AAT1155

AAT1155 1MHz 2.5A Step-Down DC/DC Converter General Description The AAT1155 SwitchReg™ is a member of AnalogicTech's Total Power Management IC™ (TPMIC™) product family. The step-down switching converter is ideal for applications where high efficiency, small size, and low ripple are critical. Able to deliver 2.5A with an internal power MOSFET, the current-mode controlled IC provides high efficiency. Fully internally compensated, the AAT1155 simplifies system design and lowers external parts count. The AAT1155 is available in a Pb-free MSOP-8 package and is rated over the -40°C to +85°C temperature range. Features • • • • • • • • • • • • • • SwitchReg™ 5.5V Max Supply Input Fixed or Adjustable VOUT: 1.0V to 4.2V 2.5A Output Current Up to 95% Efficiency Integrated Low On Resistance Power Switches Internally Compensated Current Mode Control 1MHz Switching Frequency Constant PWM Mode Low Output Ripple With Light Load Internal Soft Start Current Limit Protection Over-Temperature Protection MSOP-8 Package -40°C to +85°C Temperature Range Applications • • • • • Cable/DSL Modems Computer Peripherals High Efficiency Conversion from 5V or 3.3V Supply Network Cards Set-Top Boxes Typical Application INPUT 10µF VP FB AAT1155 ENABLE 100Ω VCC LX LX 1.5µH OUTPUT GND 0.1µF 120µF 1155.2005.11.1.6 1 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Pin Descriptions Pin # 1 2 3 4 5, 8 6, 7 Symbol FB GND EN VCC VP LX Function Feedback input pin. Signal ground. Converter enable pin. Small signal filtered bias supply. Input supply for converter power stage. Inductor connection pin. Pin Configuration MSOP-8 FB GND EN VCC 1 8 VP LX LX VP 1 2 2 7 3 6 4 5 2 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Absolute Maximum Ratings1 TA = 25°C, unless otherwise noted. Symbol VCC, VP VLX VFB VEN TJ TLEAD VESD Description VCC, VP to GND LX to GND FB to GND EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) ESD Rating2 - HBM Value 6 -0.3 to VP+0.3 -0.3 to VCC+0.3 -0.3 to VCC+0.3 -40 to 150 300 3000 Units V V V V °C °C V Thermal Characteristics3 Symbol ΘJA PD Description Maximum Thermal Resistance Maximum Power Dissipation Value 150 833 Units °C/W mW Recommended Operating Conditions Symbol T Description Ambient Temperature Range Rating -40 to +85 Units °C 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. Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. 3. Mounted on a demo board (FR4, in still air). 1155.2005.11.1.6 3 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Electrical Characteristics VIN = VCC = VP = 5V, TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C. Symbol VIN VOUT VIL VIH VUVLO VUVLO(HYS) IQ ISHDN ILIM RDS(ON)L η ∆VOUT (VOUT*∆VIN) ∆VOUT/VOUT FOSC TSD THYS Description Input Voltage Range Output Voltage Tolerance Input Low Voltage Input High Voltage Under-Voltage Lockout Under-Voltage Lockout Hysteresis Quiescent Supply Current Shutdown Current Current Limit High Side Switch On Resistance Efficiency Load Regulation Line Regulation Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis Conditions VIN = VOUT + 0.2 to 5.5V, IOUT = 0.5A Min Typ Max Units 2.7 -2.5 5.5 2.5 0.6 1.4 V % V V V mV µA µA A mΩ % % %/V MHz °C °C VIN Rising VIN Falling No Load, VFB = 0V VEN = 0V, VIN = 5.5V TA = 25°C TA = 25°C IOUT = 1.0A ILOAD = 0A to 2.5A VIN = 2.7V to 5.5V TA = 25°C 2.5 1.2 250 630 4.4 60 92 ±2.3 0.75 1 140 15 1000 1.0 4 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics RDS(ON) vs. Temperature 90 Oscillator Frequency Variation vs. Input Voltage (VOUT = 3.3V) 0.5 2.7V 80 3.6V 4.2V Variation (%) 120 RDS(ON) (mΩ) 0.25 70 60 0 5.0V 50 40 -20 0 20 40 60 80 5.5V -0.25 100 -0.5 3.5 4 4.5 5 5.5 Temperature (°C) Input Voltage (V) RDS(ON) vs. Input Voltage (IDS = 1A) 80 Oscillator Frequency Variation vs. Temperature (VIN = 5V) 1 75 RDS(ON) (mΩ) 70 65 60 55 50 45 40 2.5 3 3.5 4 4.5 5 5.5 0 Variation (%) -1 -2 -3 -4 -20 0 20 40 60 80 100 Input Voltage (V) Temperature (°C) Enable Threshold vs. Input Voltage 1.2 Output Voltage Variation vs. Temperature (IOUT = 2A; VO = 3.3V) 0.4 Enable Threshold (V) 1.1 1 0.9 0.8 0.7 0.6 2.5 3 3.5 4 4.5 5 5.5 Variation (%) EN(H) 0.2 0 -0.2 -0.4 -0.6 -0.8 -20 0 20 40 60 80 100 EN(L) Input Voltage (V) Temperature (°C) 1155.2005.11.1.6 5 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics Line Regulation (VOUT = 3.3V) Output Voltage Error (%) 1 AAT1155 Evaluation Board Over-Temperature Current vs. Input Voltage (VOUT = 3.3V) 3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2 1.8 1.6 Output Current (A) 0 IO = 0.3A -1 -2 -3 -4 -5 3.4 55°C 70°C 85°C 100°C 3.5 3.75 4 4.25 4.5 4.75 5 5.25 5.5 IO = 3.0A 3.7 4 4.3 4.6 4.9 5.2 5.5 Input Voltage (V) Input Voltage (V) Load Regulation (VIN = 5.0V; VOUT = 3.3V) Operating Current (mA) 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0 -7.0 -8.0 -9.0 -10.0 0.01 0.10 1.00 10.00 750 700 650 600 550 500 450 -20 0 Non-Switching Operating Current vs. Temperature (FB = 0V) Output Error (%) 2.7V 3.6V 4.2V 5.0V 5.5V 20 40 60 80 100 120 Load Current (A) Temperature (°C) (VOUT = 3.3V; VIN = 5.0V) 5 4.5 4 3.5 3 2.5 2 -20 -10 0 10 20 30 40 50 60 70 80 90 100 Enalbe and Output Voltage (V) (top trace) Over-Temperature Shutdown Current vs. Temperature Inrush and Output Overshoot Characteristics 8 6 4 2 0 -2 -4 -6 -8 0 0.4 0.8 1.2 1.6 2 14 Inductor Current (A) (bottom trace) 12 10 8 6 4 2 0 -2 Output Current (A) Temperature (°C) Time (ms) 6 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics Tantalum Output Ripple (IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V) 16 0.04 0.02 7 6 12 8 L = 1.5µHy 180 135 90 Phase 4 0 -4 -8 -12 300µF gain 100µF 6.3 Ceramic TDK P/N C3225X5R0J107M Vishay GRM43SR60J107ME20L 200µF gain 45 0 -45 -90 -135 -180 100000 Loop Crossover Gain and Phase Phase (Degrees) Inductor Current (bottom) (A) Output Ripple (top) (mV) 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 0 1 2 3 4 5 120µF 6.3V Tantalum Vishay P/N 594D127X96R3C2T 5 4 3 2 1 0 -1 Gain (dB) -16 10000 Time (µs) Frequency (Hz) Output Ripple (IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V) 16 4 2 7 6 8 12 Loop Crossover Gain and Phase 180 135 90 AC Output Ripple (top) (mV) Inductor Current (bottom) (A) Phase (Degrees) 0 -2 -4 -6 200µF 6.3V Ceramic -10 TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L -12 0 1 2 3 -8 5 4 3 2 1 0 -1 4 5 Gain (dB) 4 0 -4 -8 -12 Gain 45 0 -45 -90 120µF 6.3V Tantalum Vishay P/N 594D127X96R3C2T 100000 -135 -180 1000000 Time (µs) -16 10000 Frequency (Hz) Output Ripple (IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V) 4 7 6 5 4 3 2 300µF 6.3VCeramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L 0 1 2 3 4 5 1 0 -1 3.40 3.30 Tantalum Transient Response (IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V) 7 6 5 4 3 2 1 120µF 6.3V Tantalum Vishay P/N 594D127X96R3C2T 0 100 200 300 400 500 0 -1 AC Output Ripple (top) (mV) 2 0 -2 -4 -6 -8 Inductor Current (bottom) (A) Output Voltage (top) (mV) Inductor Current (bottom) (A) 3.20 3.10 3.00 2.90 2.80 2.70 2.60 -10 -12 Time (µs) Time (µs) 1155.2005.11.1.6 7 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics Transient Response (IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V) 3.40 3.30 7 6 2x 100µF 6.3V Ceramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L Transient Response (IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V) 3.40 3.30 7 6 3x 100µF 6.3V Ceramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L Inductor Current (bottom) (A) Inductor Current (bottom) (A) Output Voltage (top) (mV) 3.20 3.10 3.00 2.90 2.80 2.70 2.60 0 100 200 Output Voltage (top) (mV) 5 4 3 2 1 0 -1 3.20 3.10 3.00 2.90 2.80 2.70 2.60 0 100 200 5 4 3 2 1 0 -1 300 400 500 300 400 500 Time (µs) Time (µs) 8 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Functional Block Diagram VCC VP = 2.5V to 5.5V REF FB OP. AMP CMP DH LOGIC LX OSC Temp. Sensing EN Applications Information Main Control Loop The AAT1155 is a peak current mode step-down converter. The inner wide bandwidth loop controls the inductor peak current. The inductor current is sensed as it flows through the internal P-channel MOSFET. A fixed slope compensation signal is then added to the sensed current to maintain stability for duty cycles greater than 50%. The inner loop appears as a voltage-programmed current source in parallel with the output capacitor. The voltage error amplifier output programs the current loop for the necessary inductor current to force a constant output voltage for all load and line conditions. The feedback resistive divider is internal, dividing the output voltage to the error amplifier reference voltage of 1.0V. The error amplifier has a limited DC gain. This eliminates the need for external compensation components, while still providing sufficient DC loop gain for good load regulation. The crossover frequency and phase margin are set by the output capacitor value. Duty cycle extends to 100% as the input voltage approaches the output voltage. Thermal shutdown protection disables the device in the event of a short-circuit or overload condition. Soft Start/Enable Soft start controls the current limit when the input voltage or enable is applied. It limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the device into a low-power, non-switching state. The total input current during shutdown is less than 1µA. Power and Signal Source Separate small signal ground and power supply pins isolate the internal control circuitry from switching noise. In addition, the low pass filter R1 and C3 (shown in Figure 1) filters noise associated with the power switching. 1155.2005.11.1.6 9 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Vin 2.7V-5.5V VOUT 1.25V @ 2.5A R3 2.55k R1 100 C4 100µF R2 100k U1 AAT1155-1.0 FB VP L1 1.5µH GND LX EN C1 10µF C3 0.1µF R4 10.0k LX VCC VP D1 B340LA C2 120µF rtn C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3 C2 Vishay 120µF 6.3V 594D127X96R6R3C2T C3 0. µF 0603ZD104M AVX 1 C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V D1 B340LA Diodes Inc. L1 CDRH6D28-1.5µH Sumida Options C2 MuRata 100uF 6.3V GRM43-2 X5R 107M 100µF 6.3V (two or three in parallel) C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel) Figure 1: AAT1155 Evaluation Board with Adjustable Output. Current Limit and Over-Temperature Protection Over-temperature and current limit circuitry protects the AAT1155 and the external Schottky diode during overload, short-circuit, and excessive ambient temperature conditions. The junction over-temperature threshold is 140°C nominal and has 15°C of hysteresis. Typical graphs of the over-temperature load current vs. input voltage and ambient temperature are shown in the Typical Characteristics section of this document. peak ripple current is 1A. Assuming a 5V ±5% input voltage and 40% ripple, the output inductance required is: VOUT VOUT ⎞ ⎛ · k · FSW · ⎝1 - VIN(MAX)⎠ OUT L =I = 3.3V ⎞ ⎛ ⎞ ⎛ · 1 - 3.3V ⎝ 2.5A · 0.4 · 1MHz ⎠ ⎝ 5.25V⎠ = 1.23µH The factor "k" is the fraction of the full load (40%) selected for the ripple current at the maximum input voltage. The corresponding inductor RMS current is: ⎛ 2 ∆I 2 ⎞ I+ ≈ I O = 2.5A ⎝O 12 ⎠ Inductor The output inductor is selected to limit the ripple current to 20% to 40% of the full load current at the maximum input voltage. 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 inductor saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. During overload and short-circuit conditions, the inductor can exceed its peak current rating without affecting the converter performance. Some inductors may have sufficient peak and average current ratings yet result in excessive losses due to a high DCR. The losses associated with the DCR and its effect on the total converter efficiency must be considered. For a 2.5A load and the ripple current set to 40% at the maximum input voltage, the maximum peak-to10 IRMS = ∆I is the peak-to-peak ripple current which is fixed by the inductor selection above. For a peak-topeak current of 40% of the full load current, the peak current at full load will be 120% of the full load. The 1.5µH inductor selected from the Sumida CDRH6D38 series has a 11mΩ DCR and a 4.0A DC current rating with a height of 4mm. At full load, the inductor DC loss is 70mW for a 0.84% loss in efficiency. 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Schottky Freewheeling Diode The Schottky average current is the load current multiplied by one minus the duty cycle. ⎛ VO ⎞ 1⎝ VIN ⎠ 3A Surface Mount Schottky Diodes Diodes Inc. ROHM Micro Semi B340LA RB050L-40 5820SM 0.45V @ 3A 0.45 @ 3A 0.46V @ 3A Input Capacitor Selection The primary function of the input capacitor is to provide a low impedance loop for the edges of pulsed current drawn by the AAT1155. A low ESR/ESL ceramic capacitor is ideal for this function. To minimize stray inductance, the capacitor should be placed as closely as possible to the IC. This also keeps the high frequency content of the input current localized, minimizing the radiated and conducted EMI while facilitating optimum performance of the AAT1155. Proper placement of the input capacitor C1 is shown in the layout in Figure 2. Ceramic X5R or X7R capacitors are ideal. The size required will vary depending on the load, output voltage, and input voltage source impedance characteristics. Typical values range from 1µF to 10µF. The input capacitor RMS current varies with the input voltage and the output voltage. It is highest when the input voltage is double the output voltage where it is one half of the load current. VO ⎛ V · 1- O ⎞ VIN ⎝ VIN ⎠ For VIN at 5V and VOUT at 3.3V, the average diode current is: V 3.3V ⎞ = 0.85A IAVG = IO · ⎛1 - O ⎞ = 2.5A · ⎛1 ⎝ VIN ⎠ ⎝ 5.0V ⎠ With a 125°C maximum junction temperature and a 120°C/W thermal resistance, the maximum average current is: T J(MAX)- T AMB 125°C - 70°C = 1.14A 120 °C / W · 0.4V IAVG = θJ-A · VFWD = For overload, short-circuit, and excessive ambient temperature conditions, the AAT1155 enters overtemperature shutdown mode protecting the AAT1155 as well as the output Schottky. In this mode, the output current is limited internally until the junction temperature reaches the temperature limit (see over-temperature characteristics graphs). The diode reverse voltage must be rated to withstand the input voltage. Vin 3.5V-5.5V IRMS = IO · Vout 3.3V @ 2.5A R1 100 C4 100µF R2 100k U1 AAT1155-3.3 FB VP L1 1.5µH GND LX EN C1 10µF C3 0.1µF LX VCC VP D1 B340LA C2 120µF + - rtn C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3 C2 Vishay120µF 6.3V 594D127X96R6R3C2T C3 0. µF 0603ZD104M AVX 1 C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V D1 B340LA Diodes Inc. L1 CDRH6D28-1.5µH Sumida Options C2 MuRata 100µF 6.3V GRM43-2 X5R 107M 100µF 6.3V (two or three in parallel) C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel) Figure 2: 3.3V, 3A Output Efficiency. 1155.2005.11.1.6 11 AAT1155 1MHz 2.5A Step-Down DC/DC Converter A high ESR tantalum capacitor with a value about 10 times the input ceramic capacitor may also be required when using a 10µF or smaller ceramic input bypass capacitor. This dampens any input oscillations that may occur due to the source inductance resonating with the converter input impedance. Due to the ESR zero associated with the tantalum capacitor, smaller values than those required with ceramic capacitors provide more phase margin with a greater loop crossover frequency. Layout Figures 3 and 4 display the suggested PCB layout for the AAT1155. The following guidelines should be used to help ensure a proper layout. 1. The connection from the input capacitor to the Schottky anode should be as short as possible. 2. The input capacitor should connect as closely as possible to VP (Pins 5 and 8) and GND (Pin 2). 3. C1, L1, and CR1 should be connected as closely as possible. The connection from the cathode of the Schottky to the LX node should be as short as possible. 4. The feedback trace (Pin 1) should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace can degrade DC load regulation. 5. The resistance of the trace from the load return to the ground (Pin 2) 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 reference ground and the load return. 6. R1 and C3 are required in order to provide a cleaner power source for the AAT1155 control circuitry. Output Capacitor With no external compensation components, the output capacitor has a strong effect on the loop stability. Larger output capacitance will reduce the crossover frequency with greater phase margin. A 200µF ceramic capacitor provides sufficient bulk capacitance to stabilize the output during large load transitions and has ESR and ESL characteristics necessary for very low output ripple. The RMS ripple current is given by: 1 2· 3 ( VOUT+ VFWD) · (VIN - VOUT) L · F · VIN IRMS = · For a ceramic output capacitor, the dissipation due to the RMS current and associated output ripple are negligible. Tantalum capacitors with sufficiently low ESR to meet output ripple requirements generally have an RMS current rating much greater than that actually seen in this application. The maximum tantalum output capacitor ESR is: ESR ≤ VRIPPLE ∆I where ∆I is the peak-to-peak inductor ripple current. Figure 3: Evaluation Board Top Side. 12 Figure 4: Evaluation Board Bottom Side. 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Thermal Losses associated with the AAT1155 output switching MOSFET are due to switching losses and conduction losses. The conduction losses are associated with the RDS(ON) characteristics of the output switching device. At the full load condition, assuming continuous conduction mode (CCM), an accurate calculation of the RDS(ON) losses can be derived from the following equations: P = I RMS2 · RDS(ON) ON Design Example (see Figures 2 and 5 for reference) IOUT IRIPPLE VOUT VIN FS TMAX 2.5A 40% of Full Load at Max VIN 2.5V 5V ±5% 1MHz 70°C RDS(ON) losses Inductor Selection ∆I 2 ⎞ ⎛ ·D IRMS = I O2 + ⎝ 12 ⎠ Internal switch RMS current D is the duty cycle and VF is the forward voltage drop of the Schottky diode. L= = VOUT ⎛ VOUT⎞ · 1IO · k · F ⎝ VIN ⎠ 3.3V 3.3V ⎞ ⎛ · 1= 1.23µH 2.5A · 0.4 · 1MHz ⎝ 5.25V⎠ Use standard value of 1.5µH Sumida inductor Series CDRH6D38. D= VO + VF VIN + VF ∆I = ∆I is the peak-to-peak inductor ripple current. A simplified form of calculating the RDS(ON) and switching losses is given by: VO ⎛ V⎞ 1- O = L · F ⎝ VIN ⎠ 3.3V ⎛ 3.3V ⎞ = 0.82A 11.5µH · 1MHz ⎝ 5.25V⎠ I PK = IOUT + ∆I = 2 I 2 · R DS(ON) Vo + tSW ·F · I O + I Q · VIN P= O VIN where IQ is the AAT1155 quiescent current. Once the total losses have been determined, the junction temperature can be derived. The thermal resistance (ΘJA) for the MSOP-8 package mounted on an FR4 printed circuit board in still air is 150°C/W. TJ = P QJA + TAMB TAMB is the maximum ambient temperature and TJ is the resultant maximum junction temperature. Efficiency (%) 2.5A + 0.41 = 2.91A Efficiency vs. Load Current (VIN = 5.0V; VOUT = 3.3V) 100 95 90 85 80 75 70 65 60 0.01 0.1 1 10 Output Current (A) Figure 5: 5V Input, 3.3V Output. 1155.2005.11.1.6 13 AAT1155 1MHz 2.5A Step-Down DC/DC Converter AAT1155 Junction Temperature P= ON IO2 · RDS(ON) · VO ⎛ tSW · F · IO ⎞ + IQ · VIN = + VIN 2 ⎝ ⎠ The 120µF Vishay 594D tantalum capacitor has an ESR of 85mΩ and a ripple current rating of 1.48Arms in a C case size. Although smaller case sizes are sufficiently rated for this ripple current, their ESR level would result in excessive output ripple. The ESR requirement for a tantalum capacitor can be estimated by : ⎞ 2.52 · 70mΩ · 3.3V ⎛ 20ns · 1MHz · 2.5A + + 690µA ⎝ ⎠ 5V 2 0.42 Watts TJ(MAX) = TAMB + θJA · P = 70°C + 150°C/W · 0.42W = 133°C ESR ≤ IRMS = VRIPPLE 100 mV = = 111mΩ ∆I 0.9A 1 · (VOUT+ VFWD) · (VIN - VOUT) L · F · VIN 2· 3 = Diode ⎛ V⎞ IDIODE = IO · 1 - O = ⎝ VIN ⎠ 3.3V ⎞ ⎛ = 0.93A 2.5A · 1 ⎝ 5.25V⎠ VFW = 0.35V PDIODE = VFW · IDIODE = 0.35V · 0.93A = 0.33A Given an ambient thermal resistance of 120°C/W from the manufacturer's data sheet, TJ(MAX) of the diode is: 1 3.65V ·1.7 V · = 240mArms 2 · 3 1.5µH · 1MHz · 5V Two or three 1812 X5R 100µF 6.3V ceramic capacitors in parallel also provide sufficient phase margin. The low ESR and ESL associated with ceramic capacitors also reduces output ripple significantly over that seen with tantalum capacitors. Temperature rise due to ESR ripple current dissipation is also reduced. Input Capacitor The input capacitor ripple is: IRMS = I O · V⎛ V⎞ O · 1 - O = 1.82Arms VIN ⎝ VIN ⎠ T J(MAX) = T AMB + ΘJA · P = 70°C + 120°C / W · 0.33W = 109°C In the examples shown, C1 is a ceramic capacitor located as closely to the IC as possible. C1 provides the low impedance path for the sharp edges associated with the input current. C4 may or may not be required, depending upon the impedance characteristics looking back into the source. It serves to dampen any input oscillations that may arise from a source that is highly inductive. For most applications, where the source has sufficient bulk capacitance and is fed directly to the AAT1155 through large PCB traces or planes, it is not required. When operating the AAT1155 evaluation board on the bench, C4 is required due to the inductance of the wires running from the laboratory power supply to the evaluation board. Output Capacitor The output capacitor value required for sufficient loop phase margin depends on the type of capacitor selected. For a low ESR ceramic capacitor, a minimum value of 200µF is required. For a low ESR tantalum capacitor, lower values are acceptable. While the relatively higher ESR associated with the tantalum capacitor will give more phase margin and a more dampened transient response, the output voltage ripple will be higher. 14 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Adjustable Output For applications requiring an output other than the fixed outputs available, the 1V version can be programmed externally. Resistors R3 and R4 of Figure 1 force the output to regulate higher than 1 volt. For accurate results (less than 1% error for all outputs), select R4 to be 10kΩ. Once R4 has been selected, R3 can be calculated. For a 1.25 volt output with R4 set to 10kΩ, R3 is 2.5kΩ. R3 = (VO - 1) · R4 = 0.25 · 10kΩ = 2.5kΩ Capacitors Part Number C4532X5ROJ107M GRM43-2 X5R 107M 6.3 GRM43-2 X5R 476K 6.3 GRM42-6 X5R 106K 6.3 594D127X_6R3C2T 595D107X0016C Manufacturer TDK MuRata MuRata MuRata Vishay Vishay Capacitance (µF) 100 100 47 10 120 100 Voltage (V) 6.3 6.3 6.3 6.3 6.3 16 Temp Co. X5R X5R X5R X5R Case 1812 1812 1812 1206 C C Inductors Part Number CDRH6D38-4763-T055 N05D B1R5M NP06DB B1R5M LQH55DN1R5M03 LQH66SN1R5M03 Manufacturer Sumida Taiyo Yuden Taiyo Yuden MuRata MuRata Inductance (µH) 1.5 1.5 1.5 1.5 1.5 I (Amps) 4.0 3.2 3.0 3.7 3.8 DCR (Ω) 0.014 0.025 0.022 0.022 0.016 Height (mm) 4.0 2.8 3.2 4.7 4.7 Type Shielded Non-Shielded Shielded Non-Shielded Shielded Diodes Manufacturer Diodes Inc. ROHM Micro Semi Part Number B340LA RB050L-40 5820SM VF 0.45V @ 3A 0.45 @ 3A 0.46V @ 3A 1155.2005.11.1.6 15 AAT1155 1MHz 2.5A Step-Down DC/DC Converter Ordering Information Output Voltage 1.0V (Adj. VOUT ≥ 1.0V) 1.8V 2.5V 3.3V Package MSOP-8 MSOP-8 MSOP-8 MSOP-8 Marking1 KXXYY KYXYY ILXYY IKXYY Part Number (Tape and Reel)2 AAT1155IKS-1.0-T1 AAT1155IKS-1.8-T1 AAT1155IKS-2.5-T1 AAT1155IKS-3.3-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 MSOP-8 4° ± 4° 1.95 BSC 3.00 ± 0.10 4.90 ± 0.10 0.60 ± 0.20 PIN 1 0.254 BSC 0.95 REF 3.00 ± 0.10 10° ± 5° 0.95 ± 0.15 0.85 ± 0.10 0.65 BSC 0.30 ± 0.08 0.075 ± 0.075 All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. 16 GAUGE PLANE 0.155 ± 0.075 1155.2005.11.1.6 AAT1155 1MHz 2.5A Step-Down DC/DC Converter © 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. Advanced Analogic Technologies, Inc. 830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 1155.2005.11.1.6 17