PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter General Description Features The AAT1145 SwitchReg™ is a 1.2MHz constant frequency current mode PWM step-down converter. It is ideal for portable equipment requiring very high current up to 1.5A from single-cell Lithium-ion batteries while still achieving over 90% efficiency during peak load conditions. The AAT1145 also can run at 100% duty cycle for low dropout operation, extending battery life in portable systems while light load operation provides very low output ripple for noise sensitive applications. • • • • • • • • The AAT1145 can supply up to 1.5A output load current from a 2.5V to 5.5V input voltage and the output voltage can be regulated as low as 0.6V. The high switching frequency minimizes the size of external components while keeping switching losses low. The internal slope compensation setting allows the device to operate with smaller inductor values to optimize size and provide efficient operation. The AAT1145 is available in adjustable (0.6V to VIN) and fixed (1.8V) output voltage versions. The device is available in a Pb-free, 3x3mm 10-lead TDFN package and is rated over the -40°C to +85°C temperature range. • • • • • • Input Voltage Range: 2.5V to 5.5V Output Voltages from 0.6V to VIN 1.5A Output Current High Efficiency: Up to 95% 1.2MHz Constant Switching Frequency Low RDS(ON) Internal Switches: 135mΩ, 95mΩ Allows Use of Ceramic Capacitors Current Mode Operation for Excellent Line and Load Transient Response Short-Circuit and Thermal Fault Protection Soft Start Low Dropout Operation: 100% Duty Cycle Low Shutdown Current: ISHUTDOWN < 1μA TDFN33-10 Package -40°C to +85°C Temperature Range Applications • • • • • • Cellular Phones Digital Cameras DSP Core Supplies PDAs Portable Instruments Smart Phones Typical Application VIN 2.5V-5.5V C1 10μF 1145.2008.01.1.2 L1 2.2μH 1 2 3 8 EN LX IN LX 7 AAT1145-1.8 OUT 5 AIN 6 AGND PGND 4 AGND PGND VOUT 1.8V,1.5A C2 22μF 10 9 www.analogictech.com 1 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Pin Descriptions Pin # Symbol 1 EN 2 3 4, 6 IN AIN AGND 5 FB/OUT 7, 8 9, 10 LX PGND EP Function Enable pin. Active high. In shutdown, all functions are disabled drawing <1μA supply current. Do not leave EN floating. Power supply input pin. Must be closely decoupled to AGND with a 2.2μF or greater ceramic capacitor. Analog supply input pin. Provides bias for internal circuitry. Analog ground pin FB pin (AAT1145IDE-0.6): Adjustable version feedback input. Connect FB to the center point of the external resistor divider. The feedback threshold voltage is 0.6V. OUT pin (AAT1145IDE-1.8): Fixed version feedback input. Connect OUT to the output voltage, VOUT. Switching node pin. Connect the output inductor to this pin. Power ground pin Power ground exposed pad. Must be connected to bare copper ground plane. Pin Configuration TDFN-10 (Top View) EN IN AIN AGND FB/OUT1 1 10 2 9 3 8 4 7 5 6 PGND PGND LX LX AGND 1. FB pin for the adjustable voltage version (AAT1145IDE-0.6), OUT pin for the fixed voltage version (AAT1145IDE-1.8). 2 www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Absolute Maximum Ratings1 Symbol IN, AIN VFB, VLX VEN PGND, AGND TA TSTORAGE TLEAD Description Input Supply Voltages FB, LX Voltages EN Voltage Ground Voltages Operating Temperature Range Storage Temperature Lead Temperature (Soldering, 10s) Value Units -0.3 to 6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to 6.0 -40 to +85 -65 to 150 300 V V V V °C °C °C Value Units 45 2.2 °C/W W Thermal Information3 Symbol θJA PD Description Thermal Resistance2 Maximum Thermal Dissipation at TA = 25°C 1. Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. 2. TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + PD · θJA. 3. Thermal Resistance is specified with approximately 1 square inch of 1 oz. copper. 1145.2008.01.1.2 www.analogictech.com 3 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Electrical Characteristics1 VIN = 3.6V, TA = -40°C to +85°C unless otherwise noted; typical values are TA = 25°C. Symbol VIN VOUT Description Conditions IFB Input Voltage Range Output Voltage Range Input DC Supply Active Mode Current Shutdown Mode Feedback Input Bias Current VFB Regulated Feedback Voltage3 IQ ΔVLINEREG/ ΔVIN ΔVLOADREG/ ΔIOUT VFB VOUT FOSC TS TSD THYS ILIM RDS(ON) VEN(L) VEN(H) IEN Min Typ Max Units V V μA μA nA 0.6000 0.6000 0.6000 5.5 VIN 500 1 30 0.6120 0.6135 0.6150 0.20 %/V 2.5 0.6 2 VFB = 0.5V VEN = 0V, VAIN = 5.5V VFB = 0.65V TA = 25°C TA = 0°C ≤ TA ≤ 85°C TA = -40°C ≤ TA ≤ 85°C 300 0.1 0.5880 0.5865 0.5850 Line Regulation VIN = 2.5V to 5.5V, IOUT = 10mA 0.10 Load Regulation IOUT = 10mA to 1500mA 0.20 Output Voltage Accuracy VIN = 2.5 to 4.2V, IOUT = 10 to 1500mA Oscillator Frequency Startup Time Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis Peak Switch Current P-CH MOSFET N-CH MOSFET Enable Threshold Low Enable Threshold High Input Low Current VFB = 0.6V From Enable to Output Regulation -3 0.96 1.5 -1.0 %/A +3 % 1.44 MHz ms 170 °C 10 °C 2.5 135 95 VIN = 3.6V VIN = 3.6V VIN = VEN = 5.5V 1.2 1.3 V A 200 150 0.3 1.0 mΩ V V μA 1. The AAT1145 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. VIN should be not less than VOUT + VDROPOUT, where VDROPOUT = IOUT · (RDS(ON)PMOS + ESRINDUCTOR), typically VDROPOUT = 0.3V. 3. The regulated feedback voltage is tested in an internal test mode that connects VFB to the output of the error amplifier. 4 www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Typical Characteristics Efficiency vs. Output Current Efficiency vs. Output Current (VOUT = 3.3V; TA = 25°°C; L = 2.2µH) (VOUT = 1.8V; TA = 25°°C; L = 2.2µH) 90 90 80 80 Efficiency (%) 100 Efficiency (%) 100 70 60 50 40 VIN = 3.7V VIN = 4.2V VIN = 5V VIN = 5.5V 30 20 10 0 0.1 1 10 100 1000 70 60 50 VIN = 2.5V VIN = 3.6V VIN = 4.2V VIN = 5V VIN = 5.5V 40 30 20 10 0 0.1 10000 1 Output Current (mA) 80 Efficiency (%) 90 80 Efficiency (%) 100 90 70 60 50 VIN = 2.7V VIN = 3.6V VIN = 4.2V VIN = 5V VIN = 5.5V 10 0 0.1 1 10 100 1000 70 60 50 40 VIN = 2.5V VIN = 3.6V VIN = 4.2V VIN = 5V 30 20 10 0 10000 0.1 1 100 1000 10000 DC Regulation DC Regulation (VOUT = 3.3V; TA = 25°C; L = 2.2µH; COUT = 22µF) (VOUT = 1.8V; TA = 25°C; L = 2.2µH; COUT = 22µF) 1.854 VIN = 5.5V VIN = 5V VIN = 4.2V VIN = 3.7V 3.366 3.333 3.300 3.267 3.234 0 300 600 900 1200 1500 Output Voltage (V) 3.399 Output Voltage (V) 10 Output Current (mA) Output Current (mA) 3.201 10000 (VOUT = 1.2V; TA = 25°C; L = 2.2µH) 100 20 1000 Efficiency vs. Output Current (VOUT = 1.5V; TA = 25°°C; L = 2.2µH) 30 100 Output Current (mA) Efficiency vs. Output Current 40 10 1.818 1.800 1.782 1.764 1.746 Output Current (mA) 1145.2008.01.1.2 VIN = 5.5V VIN = 5V VIN = 4.2V VIN = 3.6V VIN = 2.5V 1.836 0 300 600 900 1200 1500 Output Current (mA) www.analogictech.com 5 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Typical Characteristics DC Regulation DC Regulation (VOUT = 1.5V; TA = 25°C; L = 2.2µH; COUT = 22µF) (VOUT = 1.2V; TA = 25°C; L = 2.2µH; COUT = 22µF) 1.236 VIN = 5.5V VIN = 5V VIN = 4.2V VIN = 3.6V VIN = 2.5V 1.530 1.515 Output Voltage (V) Output Voltage (V) 1.545 1.500 1.485 1.470 1.455 1.224 1.212 1.200 1.188 1.164 0 300 600 900 1200 VIN = 5V VIN = 4.2V VIN = 3.6V VIN = 2.5V 1.176 1500 0 300 600 Output Current (mA) Load Transient Response (VIN = 3.6V; VOUT = 1.8V; COUT = 22µF; CFF = 22pF) Output Voltage (top) (V) Input Current (mA) 0.20 0.10 4.0 4.4 4.8 2.2 1.8 1.4 1.0 0.6 0.2 -0.2 1.5A 0.2A 5.2 Time (200µs/div) Soft Start (VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF) VOUT (1V/div) 6 4 2 0 -2 1.8 1.3 0.8 Inductor Current (bottom) (A) Enable Voltage (top) (V) Output Voltage (middle) (V) Start-Up Response (VOUT = 1.8V; VIN = 3.6V; No Load; CIN = 10µF; COUT = 22µF; L = 2.2µH) VENABLE (2V/div) Output Current (bottom) (A) 0.30 2.2 2.0 1.8 1.6 Input Voltage (V) 0.3 -0.2 Time (400µs/div) Time (1ms/div) 6 1500 (VOUT = 3.3V; L = 2.2µH) 0.40 3.6 1200 Input Current vs. Input Voltage 0.50 0.00 900 Output Current (mA) www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Typical Characteristics Output Ripple (VOUT = 1.8V; VIN = 3.6V; IOUT = 0A; L = 2.2µH) 5.00 4.00 3.00 3.6V 2.00 0.40 1.00 0.20 0.00 0.00 -1.00 Output Voltage (top) (V) 4.2V 1.81 1.80 1.79 0.300 0.200 0.100 0.000 -0.100 Inductor Current (bottom) (A) 1.82 6.00 Line Transient (top) (V) Output Voltage (bottom) (VAC) Line Transient Response (VOUT = 1.8V; IOUT = 1.5A; L = 2.2µH; CIN = 10µF; COUT = 22µF) Time (10µs/div) Time (40µs/div) Output Ripple (VOUT = 1.8V; VIN = 3.6V; IOUT = 1.5A; L = 2.2µH) Output Voltage (top) (V) 1.81 1.80 1.79 2.0 1.8 1.6 1.4 1.2 1.0 Inductor Current (bottom) (A) 1.82 Time (400ns/div) 1145.2008.01.1.2 www.analogictech.com 7 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Functional Block Diagram OSC SLOPE COMP IN VIN 2.5V to 5.5V ISENSE AMP 0.6V Softstart SET RESET ICOMP PWM LOGIC NON-OVERLAP CONTROL VOUT LX L1 OUT R1* R1* COUT OVDET 0.65V Over-Temperature and Short-Circuit Protection R2* R2* VIN IZERO COMP PGND 0.6V EN REF SHUTDOWN AIN AGND *The resistor divider R1 + R2 is internally set for the fixed output versions, and is externally set for the adjustable output versions. Functional Description The AAT1145 is a high output current monolithic switchmode step-down DC-DC converter. The device operates at a fixed 1.2MHz switching frequency, and uses a slope compensated current mode architecture. This step-down DC-DC converter can supply up to 1500mA output current at VIN = 3.6V and has an input voltage range from 2.5V to 5.5V. It minimizes external component size and optimizes efficiency at the heavy load range. The slope compensation allows the device to remain stable over a wider range of inductor values so that smaller values (1μH to 4.7μH) with lower DCR can be used to achieve higher efficiency. Apart from the small bypass input capacitor, only a small L-C filter is required at the output. The fixed output version requires only three external power components (CIN, COUT, and L). The adjustable version can be programmed with external feedback to any voltage, ranging from 0.6V to near the input voltage. It uses internal MOSFETs to achieve high efficiency and can generate very low output voltages by using an internal reference of 0.6V. At dropout, the converter duty cycle 8 increases to100% and the output voltage tracks the input voltage minus the low RDS(ON) drop of the P-channel high-side MOSFET and the inductor DCR. The internal error amplifier and compensation provides excellent transient response, load and line regulation. Internal soft start eliminates any output voltage overshoot when the enable or the input voltage is applied. Current Mode PWM Control Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for excellent load and line response with protection of the internal main switch (P-channel MOSFET) and synchronous rectifier (N-channel MOSFET). During normal operation, the internal P-channel MOSFET is turned on for a specified time to ramp the inductor current at each rising edge of the internal oscillator, and switched off when the peak inductor current is above the error voltage. The current comparator, ICOMP, limits the peak inductor current. When the main switch is off, the synchronous rectifier turns on immediately and stays on www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter until either the inductor current starts to reverse, as indicated by the current reversal comparator, IZERO, or the beginning of the next clock cycle. Control Loop The AAT1145 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 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 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. Soft Start / Enable Soft start limits the current surge seen at the input and eliminates output voltage overshoot. The enable pin is active high. When pulled low, the enable input (EN) forces the AAT1145 into a low-power, non-switching state. The total input current during shutdown is less than 1μA. Current Limit and Over-Temperature Protection For overload conditions, the peak input current is limited to 2.5A. 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. The termination lasts for seven consecutive clock cycles after a current limit has been sensed during a series of four consecutive clock cycles. 1145.2008.01.1.2 Thermal protection completely disables switching when internal dissipation becomes excessive. The junction over-temperature threshold is 170°C with 10°C of hysteresis. Once an over-temperature or over-current fault conditions is removed, the output voltage automatically recovers. Dropout Operation When the battery input voltage decreases near the value of the output voltage, the AAT1145 allows the main switch 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: D = TON · FOSC · 100% ≈ VOUT · 100% VIN Where TON is the main switch on time and FOSC is the oscillator frequency. 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. Maximum Load Current The AAT1145 will operate with an input supply voltage as low as 2.5V, however, the maximum load current decreases at lower input voltages due to a large IR drop on the main switch and synchronous rectifier. The slope compensation signal reduces the peak inductor current as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely the current limit increases as the duty cycle decreases. www.analogictech.com 9 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Applications Information VIN 2.5V-5.5V 1 2 C1 10μF 3 EN LX IN LX AIN AAT1145-0.6 6 AGND 4 AGND FB PGND PGND L1 2.2μH 8 7 VOUT 1.8V,1.5A C3 22pF 5 10 R1 634kΩ C2 22μF delivers enhanced transient response for extreme pulsed load applications. The addition of the feed forward capacitor typically requires a larger output capacitor C2 for stability. The external resistor sets the output voltage according to the following equation: ⎛ R1 ⎞ VOUT = 0.6V · 1 + ⎝ R2 ⎠ R2 316kΩ 9 R1 = Figure 1: Basic Application Circuit for the Adjustable Output Version. VIN 2.5V-5.5V C1 10μF 1 2 3 EN LX IN LX AIN AAT1145-1.8 OUT 6 AGND 4 AGND PGND PGND 8 L1 2.2μH VOUT 1.8V,1.5A Table 1 shows the resistor selection for different output voltage settings. VOUT (V) R2 = 59kΩ R1 (kΩ) R2 = 316kΩ R1 (kΩ) 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.3 19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 267 105 158 210 261 316 365 422 475 634 655 732 1000 1430 7 5 C2 22μF 10 9 Figure 2: Basic Application Circuit for the Fixed Output Versions. Setting the Output Voltage Figure 1 shows the basic application circuit with the AAT1145 adjustable output version while Figure 2 shows the application circuit with the AAT1145 fixed output version. For applications requiring an adjustable output voltage, the AAT1145-0.6 adjustable version can be externally programmed. Resistors R1 and R2 in Figure 1 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. 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. Table 1 summarizes the resistor values for various output voltages with R2 set to either 59kΩ for good noise immunity or 316kΩ for reduced no load input current. Table 1: Resistor Selections for Different Output Voltage Settings (Standard 1% Resistors Substituted For Calculated Values). Inductor Selection For most designs, the AAT1145 operates with inductor values of 1μH to 4.7μH. Low inductance values are physically smaller but require faster switching, which results in some efficiency loss. The inductor value can be derived from the following equation: The adjustable version of the AAT1145, combined with an external feed forward capacitor (C3 in Figure 1), 10 ⎛ VOUT ⎞ - 1 · R2 ⎝ 0.6V ⎠ www.analogictech.com L= VOUT · (VIN - VOUT) VIN · ΔIL · fOSC 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Where ΔIL is inductor ripple current. 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 1500mA, or ΔIL = 450mA For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor is 2.2μH. 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. For optimum voltage-positioning load transients, choose an inductor with DC series resistance in the 20mΩ to 100mΩ range. For higher efficiency at heavy loads (above 200mA), or minimal load regulation (but some transient overshoot), the resistance should be kept below 100mΩ. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation (1500mA + 225mA). Table 2 lists some typical surface mount inductors that meet target applications for the AAT1145. The worst case external current slope (m) using the 2.2μH inductor is when VOUT = 3.3V and is: m= To keep the power supply stable when the duty cycle is above 50%, the internal slope compensation (mA) should be: ma ≥ Input Capacitor Selection The input capacitor reduces the surge current drawn from the input and switching noise from the device. The input capacitor impedance at the switching frequency should be less than the input source impedance to prevent high frequency switching current passing to the input. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage. CIN = CIN(MIN) = Slope Compensation 1145.2008.01.1.2 1 · m = 0.75A/µs 2 Therefore, to guarantee current loop stability, the slope of the compensation ramp must be greater than one-half of the down slope of the current waveform. So the internal slope compensated value of 1A/μs will guarantee stability using a 2.2μH inductor value for all output voltages from 0.6V to 3.3V. For example, the 2.2μH SD3118-2R2-R inductor selected from Coiltronics has a 74mΩ DCR and a 2.00ADC current rating. At full load, the inductor DC loss is 106mW which gives a 5% loss in efficiency for a 1200mA, 1.8V output. The AAT1145 step-down converter uses peak current mode control with slope compensation for stability when duty cycles are greater than 50%. The slope compensation is set to maintain stability with lower value inductors which provide better overall efficiency. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. As an example, the value of the slope compensation is set to 1A/μs which is large enough to guarantee stability when using a 2.2μH inductor for all output voltage levels from 0.6V to 3.3V. VOUT 3.3 = = 1.5A/µs L 2.2 V ⎞ VO ⎛ · 1- O VIN ⎝ VIN ⎠ ⎛ VPP ⎞ - ESR · fS ⎝ IO ⎠ 1 ⎛ VPP ⎞ - ESR · 4 · fS ⎝ IO ⎠ A low ESR input capacitor sized for maximum RMS current must be used. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. A 10μF ceramic capacitor for most applications is sufficient. A large value may be used for improved input voltage filtering. The maximum input capacitor RMS current is: www.analogictech.com IRMS = IO · VO ⎛ V ⎞ · 1- O VIN ⎝ VIN ⎠ 11 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter 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. IRMS(MAX) = 1 · IO 2 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 Figures 3 and 4. 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. 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. This dampens the high Q network and stabilizes the system. Output Capacitor Selection The function of output capacitance is to store energy to attempt to maintain a constant voltage. The energy is stored in the capacitor’s electric field due to the voltage applied. 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 12 ripple) are equivalent series resistance (ESR), equivalent series inductance (ESL), and capacitance (C). 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 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 two switching cycles to the output capacitance can be estimated by: COUT = 2 · ΔILOAD VDROOP · fS In many practical designs, to get the required ESR, a capacitor with much more capacitance than is needed must be selected. For both continuous or discontinuous inductor current mode operation, the ESR of the COUT needed to limit the ripple to ∆VO, V peak-to-peak is: ESR ≤ ΔVO ΔIL Ripple current flowing through a capacitor’s ESR causes power dissipation in the capacitor. This power dissipation causes a temperature increase internal to the capacitor. Excessive temperature can seriously shorten the expected life of a capacitor. Capacitors have ripple current ratings that are dependent on ambient temperature and should not be exceeded. The output capacitor ripple current is the inductor current, IL, minus the output current, IO. The RMS value of the ripple current flowing in the output capacitance (continuous inductor current mode operation) is given by: IRMS = ΔIL · 3 = ΔIL · 0.289 6 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. www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter In conclusion, in order to meet the requirement of output voltage ripple small and regulation loop stability, ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR and high ripple current ratings. The output ripple VOUT is determined by: Layout Guidance When laying out the PC board, the following layout guideline should be followed to ensure proper operation of the AAT1145: 1. 1 VOUT · (VIN - VOUT) ⎛ ⎞ ΔVOUT ≤ · ⎝ESR + 8 · fOSC · COUT ⎠ VIN · fOSC · L 2. A 22μF ceramic capacitor can satisfy most applications. Thermal Calculations There are three types of losses associated with the AAT1145 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 = 3. 4. 5. IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO]) VIN 6. + (tsw · F · IO + IQ) · VIN 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: 7. The exposed pad (EP) must be reliably soldered to the GND plane. A PGND pad below EP is strongly recommended. The power traces, including the GND trace, the LX trace and the IN trace should be kept short, direct and wide to allow large current flow. The L1 connection to the LX pins should be as short as possible. Use several VIA pads when routing between layers. The input capacitor (C1) should connect as closely as possible to IN (Pin 2) and AGND (Pins 4 and 6) to get good power filtering. Keep the switching node, LX (Pins 7 and 8), away from the sensitive FB/OUT node. The feedback trace or OUT pin (Pin 2) 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. If external feedback resistors are used, they should be placed as closely as possible to the FB pin (Pin 5) to minimize the length of the high impedance feedback trace. The output capacitor C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible and there should not be any signal lines under the inductor. 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. Figures 4 and 5 show an example of a layout with 2 layers. PTOTAL = IO2 · RDSON(HS) + 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 θJA for the DFN-10 package which is 45°C/W. TJ(MAX) = PTOTAL · ΘJA + TAMB 1145.2008.01.1.2 www.analogictech.com 13 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Manufacturer Part Number Inductance (μH) Max DC Current (A) DCR (mΩ) Size (mm) LxWxH Type Sumida Sumida Sumida Coiltronics Coiltronics CDRH3D16 CDRH4D15 CDRH5D16 SD3118-2R2-R SD3114-2R2-R 2.2 3.3 4.7 2.2 2.2 1.75 1.6 2.15 2.00 1.74 47 71 51.3 74 110 4.0x4.0x1.8 4.7x4.7x1.7 5.8x5.8x1.8 3.1x3.1x1.8 3.1x3.1x1.4 Shielded Shielded Shielded Shielded Shielded Manufacturer Part Number Value Voltage (V) Temp. Co. Case Murata Murata Murata GRM219R60J106KE19 GRM21BR60J226ME39 GRM1551X1E220JZ01B 10μF 22μF 22pF 6.3 6.3 25 X5R X5R JIS 0805 0805 0402 Table 2. Suggested Component Selection Information L1 2.2μH VIN 2.5V-5.5V C1 10μF EN LX IN LX AIN AAT1145 C3 22pF R1 634k FB EP C2 22μF R2 316k PGND AGND AGND VOUT 1.8V,1.5A PGND U1: AAT1145 TDFN33-10 L1: SD3118-2R2-R C1: 10μF 6.3V 0805 X5R C2: 22μF 6.3V 0805 X5R Figure 3: AAT1145 Adjustable Voltage Version Recommended Evaluation Board Schematic. Figure 4: AAT1145 Evaluation Board Top Layer. 14 Figure 5: Exploded View of AAT1145 Evaluation Board Top Layer. www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Step-Down Converter Design Example Specifications VO = 1.8V @1.5A VIN = 2.7V to 4.2V (3.6V nominal) fS = 1.2MHz Transient droop = 100mV ∆VO = 50mV 1.8V Output Inductor ΔIL = 30% ⋅ IO = 0.3 · 1.5 = 450mA L= VOUT · (VIN(MAX) - VOUT) 1.8 · (4.2 - 1.8) = =1.90µH VIN(MAX) ⋅ ΔIL ⋅ fOSC 4.2 ⋅ 0.45 · 1.2 · 106 For Cooper 2.2μH inductor (SD3118-2R2-R) with DCR 74mΩ, the ∆IL should be ΔIL = VO ⎛ VO ⎞ ⋅ 1· T = 341mA L ⎝ VIN ⎠ IPKL = IO + 0.341 ΔIL = 1.5 + = 1.67A 2 2 PL = IO2 ⋅ DCR = 1.52 ⋅ 0.074 = 166.5mW 1.8V Output Capacitor COUT = 2 · ΔILOAD 2 · 1.2 = = 20µF; use 22µF VDROOP · fS 0.1 · 1.2 · 106 ESR ≤ ΔVO 0.05 = = 0.15Ω ΔIL 0.341 Select a 22μF, 10mΩ ESR ceramic capacitor to meet the ripple 50mV requirement. ΔVOUT ≤ = 1 VOUT · (VIN - VOUT) ⎛ ⎞ · ⎝ESR + 8 · fOSC · COUT ⎠ VIN · fOSC · L 1.8 · (4.2 - 1.8) 1 ⎛ ⎞ · ⎝ 0.01 + = 6mV 6 -6 6 -6 ⎠ 4.2 · 1.2 · 10 · 2.2 · 10 8 · 1.2 · 10 · 22 · 10 IRMS = ΔIL ·0.289 = 0.341 · 0.289 = 98.5mArms PCOUT = ESR · IRMS2 = 0.01 · 0.09852 = 97μW 1145.2008.01.1.2 www.analogictech.com 15 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Input Capacitor Input ripple VPP = 50mV CIN(MIN) = IRMS = 1 ⎛ VPP ⎞ - ESR · 4 · fS ⎝ IO ⎠ = 1 ⎛ 0.05 ⎞ - 0.01 · 4 · 1.2 · 106 ⎝ 1.5 ⎠ = 8.9µF; use 10µF IO 1.5 = = 750mArms 2 2 PCIN = ESR · IRMS2 = 0.01 · 0.752 = 5.6mW AAT1145 Losses PTOTAL = IO2 · RDS(ON)P · D + IO2 · RDS(ON)N · (1 - D) + (tSW · fS · IO) · VIN = 1.52 · 0.135 · 16 1.8 1.8⎞ ⎛ + 1.52 · 0.095 · 1 + (5 · 10-9 · 1.2 · 106 · 1.5) · 4.2 = 290mW ⎝ 4.2 4.2⎠ www.analogictech.com 1145.2008.01.1.2 PRODUCT DATASHEET AAT1145 SwitchRegTM 1.5A Step-Down Converter Ordering Information Output Voltage Package Marking1 Part Number (Tape and Reel)2 Adj. 0.6V to VIN Fixed 1.8V TDFN33-10 TDFN33-10 QNXYY WUXYY AAT1145IDE-0.6-T1 AAT1145IDE-1.8-T1 Package Information3 TDFN33-10 Pin 1 dot by marking 0.500 BSC 1.70 ± 0.05 3.00 ± 0.05 0.23 ± 0.05 Pin 1 identification R0.200 0.40 ± 0.05 3.00 ± 0.05 2.40 ± 0.05 Top View 0.05 ± 0.05 0.203 REF 0.75 ± 0.05 Bottom View Side View All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on all 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. Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611 © Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Except as provided in AnalogicTech’s terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer’s applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders. 1145.2008.01.1.2 www.analogictech.com 17