. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Typical Characteristics Efficiency vs. Output Current Load Regulation (VOUT = 3.3V) 100 1.00 90 0.75 VOUT Error (%) Efficiency (%) (VOUT = 3.3V) 80 VIN = 4.2V 70 VIN = 5.0V VIN = 3.6V 60 50 0.50 VIN = 4.2V 0.25 0.00 -0.25 VIN = 3.6V -0.50 -0.75 40 30 VIN = 5.0V 0 1 10 100 1000 -1.00 0.1 10000 1 10 Output Current (mA) 100 Load Regulation (VOUT = 1.8V) (VOUT = 1.8V) VIN = 4.2V 0.75 80 VIN = 4.2V VIN = 3.6V 60 50 40 30 0.50 VIN = 3.6V 0.25 0.00 -0.25 VIN = 2.7V -0.50 -0.75 0 1 10 100 1000 -1.00 10000 0.1 1 10 Output Current (mA) 1000 10000 Efficiency vs. Output Current Load Regulation (VOUT = 1.2V) (VOUT = 1.2V) 1000 10000 1.00 90 0.75 VIN = 2.7V VOUT Error (%) Efficiency (%) 100 Output Current (mA) 100 80 70 VIN = 4.2V VIN = 3.6V 60 50 40 30 10000 1.00 VOUT Error (%) Efficiency (%) Efficiency vs. Output Current 70 1000 Output Current (mA) VIN = 2.7V 90 100 VIN = 4.2V 0.50 0.25 VIN = 2.7V 0.00 -0.25 VIN = 3.6V -0.50 -0.75 0 1 10 100 Output Current (mA) 1000 10000 -1.00 0.1 1 10 100 Output Current (mA) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 5 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Typical Characteristics Quiescent Current vs. Input Voltage Output Voltage vs. Temperature (VOUT = 1.8V; No Load) (VOUT = 1.8V; IOUT = 1A) 1.5 70 Output Voltage Error (%) Quiescent Current (µA) 80 85°C 60 50 25°C 40 30 -40°C 20 10 0 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -40 -30 -20 -10 0 Input Voltage (V) Output Voltage (V) 1.80 1.79 25°C -40°C 1.75 1.74 1.73 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 Switching Frequency (MHz) 85°C 1.81 60 80 90 1.54 1.52 1.5 1.48 1.46 1.44 1.42 -40 -30 -20 -10 0 10 20 30 40 50 60 Load Transient Response (VOUT = 1.8V) (VOUT = 1.8V; CFF = 100pF) 70 80 90 2.4 0.10 2.2 0.00 2.0 -0.10 1.8 -0.20 1.6 -0.30 1.4 -0.40 1.2 -0.50 1.0 -0.60 0.8 0.20 2.4 0.10 2.2 0.00 2.0 -0.10 1.8 -0.20 1.6 -0.30 1.4 -0.40 1.2 -0.50 1.0 -0.60 0.8 Time (100µs/div) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 Output Current (bottom) (A) 0.20 Output Voltage (AC coupled) (top)(mV) Load Transient Response Time (100µs/div) 70 Temperature (°C) Output Current (bottom) (A) Output Voltage (AC coupled) (top)(mV) 50 1.56 Input Voltage (V) 6 40 (VOUT = 1.8V; IOUT = 1A) 1.82 1.76 30 Switching Frequency vs. Temperature (VOUT = 1.8V; IOUT = 1A) 1.77 20 Temperature (°C) Output Voltage vs. Input Voltage 1.78 10 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Typical Characteristics Line Transient Response Line Regulation (VOUT = 1.8V; IOUT = 1A) 0.12 4.5 0.10 4.0 0.08 3.5 0.06 3.0 0.04 2.5 0.02 2.0 0.00 1.5 -0.02 1.0 -0.04 0.40 0.30 VOUT Error (%) 5.0 Output Voltage (AC coupled) (bottom) (V) Input Voltage (top) (V) (VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF) 0.20 0.10 0.00 -0.10 -0.20 -0.30 -0.40 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 Time (100µs/div) Input Voltage (V) Enable Soft Start Heavy Load Switching Waveform (VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A) EN (2V/div) VOUT (1V/div) IIN (500mA/div) 4.0 2.6 2.0 2.4 0.0 2.2 -2.0 2.0 -4.0 1.8 -6.0 1.6 -8.0 1.4 -10.0 1.2 -12.0 1.0 Time (500ns/div) Time (100µs/div) Light Load Switching Waveform Light Load Switching Waveform 40 0.6 0 0.5 -40 0.4 -80 0.3 -120 0.2 -160 0.1 -200 0.0 -240 -0.1 Output Voltage (AC coupled) (top)(mV) 0.7 80 0.7 40 0.6 0 0.5 -40 0.4 -80 0.3 -120 0.2 -160 0.1 -200 0.0 -240 -0.1 Inductor Ripple Current (bottom) (A) 80 (VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF) Inductor Ripple Current (bottom) (A) Output Voltage (AC coupled) (top)(mV) (VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF) Time (5µs/div) Inductor Ripple Current (bottom) (A) Output Voltage (AC Coupled) (top) (mV) (VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A) Time (200µs/div) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 7 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Typical Characteristics Light Load Switching Waveform Light Load Switching Waveform (VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF) 0.6 0 0.5 -40 0.4 0.3 -80 -120 0.2 -160 0.1 -200 0.0 -240 -0.1 Time (5µs/div) 8 Output Voltage (AC coupled) (top)(mV) 0.7 40 80 0.7 40 0.6 0 0.5 -40 0.4 -80 0.3 -120 0.2 -160 0.1 -200 0.0 -240 -0.1 Time (50µs/div) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 Inductor Ripple Current (bottom) (A) 80 Inductor Ripple Current (bottom) (A) Output Voltage (AC coupled) (top)(mV) (VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF) DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Functional Block Diagram VCC VP 0.6V REF OP. AMP CMP DH FB LOGIC LX DL Temp. Sensing OSC SGND EN Functional Description The AAT2158 is a high performance 1.5A monolithic step-down converter operating at a 1.4MHz switching frequency. It minimizes external component size, optimizes efficiency over the complete load range, and produces reduced ripple and spectral noise. Apart from the small bypass input capacitor, only a small L-C filter is required at the output. Typically, a 3.3μH inductor and a 22μF ceramic capacitor are recommended for a 3.3V output (see table of recommended values). At dropout, the converter duty cycle increases to 100% and the output voltage tracks the input voltage minus the RDS(ON) drop of the P-channel high-side MOSFET (plus the DC drop of the external inductor). The device integrates extremely low RDS(ON) MOSFETs to achieve low dropout voltage during 100% duty cycle operation. This is advantageous in applications requiring high output voltages (typically > 2.5V) at low input voltages. PGND The integrated low-loss MOSFET switches can provide greater than 95% efficiency at full load. Light load operation maintains high efficiency, low ripple and low spectral noise even at lower currents (typically <150mA). In battery-powered applications, as VIN decreases, the converter dynamically adjusts the operating frequency prior to dropout to maintain the required duty cycle and provide accurate output regulation. Output regulation is maintained until the dropout voltage, or minimum input voltage, is reached. At 1.5A output load, dropout voltage headroom is approximately 200mV. The AAT2158 typically achieves better than ±0.5% output regulation across the input voltage and output load range. A current limit of 2.0A (typical) protects the IC and system components from short-circuit damage. Typical no load quiescent current is 42μA. Thermal protection completely disables switching when the maximum junction temperature is detected. The Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 9 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Soft Start/Enable junction over-temperature threshold is 140°C with 15°C of hysteresis. Once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers. Soft start limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the AAT2158 into a low-power, non-switching state. The total input current during shutdown is less than 1μA. Peak current mode control and optimized internal compensation provide high loop bandwidth and excellent response to input voltage and fast load transient events. Soft start eliminates output voltage overshoot when the enable or the input voltage is applied. Under-voltage lockout prevents spurious start-up events. Current Limit and Over-Temperature Protection For overload conditions, the peak input current is limited. To minimize power dissipation and stresses under current limit and short-circuit conditions, switching is terminated after entering current limit for a series of pulses. Switching is terminated for seven consecutive clock cycles after a current limit has been sensed for a series of four consecutive clock cycles. Control Loop The AAT2158 is a peak current mode step-down converter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as shortcircuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor. Thermal protection completely disables switching when internal dissipation becomes excessive. The junction over-temperature threshold is 140°C with 15°C of hysteresis. Once an over-temperature or over-current fault conditions is removed, the output voltage automatically recovers. 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. The reference voltage is internally set to program the converter output voltage greater than or equal to 0.6V. Enable VIN+ R1 100 C1 10µF 11 10 R2 7 100K 9 C2 0.1µF 6 8 5 GND VP FB VP LX VP LX EN VCC N/C N/C LX N/C PGND PGND SGND PGND Internal bias of all circuits is controlled via the VCC input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation. LX U1 AAT2158 12 Under-Voltage Lockout C8 VOUT+ 4 R3 15 14 13 L1 3.0µH 16 3 2 R4 59.0k C3 22µF 1 VOUT(V) 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 2.0 2.5 3.3 R3 (kΩ) 19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 137 187 267 GND C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3 C3 Murata 22µF 6.3V GRM21BR60J226ME39L X5R 0805 L1 Sumida CDRH5D28-3R0NC (see Table 2) R1 and C2 are an optional noise filter for internal VCC. R6, C4, C5-C7 are not populated C8 100pF feed forward Figure 1: AAT2158 Evaluation Schematic. 10 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Component Selection 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 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. The 3.3μH CDRH4D28 series Sumida inductor has a 49.2m worst case DCR and a 1.57A DC current rating. At full 1.5A load, the inductor DC loss is 97mW which gives less than 1.5% loss in efficiency for a 1.5A, 3.3V output. Input Capacitor Select a 10μF to 22μF X7R or X5R ceramic capacitor for the input. To estimate the required input capacitor size, determine the acceptable input ripple level (VPP) and solve for C. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage. CIN = V ⎞ VO ⎛ · 1- O VIN ⎝ VIN ⎠ ⎛ VPP ⎞ - ESR · FS ⎝ IO ⎠ VO ⎛ V ⎞ 1 · 1 - O = for VIN = 2 · VO VIN ⎝ VIN ⎠ 4 CIN(MIN) = 1 ⎛ VPP ⎞ - ESR · 4 · FS ⎝ IO ⎠ Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value. For example, the capacitance of a 10μF, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6μF. The maximum input capacitor RMS current is: IRMS = IO · VO ⎛ V ⎞ · 1- O VIN ⎝ VIN ⎠ The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current. VO ⎛ V ⎞ · 1- O = VIN ⎝ VIN ⎠ D · (1 - D) = 0.52 = 1 2 for VIN = 2 · VO IRMS(MAX) = VO IO 2 ⎛ V ⎞ · 1- O The term VIN ⎝ VIN ⎠ appears in both the input voltage ripple and input capacitor RMS current equations and is a maximum when VO is twice VIN. This is why the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2158. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function. To minimize stray inductance, the capacitor should be placed as closely 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 2). A laboratory test set-up typically consists of two long wires running from the bench power supply to the evaluation 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. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 11 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Since the inductance of a short PCB trace feeding the input voltage is significantly lower than the power leads from the bench power supply, most applications do not exhibit this problem. In applications where the input power source lead inductance cannot be reduced to a level that does not affect the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the low ESR/ESL bypass ceramic capacitor. This dampens the high Q network and stabilizes the system. Output Capacitor The output capacitor limits the output ripple and provides holdup during large load transitions. A 10μF to 22μF X5R or X7R ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the ESR and ESL characteristics necessary for low output ripple. The output voltage droop due to a load transient is dominated by the capacitance of the ceramic output capacitor. During a step increase in load current, the ceramic output capacitor alone supplies the load current until the loop responds. Within two or 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 = 3 · ΔILOAD VDROOP · FS Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum value for the output capacitor with respect to load transients. The internal voltage loop compensation also limits the minimum output capacitor value to 10μF. This is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater phase margin. Adjustable Output Resistor Selection The output voltage on the AAT2158 is programmed with external resistors R3 and R4. To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum suggested value for R4 is 59k. Although a larger value will further reduce quiescent current, it will also increase the 12 impedance of the feedback node, making it more sensitive to external noise and interference. Table 1 summarizes the resistor values for various output voltages with R4 set to either 59k for good noise immunity or 221k for reduced no load input current. The external resistor R3, combined with an external 100pF feed forward capacitor (C8 in Figure 1), 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 C3-C4 for stability. The external resistors set the output voltage according to the following equation: ⎛ R3 ⎞ VOUT = 0.6V 1 + ⎝ R4 ⎠ or R3 = ⎡⎛VOUT ⎣⎝VREF ⎞ - 1 ⎤ R4 ⎠⎦ VOUT (V) R4 = 59k R3 (k) R4 = 221k R3 (k) 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.0 3.3 19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 237 267 75 113 150 187 221 261 301 332 442 464 523 715 887 1000 Table 1: AAT2158 Resistor Values for Various Output Voltages. The typical circuit shown in the AAT2158 evaluation schematic is intended to be general purpose and suitable for most applications. An additional example schematic is shown in Figure 2 according to the design guidelines for cases where transient load steps are more severe and the restriction on output voltage deviation is more stringent. To handle these cases some simple adjustments can be made. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter U1 V IN 12 2.4V - 5.5V 11 R1 0 R2 100K 10 9 EN C1 10μF C2 open U1 C1 C3 C4 C8 R1-R4 L1 LX VP LX VP LX VCC FB 15 V OUT L1 1.0 -1.5μH 1.2V/1.5A 14 13 R3 100K C8 1nF (optional) 4 C3 22μF AAT2158 3 2 VP 7 EN N/C 16 PGND N/C 6 2 PGND N/C 8 1 PGND SGND 1 3 C4 22μF R4 100K 5 AAT2158 AnalogicTech, Hi-Voltage Buck, TDFN34-16 Cap, MLC, 10μF/6.3V,X7R, 0805 Cap, MLC, 22μF/6.3V, X7R,0805 Cap, MLC, 22μF/6.3V, X7R, 0805 Cap, MLC, 1nF/6.3V, 0402 Carbon film resistor, 0402 TDK, VLS3012T-1R5N1R7, 1.5μH, ISAT = 1.9A, DCR = 68mΩ Figure 2: AAT2158 Alternate Application Schematic. The schematic in Figure 2 shows the configuration for improved transient response in an application where the output is stepped down to 1.2V. The adjustments consist of adding an additional 22μF output capacitor C4, increasing the value of the feed forward capacitor C8 to 1nF, and eliminating the RC filter network R1 and C2. IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses. For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to: Thermal Calculations There are three types of losses associated with the AAT2158 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction losses are associated with the RDS(ON) characteristics of the power output switching devices. Switching losses are dominated by the gate charge of the power output switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the losses is given by: PTOTAL = IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO]) VIN PTOTAL = IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO]) VIN + (tsw · FS · IO + IQ) · VIN Since RDS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. Given the total losses, the maximum junction temperature can be derived from the JA for the QFN33-16 package, which is 50°C/W. TJ(MAX) = PTOTAL · ΘJA + TAMB + (tsw · FS · IO + IQ) · VIN Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 13 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Layout 3. The suggested PCB layout for the AAT2158 is shown in Figures 3 and 4. The following guidelines should be used to help ensure a proper layout. 4. 1. 2. The input capacitor (C1) should connect as closely as possible to VP and PGND. C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible. Figure 3: AAT2158 Evaluation Board Top Side Layout. 14 5. The feedback trace or FB pin 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. The resistance of the trace from the load return to PGND 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. Connect unused signal pins to ground to avoid unwanted noise coupling. Figure 4: AAT2158 Evaluation Board Bottom Side Layout. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Design Example Specifications VO VIN FS TAMB 3.3V @ 1.4A, Pulsed Load ILOAD = 1.4A 2.7V to 4.2V (3.6V nominal) 1.2MHz 85°C in QFN33-16 Package Output Inductor L1 = VO(μH) = 3.3μH; see Table 2. For Sumida inductor CDRH4D28 3.3μH DCR = 49.2m max. ⎛ VO V ⎞ 3.3V 3.3V ⎞ ⎛ ⋅ 1 - O1 = ⋅ ⎝1 = 179mA L1 ⋅ FS ⎝ VIN ⎠ 3.3µH ⋅ 1.2MHz 4.2V ⎠ ΔI1 = IPK1 = IO1 + ΔI1 = 1.4A + 0.089A = 1.49A 2 PL1 = IO12 ⋅ DCR = 1.4A2 ⋅ 49.2mΩ = 96mW Output Capacitor VDROOP = 0.2V COUT = 3 · ΔILOAD 3 · 1.4A = = 17.5µF; use 22µF VDROOP · FS 0.2V · 1.2MHz IRMS(MAX) = (VOUT) · (VIN(MAX) - VOUT) 1 3.3V · (4.2V - 3.3V) · = 52mArms = L · FS · VIN(MAX) 2 · 3 3.3µH · 1.2MHz · 4.2V 2· 3 1 · Pesr = esr · IRMS2 = 5mΩ · (52mA)2 = 13.3µW Input Capacitor Input Ripple VPP = 50mV CIN = 1 ⎛ VPP ⎞ - ESR · 4 · FS ⎝ IO1 + IO2 ⎠ IRMS(MAX) = = 1 = 6.8µF; use 10µF ⎛ 50mV ⎞ - 5mΩ · 4 · 1.2MHz ⎝ 1.4A ⎠ IO = 0.7Arms 2 P = esr · IRMS2 = 5mΩ · (0.7A)2 = 2.45mW Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 15 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter AAT2158 Losses Total losses can be estimated by calculating the dropout (VIN = VO) losses where the power MOSFET RDS(ON) will be at the maximum value. All values assume an 85°C ambient temperature and a 120°C junction temperature with the QFN 50°C/W package. PLOSS = IO12 · RDS(ON)H = 1.4A2 · 0.16Ω = 0.31W TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 310mW = 101°C The total losses are also investigated at the nominal lithium-ion battery voltage (3.6V). The simplified version of the RDS(ON) losses assumes that the N-channel and P-channel RDS(ON) are equal. PTOTAL = IO2 · RDS(ON) + [(tsw · FS · IO + IQ) · VIN] = 1.4A2 · 152mΩ + [(5ns · 1.2MHz · 1.4A + 50μA) · 3.6V] = 328mW TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 328mW = 101°C VOUT (V) Inductance (μH) Part Number Manufacturer Size (mm) 3.3 3.3 2.5 1.8 1.5 1.2 1.0 0.8 0.6 3.0 3.3 2.2 1.8 1.8 1.2 1.0 1.0 1.0 CDRH5D28 CDRH4D28 CDRH4D28 CDRH4D28 CDRH4D28 CDRH4D28 SD3114-1.0 SD3114-1.0 SD3114-1.0 Sumida Sumida Sumida Sumida Sumida Sumida Cooper Cooper Cooper 6x6x3 5x5x3 5x5x3 5x5x3 5x5x3 5x5x3 3.1x3.1x1.45 3.1x3.1x1.45 3.1x3.1x1.45 Rated Current (A) ISAT (A) DCR () 2.07 2.07 2.07 24.0 49.2 31.3 27.5 27.5 23.6 0.042 0.042 0.042 IRMS (A) 2.4 1.57 2.04 2.2 2.2 2.56 1.67 1.67 1.67 Table 2: Surface Mount Inductors. Manufacturer Part Number Value Voltage Temp. Co. Case Murata Murata GRM21BR60J106KE19 GRM21BR60J226ME39 10μF 22μF 6.3V 6.3V X5R X5R 0805 0805 Table 3: Surface Mount Capacitors. 16 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 DATA SHEET AAT2158 1.5A Low-Noise,Step-Down Converter Ordering Information Package Marking1 Part Number (Tape and Reel)2 QFN33-16 XSXYY AAT2158IVN-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 Information3 QFN33-16 0.230 ± 0.050 Pin 1 Identification 0.500 ± 0.050 C0.3 13 1.250 ± 0.050 Top View Bottom View 0.214 ± 0.036 0.900 ± 0.100 9 3.000 ± 0.050 0.025 ± 0.025 All dimensions in millimeters. 5 1.250 ± 0.050 0.400 ± 0.100 Pin 1 Dot By Marking 3.000 ± 0.050 1 Side View 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. Copyright © 2012 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 202011A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012 17