AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter General Description Features The AAT1147 SwitchReg is a member of AnalogicTech's Total Power Management IC™ (TPMIC™) product family. It is a fixed frequency 1.4MHz step-down converter with an input voltage range of 2.7V to 5.5V and output voltage as low as 0.6V. • • • • • • • • The AAT1147 is optimized for low noise portable applications, reacts quickly to load variations, and reaches peak efficiency at heavy load. SwitchReg™ VIN Range: 2.7V to 5.5V VOUT Adjustable from 0.6V to VIN 400mA Output Current Up to 98% Efficiency Low Noise, 1.4MHz Fixed Frequency PWM Operation Fast Load Transient 150µs Soft Start Over-Temperature and Current Limit Protection 100% Duty Cycle Low Dropout Operation <1µA Shutdown Current 8-Pin SC70JW Package Temperature Range: -40°C to +85°C The AAT1147 output voltage is programmable with external feedback resistors. It can deliver 400mA of load current while maintaining high power efficiency. The 1.4MHz switching frequency minimizes the size of external components while keeping switching losses low. • • • • The AAT1147 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. Applications • • • • • • Cellular Phones Digital Cameras Handheld Instruments Microprocessor/DSP Core /IO Power PDAs and Handheld Computers USB devices Typical Application VIN 3 1 C2 4.7μF 5 8 1147.2006.05.1.0 VO = 1.8V U1 AAT1147 VIN LX EN OUT AGND PGND PGND PGND 4 2 L1 4.7μH 118k 7 6 R1 R2 59k C1 4.7μF 1 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Pin Descriptions Pin # Symbol Function 1 EN Enable pin. 2 OUT Feedback input pin. This pin is connected to an external resistive divider for an adjustable output. 3 VIN Input supply voltage for the converter. 4 LX Switching node. Connect the inductor to this pin. It is connected internally 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 OUT VIN LX 2 1 8 2 7 3 6 4 5 PGND PGND PGND AGND 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Absolute Maximum Ratings1 Symbol VIN VLX VOUT VEN TJ TLEAD Description Input Voltage to GND LX to GND OUT 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 0.625 160 W °C/W Thermal Information2 Symbol PD θJA Description Maximum Power Dissipation Thermal Resistance 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. 1147.2006.05.1.0 3 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Electrical Characteristics1 TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C, VIN = 3.6V. Symbol Description Conditions Step-Down Converter VIN Input Voltage VUVLO UVLO Threshold VOUT Output Voltage Tolerance VOUT IQ ISHDN ILIM 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 FOSC TSD THYS Start-Up Time 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 300 1.0 V µA µA mA Ω Ω 1 µA 100 1.8 -3.0 0.6 No Load EN = AGND = PGND 160 600 0.45 0.40 VIN = 5.5V, VLX = 0 to VIN, EN = GND VIN = 2.7V to 5.5V 0.6V Output, No Load, TA = 25°C 0.6V Output From Enable to Output Regulation Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis 0.1 591 600 %/V 609 mV 0.2 µA 150 1.0 1.4 140 µs 2.0 15 MHz °C °C EN VEN(L) VEN(H) IEN Enable Threshold Low Enable Threshold High Input Low Current 0.6 VIN = VOUT = 5.5V 1.4 -1.0 1.0 V V µA 1. The AAT1147 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 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Typical Characteristics Efficiency vs. Load DC Regulation (VOUT = 3.3V; L = 6.8µH) (VOUT = 3.3V) 100 2.0 VIN = 3.6V 1.5 Output Error (%) Efficiency (%) 80 60 VIN = 4.2V 40 VIN = 5.0V 20 1.0 VIN = 4.2V 0.5 0.0 -0.5 VIN = 3.6V -1.0 VIN = 5.0V -1.5 -2.0 0 1 10 100 1000 0.1 1 Output Current (mA) 100 1000 Output Current (mA) Efficiency vs. Load DC Regulation (VOUT = 2.5V; L = 6.8µH) (VOUT = 2.5V) 100 2.0 1.5 Output Error (%) VIN = 3.6V 80 Efficiency (%) 10 60 VIN = 4.2V 40 VIN = 5.0V 20 1.0 VIN = 4.2V 0.5 0.0 -0.5 VIN = 5.0V -1.0 VIN = 3.6V -1.5 0 1 10 100 -2.0 0.1 1000 1 Output Current (mA) 100 1000 Output Current (mA) Efficiency vs. Load DC Regulation (VOUT = 1.8V; L = 4.7µH) (VOUT = 1.8V) 100 2.0 1.5 Output Error (%) VIN = 3.0V 80 Efficiency (%) 10 60 VIN = 3.6V 40 VIN = 4.2V 20 1.0 VIN = 4.2V 0.5 0.0 VIN = 3.6V -0.5 -1.0 VIN = 3.0V -1.5 0 1 10 100 Output Current (mA) 1147.2006.05.1.0 1000 -2.0 0.1 1 10 100 1000 Output Current (mA) 5 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Typical Characteristics Line Regulation Line Regulation (VOUT = 3.3V) 0.5 0.4 0.4 0.3 0.3 IOUT = 10mA IOUT = 1mA 0.2 Accuracy (%) Accuracy (%) (VOUT = 2.5V) 0.5 0.1 0 -0.1 -0.2 IOUT = 400mA -0.3 IOUT = 10mA IOUT = 1mA 0.2 0.1 0.0 -0.1 -0.2 IOUT = 400mA -0.3 -0.4 -0.4 -0.5 -0.5 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 3.0 3.2 3.4 3.6 Input Voltage (V) 3.8 4.0 4.2 4.4 4.6 4.8 5.0 Input Voltage (V) Line Regulation Frequency vs. Input Voltage (VOUT = 1.8V) 0.5 Frequency Variation (%) 2.0 0.4 Accuracy (%) 0.3 0.2 0.1 IOUT = 10mA 0.0 IOUT = 1mA -0.1 -0.2 -0.3 IOUT = 400mA -0.4 -0.5 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 1.0 VOUT = 1.8V 0.0 -1.0 -2.0 VOUT = 2.5V -3.0 -4.0 2.5 5.0 2.9 3.3 4.5 4.9 Output Voltage Error vs. Temperature Switching Frequency vs. Temperature (VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA) (VIN = 3.6V; VOUT = 1.8V) 2.0 15 1.5 12 0.5 0.0 -0.5 -1.0 -2.0 -40 5.3 9 1.0 6 3 0 -3 -6 -9 -1.5 -12 -15 -25 -10 5 20 35 50 Temperature (°°C) 6 4.1 Input Voltage (V) Variation (%) Output Error (%) Input Voltage (V) 3.7 VOUT = 3.3V 65 80 95 -40 -25 -10 5 20 35 50 65 80 95 Temperature (°°C) 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Typical Characteristics Line Transient Response No-Load Quiescent Current vs. Input Voltage (40mA to 400mA; VIN = 3.6V; VOUT = 1.8V; C1 = 4.7µF; CFF = 100pF) 220 190 85°C 180 25°C Output Voltage (top) (V) Supply Current (µA) 200 170 160 -40°C 150 140 130 120 2.0 1.4 1.9 1.2 1.8 1.0 1.7 0.8 1.6 0.6 1.5 0.4 1.4 0.2 400 mA 1.3 1.2 40 mA 1.1 2.5 3.0 3.5 4.0 4.5 5.0 1.0 5.5 1.4 1.9 1.2 1.8 1.0 1.7 0.8 1.6 0.6 1.5 0.4 1.4 0.2 0.0 400mA 40mA 1.1 1.0 -0.2 -0.4 -0.6 5.6 3.6 4.8 3.2 4.0 2.8 3.2 2.4 2.4 2.0 1.6 1.6 0.8 1.2 0.0 0.8 -0.8 0.4 -1.6 0.0 -2.4 -0.4 Time (25µs/div) Time (25µs/div) Output Ripple (VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA) 5.0 1.80 4.8 1.75 4.6 1.70 4.4 1.65 4.2 1.60 4.0 1.55 3.8 1.50 3.6 1.45 3.4 1.40 3.2 1147.2006.05.1.0 40 0.9 20 0.8 0 0.7 -20 0.6 -40 0.5 -60 0.4 -80 0.3 -100 0.2 -120 0.1 Inductor Current (bottom) (A) 5.2 1.85 Input Voltage (bottom) (V) 1.90 Output Voltage (AC Coupled) (top) (mV) Line Response (VOUT = 1.8V @ 400mA) Time (25µs/div) Inductor Current (bottom) (A) 2.0 Enable and Output Voltage (top) (V) (VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA) Load and Inductor Current (bottom) (200mA/div) Output Voltage (top) (V) Soft Start (40mA to 400mA; VIN = 3.6V; VOUT = 1.8V; C1 = 4.7µF) 1.2 -0.4 -0.6 Line Transient Response 1.3 -0.2 Time (25µs/div) Input Voltage (V) Output Voltage (top) (V) 0.0 Load and Inductor Current (bottom) (200mA/div) 210 Time (500ns/div) 7 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Functional Block Diagram VIN OUT Err Amp . DH Voltage Reference EN INPUT LX Logic DL PGND AGND Functional Description The AAT1147 is a high performance 400mA 1.4MHz monolithic step-down converter. It has been designed with the goal of minimizing external component size and optimizing efficiency at heavy load. Apart from the small bypass input capacitor, only a small L-C filter is required at the output. Typically, a 4.7µH 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 resistors and ranges from 0.6V to the input voltage. An additional feed-forward capacitor can also be added to the external feedback to pro- 8 vide improved transient response (see Figure 1). 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 heavy load conditions up to 400mA. The internal error amplifier and compensation provide excellent transient response, load, and line regulation. Soft start eliminates any output voltage overshoot when the enable or the input voltage is applied. 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter 1 2 3 Enable VIN C4 100pF U1 AAT1147 1 VOUT =1.8V R1 2 L1 118k 4.7μH C1 10μF C3 n/a 3 4 R2 59k EN PGND OUT PGND VIN PGND LX AGND 8 7 6 5 C2 4.7μF GND LX GND2 U1 AAT1147 SC70JW-8 L1 CDRH3D16-4R7 C2 4.7μF 10V 0805 X5R C1 10μF 6.3V 0805 X5R Figure 1: Enhanced Transient Response Schematic. Control Loop The AAT1147 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. The error amplifier reference is 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 AAT1147 into a low-power, non-switching state. The total input current during shutdown is less than 1µA. 1147.2006.05.1.0 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 VIN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation. 9 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter 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. The internal slope compensation for the AAT1147 is 0.24A/µsec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.5V output and 4.7µH inductor. m= 0.75 ⋅ VO 0.75 ⋅ 1.5V A = = 0.24 L 4.7μH μsec This is the internal slope compensation. When externally programming the 0.6V version to 2.5V, the calculated inductance is 7.5µH. 0.75 ⋅ VO L= = m =3 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 4.7µH CDRH3D16 series inductor selected from Sumida has a 105mΩ DCR and a 900mA DC current rating. At full load, the inductor DC loss is 17mW which gives a 2.8% loss in efficiency for a 400mA, 1.5V 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 = μsec 0.75 ⋅ VO ≈ 3 A ⋅ VO A 0.24A μsec In this case, a standard 6.8µH value is selected. CIN(MIN) = Table 1 displays inductor values for the AAT1147. 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 0.6V Adjustable With External Feedback ⎛ VPP ⎞ - ESR · FS ⎝ IO ⎠ VO ⎛ V ⎞ 1 · 1 - O = for VIN = 2 · VO VIN ⎝ VIN ⎠ 4 μsec ⋅ 2.5V = 7.5μH A Configuration VO ⎛ V ⎞ · 1- O VIN ⎝ VIN ⎠ 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. Output Voltage Inductor 1V, 1.2V 2.2µH 1.5V, 1.8V 4.7µH 2.5V, 3.3V 6.8µH Table 1: Inductor Values. 10 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter 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 AAT1147. 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 (C2) can be seen in the evaluation board layout in 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. 1147.2006.05.1.0 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 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 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. 11 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Figure 2: AAT1147 Evaluation Board Top Side. Figure 3: Exploded View of Evaluation Board Top Side Layout. Figure 4: AAT1147 Evaluation Board Bottom Side. The maximum output capacitor RMS ripple current is given by: IRMS(MAX) = 1 VOUT · (VIN(MAX) - VOUT) L · FS · VIN(MAX) 2· 3 · 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. Output Resistor Selection The output voltage of the AAT1147 0.6V version can be externally programmed. Resistors R1 and R2 of Figure 5 program the output to regulate at a voltage 12 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. ⎛ VOUT ⎞ ⎛ 1.5V ⎞ R1 = V -1 · R2 = 0.6V - 1 · 59kΩ = 88.5kΩ ⎝ REF ⎠ ⎝ ⎠ 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Thermal Calculations The AAT1147, combined with an external feedforward capacitor (C4 in Figure 1), delivers enhanced transient response for extreme pulsed load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C1 for stability. Ω R2 = 59kΩ Ω R2 = 221kΩ VOUT (V) Ω) R1 (kΩ Ω) 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 75 113 150 187 221 261 301 332 442 464 523 715 1000 There are three types of losses associated with the AAT1147 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 · (RDSON(H) · VO + RDSON(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. Table 2: Resistor Values For Use With 0.6V Step-Down Converter. 1 2 3 Enable VIN U1 AAT1147 1 R1 VOUT C1 10μF 2 118k 3 L1 4.7μH 4 EN PGND OUT PGND VIN PGND LX AGND 8 7 6 5 C2 4.7μF R2 59k GND GND2 LX U1 AAT1147 SC70JW-8 L1 CDRH3D16-4R7 C1 10μF 10V 0805 X5R C2 4.7μF 10V 0805 X5R Figure 5: AAT1147 Evaluation Board Schematic. 1147.2006.05.1.0 13 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to: PTOTAL = IO2 · RDSON(H) + 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 SC70JW-8 package which is 160°C/W. TJ(MAX) = PTOTAL · ΘJA + TAMB 3. 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. External feedback resistors should be placed as closely as possible to the OUT 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. An evaluation board is available with this inductor and is shown in Figure 6. The total solution footprint area is 40mm2. Layout The suggested PCB layout for the AAT1147 is shown in Figures 2, 3, and 4. The following guidelines should be used to help ensure a proper layout. 1. The input capacitor (C2) should connect as closely as possible to VIN (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. Figure 6: Minimum Footprint Evaluation Board Using 2.0mm x 1.6mm x 0.95mm Inductor. 14 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, 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 = 1.4MHz TAMB = 85°C 1.8V Output Inductor L1 = 3 μsec μsec ⋅ VO2 = 3 ⋅ 1.8V = 5.4μH A A (use 4.7µH; see Table 1) For Sumida inductor CDRH3D16, 4.7µH, DCR = 105mΩ. ΔIL1 = ⎛ VO V ⎞ 1.8V 1.8V ⎞ ⎛ ⋅ 1- O = ⋅ 1= 156mA 4.2V ⎠ L1 ⋅ FS ⎝ VIN⎠ 4.7μH ⋅ 1.4MHz ⎝ IPKL1 = IO + ΔIL1 = 0.4A + 0.068A = 0.468A 2 PL1 = IO2 ⋅ DCR = 0.4A2 ⋅ 105mΩ = 17mW 1.8V Output Capacitor VDROOP = 0.1V COUT = 3 · ΔILOAD 3 · 0.3A = = 6.4μF; use 10µF 0.1V · 1.4MHz VDROOP · FS IRMS = (VO) · (VIN(MAX) - VO) 1 1.8V · (4.2V - 1.8V) · = 45mArms = 4.7μH · 1.4MHz · 4.2V · V L1 · F 2· 3 2· 3 S IN(MAX) 1 · Pesr = esr · IRMS2 = 5mΩ · (45mA)2 = 10μW 1147.2006.05.1.0 15 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Input Capacitor Input Ripple VPP = 25mV CIN = IRMS = ⎛ VPP ⎝ IO 1 1 = = 3.11μF; use 4.7μF ⎞ ⎛ 25mV ⎞ - 5mΩ · 4 · 1.4MHz - ESR · 4 · FS ⎠ ⎝ 0.4A ⎠ IO = 0.2Arms 2 P = esr · IRMS2 = 5mΩ · (0.2A)2 = 0.2mW AAT1147 Losses PTOTAL = IO2 · (RDSON(H) · VO + RDSON(L) · [VIN -VO]) VIN + (tsw · FS · IO + IQ) · VIN = 0.42 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V]) 4.2V + (5ns · 1.4MHz · 0.4A + 70μA) · 4.2V = 126mW TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (160°C/W) · 126mW = 105.1°C 16 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, 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) 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 75.0 113 150 187 221 261 301 332 442 464 523 715 1000 2.2 2.2 2.2 2.2 2.2 2.2 4.7 4.7 4.7 4.7 6.8 6.8 6.8 Table 3: Evaluation Board Component Values. Manufacturer Sumida Sumida Sumida MuRata MuRata Coilcraft Coiltronics Coiltronics Coiltronics Part Number Inductance (µH) Max DC Current (A) DCR Ω) (Ω Size (mm) LxWxH Type CDRH3D16-2R2 CDRH3D16-4R7 CDRH3D16-6R8 LQH2MCN4R7M02 LQH32CN4R7M23 LPO3310-472 SD3118-4R7 SD3118-6R8 SDRC10-4R7 2.2 4.7 6.8 4.7 4.7 4.7 4.7 6.8 4.7 1.20 0.90 0.73 0.40 0.45 0.80 0.98 0.82 1.30 0.072 0.105 0.170 0.80 0.20 0.27 0.122 0.175 0.122 3.8x3.8x1.8 3.8x3.8x1.8 3.8x3.8x1.8 2.0x1.6x0.95 2.5x3.2x2.0 3.2x3.2x1.0 3.1x3.1x1.85 3.1x3.1x1.85 5.7x4.4x1.0 Shielded Shielded Shielded Non-Shielded Non-Shielded 1mm Shielded Shielded 1mm Shielded Table 4: Typical Surface Mount Inductors. 1. For reduced quiescent current, R2 and R4 = 221kΩ. 1147.2006.05.1.0 17 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Manufacturer MuRata MuRata MuRata Part Number Value Voltage Temp. Co. Case GRM219R61A475KE19 GRM21BR60J106KE19 GRM21BR60J226ME39 4.7µF 10µF 22µF 10V 6.3V 6.3V X5R X5R X5R 0805 0805 0805 Table 5: Surface Mount Capacitors. 18 1147.2006.05.1.0 AAT1147 High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter Ordering Information Package Marking1 Part Number (Tape and Reel)2 SC70JW-8 SCXYY AAT1147IJS-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. XYY = assembly and date code. 2. 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 1147.2006.05.1.0 19