AAT1149 3MHz Fast Transient 400mA Step-Down Converter General Description Features The AAT1149 SwitchReg is a 3.0MHz step-down converter with an input voltage range of 2.7V to 5.5V and output voltage as low as 1.0V. It is optimized to react quickly to load variations and operate with a tiny 0603 inductor that is only 1mm tall. • • • • • • • • • • • • • • • The AAT1149 output voltage is programmable via external feedback resistors. It can deliver 400mA of load current while maintaining a low 45μA no load quiescent current. The 3.0MHz switching frequency minimizes the size of external components while keeping switching losses low. The AAT1149 maintains high efficiency throughout the operating range, which is critical for portable applications. The AAT1149 is available in a Pb-free, space-saving 2.0x2.1mm SC70JW-8 package and is rated over the -40°C to +85°C temperature range. SwitchReg™ Ultra-Small 0603 Inductor (Height = 1mm) VIN Range: 2.7V to 5.5V VOUT Adjustable from 1.0V to VIN 400mA Max Output Current Up to 98% Efficiency 45μA No Load Quiescent Current 3.0MHz Switching Frequency 70μs Soft Start Fast Load Transient Over-Temperature Protection Current Limit Protection 100% Duty Cycle Low-Dropout Operation <1μA Shutdown Current SC70JW-8 Package Temperature Range: -40°C to +85°C Applications • • • • • • Cellular Phones Digital Cameras Handheld Instruments Microprocessor / DSP Core / IO Power PDAs and Handheld Computers USB Devices Typical Application VIN = 3.6V C2 4.7µF 1149.2006.11.1.0 VOUT = 1.8V U1 AAT1149 L1 1.8µH IN LX EN FB AGND PGND PGND PGND R1 118k R2 59k C1 4.7µF 1 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Pin Descriptions Pin # Symbol Function 1 EN Enable pin. 2 FB Feedback input pin. This pin is connected to an external resistive divider for an adjustable output. 3 IN Input supply voltage for the converter. 4 LX Switching node. Connect the inductor to this pin. It is internally connected to the drain of both high- and low-side MOSFETs. 5 AGND Non-power signal ground pin. 6, 7, 8 PGND Main power ground return pins. Connect to the output and input capacitor return. Pin Configuration SC70JW-8 (Top View) EN FB IN LX 2 1 8 2 7 3 6 4 5 PGND PGND PGND AGND 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Absolute Maximum Ratings1 Symbol VIN VLX VFB VEN TJ TLEAD Description Input Voltage to GND LX to GND FB to GND EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) Value Units 6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to 6.0 -40 to 150 300 V V V V °C °C Value Units 625 160 mW °C/W Thermal Information Symbol PD θJA Description Maximum Power Dissipation Thermal Resistance2 2, 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 6.25mW/°C above 25°C. 1149.2006.11.1.0 3 AAT1149 3MHz Fast Transient 400mA 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 Conditions Step-Down Converter VIN Input Voltage VUVLO UVLO Threshold VOUT Output Voltage Tolerance VOUT IQ ISHDN ILIM Adjustable Output Voltage Range Quiescent Current Shutdown Current P-Channel Current Limit High Side Switch On Resistance Low Side Switch On Resistance RDS(ON)H RDS(ON)L ILXLEAK ΔVLinereg LX Leakage Current Line Regulation VOUT Out Threshold Voltage Accuracy IOUT Out Leakage Current TS Start-Up Time FOSC TSD THYS Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis VEN(L) VEN(H) IEN Enable Threshold Low Enable Threshold High Input Low Current Min Typ 2.7 VIN Rising Hysteresis VIN Falling IOUT = 0 to 400mA, VIN = 2.7V to 5.5V Max Units 5.5 2.7 V V mV V 3.0 % VIN 70 1.0 V μA μA mA Ω Ω 1 μA 100 1.8 -3.0 1.0 No Load VEN = GND 45 600 0.45 0.40 VIN = 5.5V, VLX = 0 to VIN, VEN = GND VIN = 2.7V to 5.5V 0.6V Output, No Load TA = 25°C 0.6V Output From Enable to Output Regulation TA = 25°C 0.1 591 600 %/V 609 mV 0.2 μA 70 μs 3.0 140 15 MHz °C °C EN 0.6 VIN = VOUT = 5.5V 1.4 -1.0 1.0 V V μA 1. The AAT1149 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 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics Efficiency vs. Load Current Load Regulation (VOUT = 3V; L = 3µH) (VOUT = 3V; L = 3µH) 1.00 100 VIN = 3.3V 0.75 Output Error (%) Efficiency (%) 90 80 VIN = 4.2V 70 VIN = 5V 60 VIN = 4.2V 0.50 VIN = 5V 0.25 0.00 -0.25 -0.50 VIN = 3.3V -0.75 50 -1.00 0.1 1 10 100 0.1 1000 1 Load Current (mA) Load Regulation (VOUT = 1.8V; L = 2.2µH) (VOUT = 1.8V; L = 2.2µH) VIN = 3V 1.00 VIN = 2.7V 0.75 VIN = 3.6V 80 VIN = 5V 70 VIN = 4.2V 60 1000 Load Current (mA) Output Error (%) Efficiency (%) 100 Efficiency vs. Load Current 100 90 10 0.50 VIN = 3V 0.25 VIN = 4.2V 0.00 -0.25 VIN = 5V VIN = 3.6V -0.50 VIN = 2.7V -0.75 50 0.1 1 10 100 -1.00 0.1 1000 1 Load Current (mA) 85°C Frequency Variation (%) Supply Current (µA) 2 25°C 50 40 -40°C 20 10 0 2.5 1000 Switching Frequency vs. Input Voltage 70 30 100 Load Current (mA) No Load Quiescent Current vs. Input Voltage 60 10 1 VOUT = 1.1V 0 -1 -2 VOUT = 1.8V -3 VOUT = 3V -4 3 3.5 4 4.5 Input Voltage (V) 1149.2006.11.1.0 5 5.5 6 2.5 3 3.5 4 4.5 5 5.5 Input Voltage (V) 5 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics Switching Frequency Variation vs. Temperature Output Voltage Error vs. Temperature (VIN = 3.6V; VO = 1.8V; IOUT = 400mA) 2.0 10 8 Output Error (%) Variation (%) 6 4 2 0 -2 -4 -6 1.0 0.0 -1.0 -8 -2.0 -40 -10 -40 -20 0 20 40 60 80 100 120 -20 0 Line Regulation (VOUT = 3V) (VOUT = 1.8V) 1 1 0.8 0.6 1mA 0.4 0.2 Accuracy (%) Accuracy (%) 0.6 400mA 300mA -0.4 100mA 0mA 600mA -0.6 100 0mA 0.2 100mA 0 -0.2 -0.4 600mA -0.8 -1 -1 2.5 3 3.5 4 4.5 5 2.5 5.5 Input Voltage (V) 3 3.5 4 4.5 5 5.5 6 Input Voltage (V) Line Regulation Line Transient (VOUT = 1.1V) (VOUT = 1.8; 400mA Load; No Feedforward Capacitor) 1.90 4.25 1.88 4.00 1.86 3.75 1.84 3.50 1.82 3.25 1.80 3.00 1.78 2.75 1.76 2.50 1.74 Input Voltage (top) (V) 4.50 0.4 1mA 0mA 0.2 0 -0.2 400mA 600mA -0.4 -0.6 -0.8 -1 2.5 3 3.5 4 4.5 Input Voltage (V) 5 5.5 Output Voltage (bottom) (V) 1 0.8 0.6 Accuracy (%) 80 400mA 0.4 -0.6 -0.8 6 60 Line Regulation 0.8 -0.2 40 Temperature (°°C) Temperature (°°C) 0 20 6 Time (50µs/div) 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics N-Channel RDS(ON) vs. Input Voltage P-Channel RDS(ON) vs. Input Voltage 750 750 700 700 120°C 600 650 100°C RDS(ON) (mΩ Ω) RDS(ON) (mΩ Ω) 650 550 500 85°C 450 400 85°C 500 450 25°C 350 300 300 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.5 3.0 3.5 4.0 4.5 5.0 Line Transient (VOUT = 1.8; CFF = 100pF) 1.94 4.25 1.92 4.00 1.90 3.75 1.88 3.50 1.86 3.25 1.84 3.00 1.82 2.75 1.80 2.50 1.78 1.86 4.50 1.85 4.25 1.84 4.00 1.83 3.75 1.82 3.50 1.81 3.25 1.80 3.00 1.79 2.75 1.78 2.50 Time (50µs/div) Time (20µs/div) Load Transient (VOUT = 1.1V; CFF = 100pF) 1.75 1.10 1.50 400mA 1.25 0.90 1.00 1mA 0.75 0.70 0.50 0.60 0.25 0.50 0.00 Time (50µs/div) 1149.2006.11.1.0 1.30 2.00 1.20 1.75 1.10 1.00 1.50 400mA 0.90 1.25 1mA 1.00 0.80 0.75 0.70 0.50 0.60 0.25 0.50 0.00 Load and Inductor Current (bottom) (A) 2.00 1.20 Load and Inductor Current (bottom) (A) 1.30 Output Voltage (top) (V) Load Transient (VOUT = 1.1V; No Feedforward Capacitor) 0.80 6.0 Input Voltage (top) (V) 4.50 Output Voltage (bottom) (V) Line Transient (VOUT = 1.8; No Load; No Feedforward Capacitor) 1.00 5.5 Input Voltage (V) Output Voltage (bottom) (V) Input Voltage (top) (V) Input Voltage (V) Output Voltage (top) (V) 100°C 550 400 25°C 350 120°C 600 Time (50µs/div) 7 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Load Transient (VOUT = 1.8V; CFF = 100pF) 2.00 2.00 1.75 1.50 1.75 400mA 1.50 1.25 1.00 1.25 10mA 1.00 0.75 0.75 0.50 0.25 0.25 0.00 2.00 2.00 1.90 1.75 1.80 1.50 1.70 1.60 0.50 1.30 0.25 1.20 0.00 1.75 1.75 1.50 400mA 1.25 1.25 1mA 1.00 1.00 0.75 0.75 0.50 0.50 0.25 0.25 0.00 2.00 2.00 1.90 1.75 1.50 1.80 400mA 1.70 1.25 1.00 1.60 1mA 1.50 0.50 1.30 0.25 1.20 0.00 Time (50µs/div) Time (50µs/div) Soft Start (VOUT = 1.8V; CFF = 100pF) 1.75 2.00 1.50 1.00 1.25 0.00 1.00 -1.00 0.75 -2.00 0.50 -3.00 0.25 -4.00 0.00 2.50 2.00 2.00 1.75 1.50 1.50 1.00 1.25 0.50 1.00 0.00 0.75 -0.50 0.50 -1.00 0.25 -1.50 0.00 Inductor Current (bottom) (250mA/div) 2.00 3.00 Inductor Current (bottom) (250mA/div) 4.00 Enable and Output Voltage (top) (V) Soft Start (VOUT = 1.8V; No Feedforward Capacitor) Time (50µs/div) 0.75 1.40 Load and Inductor Current (bottom) (A) 2.00 2.00 Output Voltage (top) (V) Load Transient (VOUT = 1.8V; CFF = 100pF) Load and Inductor Current (bottom) (A) Output Voltage (top) (V) Load Transient (VOUT = 1.8V; No Feedforward Capacitor) 1.50 0.75 1.40 Time (50µs/div) 2.25 Enable and Output Voltage (top) (V) 1.00 10mA 1.50 Time (50µs/div) 8 1.25 400mA Load and Inductor Current (bottom) (A) 2.25 Output Voltage (top) (V) Load Transient (VOUT = 1.8V; No Feedforward Capacitor) Load and Inductor Current (bottom) (A) Output Voltage (top) (V) Typical Characteristics Time (50µs/div) 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Soft Start (VOUT = 1.1V; No Feedforward Capacitor) 2.00 3.00 1.75 2.00 1.50 1.00 1.25 0.00 1.00 -1.00 0.75 -2.00 0.50 -3.00 0.25 -4.00 0.00 Time (50µs/div) 1149.2006.11.1.0 3.50 1.25 3.00 1.00 2.50 0.75 2.00 0.50 1.50 0.25 1.00 0.00 0.50 -0.25 0.00 -0.50 -0.50 -0.75 Inductor Current (bottom) (250mA/div) 4.00 Enable and Output Voltage (top) (V) Soft Start (VOUT = 3V; No Feedforward Capacitor) Inductor Current (bottom) (250mA/div) Enable and Output Voltage (top) (V) Typical Characteristics Time (20µs/div) 9 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Functional Block Diagram IN FB Err Amp . DH Voltage Reference EN INPUT LX Logic DL PGND AGND Functional Description The AAT1149 is a high performance 400mA 3.0MHz monolithic step-down converter. It minimizes external component size, enabling the use of a tiny 0603 inductor that is only 1mm tall, and optimizes efficiency over the complete load range. Apart from the small bypass input capacitor, only a small L-C filter is required at the output. Typically, a 1.8μH inductor and a 4.7μF ceramic capacitor are recommended (see table of values). Only three external power components (CIN, COUT, and L) are required. Output voltage is programmed with external feedback resistors, ranging from 1.0V to the input voltage. An additional feed-forward 10 capacitor can also be added to the external feedback to provide improved transient response (see Figure 4). 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 highside MOSFET. The input voltage range is 2.7V to 5.5V. The converter efficiency has been optimized for all load conditions, ranging from no load to 400mA. The internal error amplifier and compensation provides excellent transient response, load, and line regulation. Soft start eliminates any output voltage overshoot when the enable or the input voltage is applied. 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Control Loop The AAT1149 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 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. 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 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. When pulled low, the enable input forces the AAT1149 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 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. 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. Under-Voltage Lockout Internal bias of all circuits is controlled via the IN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation. 1149.2006.11.1.0 Applications Information 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. Table 1 displays suggested inductor values for various output voltages. 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 1.8μH CDRH2D09 series inductor selected from Sumida has a 131mΩ DCR and a 400mA saturation current rating. At full load, the inductor DC loss is 21mW which gives a 2.8% loss in efficiency for a 400mA, 1.8V output. Input Capacitor Select a 4.7μF to 10μF 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 = VO ⎛ V ⎞ · 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 ⎠ 11 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Configuration Output Voltage Typical Inductor Value 0.6V Adjustable With External Feedback 1V, 1.2V 1.5V, 1.8V 2.5V 3.3V 1.0μH to 1.2μH 1.5μH to 1.8μH 2.2μH to 2.7μH 3.3μH Table 1: Inductor Values. 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 AAT1149. 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 12 high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C2) can be seen in the evaluation board layout in Figure 1. 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 The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7μF to 10μF 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 out- 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter put 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: value for the output capacitor with respect to load transients. The internal voltage loop compensation also limits the minimum output capacitor value to 4.7μF. 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. The maximum output capacitor RMS ripple current is given by: 3 · ΔILOAD COUT = 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 Figure 1: AAT1149 Evaluation Board Top Side. IRMS(MAX) = 1 VOUT · (VIN(MAX) - VOUT) L · FS · VIN(MAX) 2· 3 · Figure 2: Exploded View of Evaluation Board Top Side. Figure 3: AAT1149 Evaluation Board Bottom Side. 1149.2006.11.1.0 13 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Dissipation due to the RMS current in the ceramic output capacitor ESR is typically minimal, resulting in less than a few degrees rise in hot-spot temperature. The AAT1149, combined with an external feedforward capacitor (C3 in Figure 4), delivers enhanced transient response for extreme pulsed load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C1 for stability. Feedback Resistor Selection Resistors R1 and R2 of Figure 4 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 2 summarizes the resistor values for various output voltages with R2 set to either 59kΩ for good noise immunity or 221kΩ for reduced no load input current. Ω R2 = 59kΩ Ω R2 = 221kΩ VOUT (V) Ω) R1 (kΩ R1 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.3 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 267 113K 150K 187K 221K 261K 301K 332K 442K 464K 523K 715K 1.00M ⎛ VOUT ⎞ ⎛ 1.5V ⎞ R1 = V -1 · R2 = 0.6V - 1 · 59kΩ = 88.5kΩ ⎝ REF ⎠ ⎝ ⎠ Table 2: Feedback Resistor Values. VIN U1 AAT1149 C3 1 R1 2 3 VOUT L1 C1 4.7μF 1 2 3 4 EN PGND OUT PGND IN PGND LX AGND 8 Enable 7 6 5 C2 R2 59k 4.7μF GND GND LX Figure 4: AAT1149 Evaluation Board Schematic. 14 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Thermal Calculations Layout There are three types of losses associated with the AAT1149 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: The suggested PCB layout for the AAT1149 is shown in Figures 1, 2, and 3. The following guidelines should be used to help ensure a proper layout. PTOTAL = IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO]) VIN + (tsw · FS · IO + IQ) · VIN IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load stepdown converter switching losses. For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to: PTOTAL = IO2 · RDS(ON)H + IQ · VIN 1. The input capacitor (C2) should connect as closely as possible to IN (Pin 3) and PGND (Pins 6-8). 2. C1 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible. 3. The feedback trace or FB 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 2) to minimize the length of the high impedance feedback trace. 4. The resistance of the trace from the load return to the PGND (Pins 6-8) 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. A high density, small footprint layout can be achieved using an inexpensive, miniature, nonshielded, high DCR inductor, as shown in Figure 5. 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 SC70JW-8 package which is 160°C/W. TJ(MAX) = PTOTAL · ΘJA + TAMB Figure 5: Minimum Footprint Evaluation Board Using 2.0x1.25x1.0mm Inductor. 1149.2006.11.1.0 15 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Step-Down Converter Design Example Specifications VO = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ΔILOAD = 300mA VIN = 2.7V to 4.2V (3.6V nominal) FS = 3.0MHz TAMB = 85°C 1.8V Output Inductor L1 = 1 μsec μsec ⋅ VO = 1 ⋅ 1.8V = 1.8μH A A (use 2.2μH; see Table 1) For Taiyo Yuden inductor CBC2518T2R2M, 2.2μH, DCR = 130mΩ. ΔIL1 = ⎛ VO V ⎞ 1.8V 1.8V⎞ ⎛ ⋅ 1- O = ⋅ ⎝1 = 156mA VIN ⎠ 2.2μH ⋅ 3.0MHz 4.2V⎠ L1 ⋅ FS ⎝ IPKL1 = IO + ΔIL1 = 0.4A + 0.078A = 0.478A 2 PL1 = IO2 ⋅ DCR = 0.4A2 ⋅ 130mΩ = 21mW 1.8V Output Capacitor VDROOP = 0.1V COUT = IRMS = 3 · ΔILOAD 3 · 0.3A = = 3.0μF; use 4.7µF 0.1V · 3.0MHz VDROOP · FS 1 2· 3 · (VO) · (VIN(MAX) - VO) 1 1.8V · (4.2V - 1.8V) · = 45mArms = L1 · FS · VIN(MAX) 2 · 3 2.2μH · 3.0MHz · 4.2V Pesr = esr · IRMS2 = 5mΩ · (45mA)2 = 10μW 16 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Input Capacitor Input Ripple VPP = 25mV CIN = IRMS = ⎛ VPP ⎝ IO 1 1 = = 1.45μF; use 2.2μF ⎞ ⎛ 25mV ⎞ - 5mΩ · 4 · 3.0MHz - ESR · 4 · FS ⎠ ⎝ 0.4A ⎠ IO = 0.2Arms 2 P = esr · IRMS2 = 5mΩ · (0.2A)2 = 0.2mW AAT1149 Losses PTOTAL = IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN -VO]) VIN + (tsw · FS · IO + IQ) · VIN = 0.42 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V]) 4.2V + (5ns · 3MHz · 0.4A + 70μA) · 4.2V = 140mW TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (160°C/W) · 140mW = 107°C 1149.2006.11.1.0 17 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Adjustable Version (0.6V device) Ω R2 = 59kΩ Ω1 R2 = 221kΩ VOUT (V) Ω) R1 (kΩ Ω) R1 (kΩ L1 (μH) 1.0 1.2 1.5 1.8 2.5 3.3 39.2 59.0 88.7 118 187 267 150 221 332 442 715 1000 1.0 1.2 1.5 1.8 2.2 3.3 Table 3: Evaluation Board Component Values. Manufacturer Part Number/ Type BRC1608 Taiyo Yuden BRL2012 CBC2518 Wire Wound Chip Sumida CDRH2D09 Shielded Murata LQH2MCN4R7M02 Unshielded Coiltronics SD3118 Shielded Inductance (μH) Rated Current (mA) DCR Ω) (Ω 0.77 1.0 1.5 1.5 2.2 3.3 1.0 2.2 1.2 1.5 1.8 2.5 3.0 1.0 1.5 2.2 3.3 0.68 0.82 1.2 1.5 2.2 3.3 660 520 410 600 550 450 1000 890 590 520 480 440 400 485 445 425 375 980 830 720 630 510 430 110 180 300 200 250 350 80 130 97.5 110 131 150 195 300 400 480 600 31 54 75 104 116 139 Size (mm) LxWxH 0603 (HMAX = 1mm) 0805 (HMAX = 1mm) 2.5x1.8x1.8 3.2x3.2x1.0 2.0x1.6x0.95 3.15x3.15x1.2 Table 4: Typical Surface Mount Inductors. 1. For reduced quiescent current, R2 = 221kΩ. 18 1149.2006.11.1.0 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Manufacturer Murata Murata Murata Part Number Value Voltage Temp. Co. Case GRM219R61A475KE19 GRM21BR60J106KE19 GRM185R60J475M 4.7μF 10μF 4.7μF 10V 6.3V 6.3V X5R X5R X58 0805 0805 0603 Table 5: Surface Mount Capacitors. 1149.2006.11.1.0 19 AAT1149 3MHz Fast Transient 400mA Step-Down Converter Ordering Information Output Voltage1 Package Marking2 Part Number (Tape and Reel)3 0.6; Adj ≥ 1.0 SC70JW-8 RGXYY AAT1149IJS-0.6-T1 All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/pbfree. Package Information SC70JW-8 2.20 ± 0.20 1.75 ± 0.10 0.50 BSC 0.50 BSC 0.50 BSC 0.225 ± 0.075 2.00 ± 0.20 0.100 7° ± 3° 0.45 ± 0.10 4° ± 4° 0.05 ± 0.05 0.15 ± 0.05 1.10 MAX 0.85 ± 0.15 0.048REF 2.10 ± 0.30 All dimensions in millimeters. 1. Contact Sales for other voltage options. 2. XYY = assembly and date code. 3. Sample stock is generally held on part numbers listed in BOLD. © 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. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech’s standard warranty. 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. Advanced Analogic Technologies, Inc. 830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 20 1149.2006.11.1.0