EV188 EVALUATION BOARD DATA SHEET

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1 Introduction The AAT2138 evaluation board demonstrates its functions and high precision current application as a 2.5A synchronous step-down converter with an integrated current-limited load switch. By guarding against excessive input current, the AAT2138 enables the system designer to maximize the output current from the step-down converter while protecting the input supply. For example, the AAT2138 protects USB ports from heavy load transient conditions commonly seen in high data rate modem applications. The AAT2138 is a 2.8MHz current mode step-down converter. It utilizes a tiny 1μH inductor and internal compensation to provide an extremely small footprint and excellent performance. The evaluation board allows users to evaluate several features of the device some of which are shown below: Wide Input Voltage Range: 2.7V to 5.5V Current-Limit Load Switch High Precision, Programmable Current Limit Range from 500mA to 3A High-Current Step-Down Converter Up to 95% Efficiency 100% Low Dropout Operation PFM/PWM or forced PWM Modes 1MHz to 3MHz External Clock Support Soft Start Independent Enable Pins for Switch and Converter Over-Temperature and Over-load Protection For additional information, please consult the AAT2138 product datasheet. The AAT2138 evaluation board is shown in Figure 1. The Getting Started section is included to help the user begin operating the evaluation board. Figure 2 and Figure 17 depict the board schematic and layout. Figure 12 shows the high precision load pulse application. Board Pictures (a) Top Side. (b) Bottom Side. Figure 1: AAT2138 Evaluation Board. 1

2 Board Schematic VOUT 3.4V C32 Option C31 22µF R1 274K C4 22pF L1 1uH SW RSET U1 AAT SW REGIN 14 9 SW 4 REGIN 11 FB SYSIN 6 12 RSET SYSIN 1 PGND ENREG PGND ENSYS 10 AGND MODE C21 10µF C22 680µF C1 1µF GND C23 0.1µF REGIN SYSIN 2.7V-5.5V R2 59K GND GND J3 J2 J1 SYSIN GND SYSIN GND SYSIN GND Figure 2: AAT2138 Evaluation Board Schematic. Getting Started The five steps below highlight the use of the AAT2138 evaluation board. Step 1: Measurement Configuration Connect the Power Supply to VIN and GND pins and set the input voltage from 2.7V to 5.5V. Connect the Jumper to the correct position as shown in figure 3 for the corresponding test. Table 1 explains the jumper selections. Power Supply 2.7V to 5.5V Figure 3: AAT2138 Evaluation Board Measurement Configuration. Jumper J1 J2 J3 Description Enable the Regulator by setting the jumper EN from OFF to ON position. Enable the Load switch by setting the jumper ENSYS from OFF to ON position. Regulator Operating Mode. Set to PFM position for selecting PFM Mode and PWM position for PWM mode. An external clock in the range of 1 MHz to 3MHz can be connected into MODE as a clock for the regulator. Table 1: Jumper Definition. 2

3 Step 2: Measurement for Load Switch Turn On When power supply is ready, the load switch Enable threshold and soft start performance can be measured as shown in figure 4. The system input voltage can be set down to 2.7V. Providing a 1Hz pulse signal to ENSYS pin and set V SYSIN to 5V, V OUT to 3.4V, C REGIN = 10μF + 680μF + 0.1μF. The waveform in figure 5 shows the soft start process. The load switch soft start time is approximately 1.5ms with no load, and the input current limit is under 500mA. When the REGIN voltage reaches 95% of system input voltage, the output of the BUCK converter starts to increase. For details, please refer to the soft start section of the AAT2138 datasheet s Functional Description. Power Supply 2.7V to 5.5V Output Signal Generator TEK AFG3102 Ch3 IIN Ch1 VOUT Ch4 ENS Oscilloscope Ch2 REGIN Figure 4: Load switch Soft Start Configuration. Figure 5: Load switch Soft Start Waveform. (CH1 = OUT, CH2 = REGIN, CH3 = I SYS, CH4 = ENSYS). 3

4 Step 3: Measurement for Regulator The AAT2138 uses a peak current-mode step-down control scheme. The converter senses the current through the highside P-channel MOSFET for current loop control as well as overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The feedback amplifier compares the FB voltage against the 0.6V reference voltage. The error amplifier internally compensates the transconductance output, and programs the current-mode loop for the necessary peak switch current to force a constant output voltage over all load and line conditions. Step 3.1: Measurement for Regulator Efficiency After configuration of step 1, connecting SYSIN and REGIN to disable the load switch for testing the single regulator. Efficiency measurement can be completed by the auto-test system based on LabView program. Figure 6 shows the basic connection principle for DC/DC efficiency measurement. POWER A Sense Line + V SYSIN VOUT Demo Board GND GND A V E-Load Sense Line - Figure 6: Auto Test System Connection Diagram for DC/DC Efficiency Measurement. The AAT2138 has a high efficiency, up to 96%, and its output error which is generated from the efficiency data can be low to 0.3%/A. Figure 7&8 show them. Efficiency (%) REGIN=3.8V 20 REGIN=4.2V REGIN=5V 10 REGIN=5.5V Cout=22uF, L=1uH Load Current (ma) Output Voltage Error (%) REGIN=3.8V REGIN=4.2V REGIN=5V REGIN=5.5V COUT=22uF, L=1uH Output Current (A) Figure 7: Efficiency vs Load. Figure 8: Output Error vs Load. 4

5 Step 3.2: Measurement for Regulator Output Ripple Voltage As switching frequency and switching speeds continue to increase, special care should be taken to ensure accurate measurement of input and output voltage ripple in switch mode DC/DC power supplies. Simple steps should be taken to assure minimum oscilloscope pick-up to the high frequency events, which can be magnified by the large ground loop formed by oscilloscope probe ground. This means that even a few inches of ground wire on the oscilloscope probe may result in hundreds of millivolts of noise spikes when improperly routed or terminated. Figure 9 shows the correct method of measurement. The ground line is winding wire around the voltage probe and the terminal is soldered to the output cap ground pad. Figure 9: Correct Method of Using Probe to Measure Ripple Voltage. 5

6 Using the correct output ripple voltage measurement method, connect the regulator output to the electronic load and power on the regulator as shown in figure 10. Set VIN = 5V, L = 1μH, Load = 2A. Figure 11 shows that the operation is stable and the output ripple voltage is under 10mV PP. For detailed operation about output ripple voltage, please refer to the datasheet. Power Supply 2.7V to 5.5V Electronic Load Ch1 Vout Ch3 IL Ch4 SW Oscilloscope Figure 10: Regulator Output Ripple Measurement Configuration. Figure 11: AAT2138 Output Ripple Voltage in Normal Operation Waveform. (CH1 = OUT, CH3 = I L, CH4 = SW). 6

7 Step 4: High Load Pulse Application Together with a large value capacitor between REGIN and GND, the AAT2138 can support a higher load pulse in lower input current limited applications such as high data rate modem applications. C REGIN is not only the load switch output capacitor but also the step-down converter input capacitor. It provides the additional input current and maintains the SYSOUT voltage for the step-down converter when load switch limits the input current from SYSIN. If the input voltage of the step-down converter is lower than the V OUT plus the dropout voltage, the AAT2138 enters dropout mode. The minimum value of C REGIN can be calculated by the following steps: First, calculate the allowed maximum delta voltage on C REGIN to keep V out in regulation: V REGIN = V IN - V OUT - V DROPOUT_SWITCH - V DROPOUT_BUCK Second, calculate the required input current at SYSOUT for the step-down converter: V I BUCKIN = OUT I OUT (VSYSIN - V DROPOUT_SWITCH ) η Next, calculate the maximum current C REGIN should provide: I C REGIN = I BUCKIN - I LIM Finally, derive the C SYSOUT at certain load on period T ON. C REGIN_MIN = I T C REGIN ON V REGIN Example: A 2A 217Hz 12.5% load pulse is applied on 3.8V V OUT in 5V V IN and 750mA load switch current limit. Under the condition, V Dropout_Switch is 0.06V. V Dropout_Buck is 0.17V, efficiency is 90%: V REGIN = = 0.97V I BUCKIN = = 1.71A (5-0.06) 0.9 I = = 0.96A C REGIN T ON is 576μs for a 217Hz 12.5% duty cycle load pulse: C REGIN_MIN = = 570µF 0.97 Considering 20% capacitance tolerance, the minimum capacitance should be 684μF. So select 680μF tantalum capacitor as C REGIN, as well as two additional 10μF & 0.1μF ceramic capacitors to closely filter the input voltage V REGIN of the step-down converter on the PCB board. 7

8 Configure the demo board as shown in figure 10, adding a load pulse created by the electronic load at the output and powering from a standard USB port. Figure 12 shows the output load transient performance. As shown, the load pulse has very little impact on SYSIN, the USB voltage drop is less than 250mV and meets USB 2.0 specification of <300mV, and the input current limit stays at about 750mA as set, while the output voltage drop is also minimal. Figure 12: Load Pulse Application Waveform. (CH1 = OUT, CH2 = SYSIN, CH3 = LOAD, CH4 = I SYS ). Step 4.1: Tantalum Capacitor Selection The rated voltage selection of tantalum capacitor must be taken into consideration in addition to the 20% capacitance tolerance. There are two main types of tantalum capacitors: MnO 2 and Polymer. Their rated voltages have different temperature features. For the MnO 2 tantalum capacitor, any stress, such as reverse voltage or large sudden current, will create a large amount of heat, which may be enough to crack or fracture the dielectric. In a worst case scenario, the capacitor may explode. For polymer tantalum capacitor, although it has the same constricted channel structure, the fill is created with a polymer, a soft, elastic material. Additionally, the polymer deposition process involves a temperature range from +25 C to +65 C, hardly enough to generate forces, even if the polymer were as hard and brittle as the MnO 2. Figure 13, quoted from KEMET Electronics Corp application note, shows the SSST (Surge Step Stress Test) result as an indicator of susceptibility to power-on failures. The sample size varied between 80 and 200 pieces for each batch. This figure indicates that when the application voltage is 50% of the rated voltage, the capacitor failure rate is 13PPM for MnO 2 and 2PPM for polymer. if there is no derating, a 100% rated voltage will cause an unacceptable, 2943PPM failure rate for MnO 2 tantalum capacitors. Figure 13: Mean PPM Failure Rates vs Stress (%Vr). 8

9 As figure 14, quoted from the T495 Surge Robust Low ESR MnO 2 Series datasheet of KEMET shows, the recommended maximum application voltage for MnO 2 tantalum capacitor is 50% of rated voltage, when the temp is below 85 C. If the temperature gets up to 125 C, the related application voltage should be kept at 33% of the rated voltage. According to the USB data card power supply application, where the input voltage is 5V±10%, a 10V rated voltage for MnO 2 tantalum capacitor is recommended. For Polymer tantalum capacitor, the derating is about 10%, so 6.3V rated voltage suits the application. Figure 14: Recommended Voltage Derating Guidelines. 9

10 Step 5: Synchronous Clock Function The step-down converter can also be synchronized to an external clock signal fed into the MODE pin. In this case, the internal oscillator is bypassed. The frequency of the external clock must be between 1MHz and 3MHz. Configure the measurement environment as shown in figure 15 and provide a 1.5MHz clock to the MODE pin from the signal generator. The AAT2138 will work with the 1.5MHz switching frequency synchronously as shown in figure 16. Power Supply 2.7V to 5.5V Output Signal Generator TEK AFG3102 Ch4 SW Ch2 MODE Oscilloscope Figure 15: Synchronous Clock Function Measurement Configuration. Figure 16: Synchronous Clock Function Waveform. (CH2 = Clock, CH4 = SW, V OUT = 3.4V, V IN = 5V). 10

11 Setting Regulator Output Voltage The output voltage of the regulator is set by the external resistor divider R1 and R2. Table 2 shows the resistor divider look-up table based on the equation V OUT V FB R1 = R2-1 = R2 V AVDD 0.6V - 1 Output Voltage (V) R1(kΩ) (R2 = 59kΩ) Table 2: Resistor Selections for Different Output Voltage Settings. Setting Load Switch Current Limit The AAT2138's load switch current limit can be programmed by an external resistor R SET connected between RSET and GND pins. In most applications, the variation in I LIM must be taken into account when determining R SET. The I LIM variation is due to processing variations from part to part, as well as variations in the voltages at SYSIN and REGIN, plus the operating temperature. Table 3 gives 1% standard metal film resistor example values for PMOS current limit programming. I LIM (A) R SET (kω) Table 3: Examples of 1% Standard Resistor Value of R SET. 11

12 Printed Circuit Board Layout Recommendations For the best performance of the AAT2138, follow the guidelines listed below when designing the PCB layout: 1. Reliably solder the exposed pad (EP) to the GND plane. A GND pad below EP is strongly recommended. 2. Keep the power traces (including GND, SW, SYSIN, and REGIN traces) short, direct and wide to allow large current flow. Keep the L connection to the SW pins as short as possible. Do not put any signal lines under the inductor. 3. Connect the input capacitors (C1 and C21) as close as possible between SYSIN/REGIN and GND to get good power filtering. 4. Connect the input capacitor C23 as close as possible between REGIN and AGND to get good power filtering and load regulation. 5. Keep the switching node, SW away from the sensitive FB node. 6. Separate the feedback trace from any power trace and connect as close as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. Place external feedback resistors as close as possible to the FB pin to minimize the length of the high impedance feedback trace. Printed Circuit Board (a) Top Layer (b) Bottom Layer Figure 17: AAT2138 Evaluation Board Layout. 12

13 Component Selection Component Part Number Description Manufacturer U1 AAT2138IWO-0.6 High Current Step-Down Converter with Adjustable Current Limit Load Switch Skyworks C1 GRM21BR71E105KA99L Cap Ceramic 1μF 0805 X7R 25V 10% C21 GRM21BR61C106KE15L Cap Ceramic 10μF 0805 X5R 16V 10% C23 GRM21BR71E105KA99L Cap Ceramic 0.1μF 0603 X7R 50V 10% Murata C31 GRM21BR60J226ME39 Cap Ceramic 22μF 0805 X5R 6.3V 10% C4 GRM0335C1H220GD01D Cap Ceramic 22pF 0603 C0J 50V 10% C22 T530X687M006ATE018 Cap 680μF 7343 X5R 6.3V 20% KEMET R1 RC0603FR-07274KL Res 274KΩ 1/10W 1% 0603 SMD R2 RC0603FR-0759KL Res 59KΩ 1/10W 1% 0603 SMD Yageo RSET RC0603FR-076K34L Res 6.34KΩ 1/10W 1% 0603 SMD L CIG22H1R0MNE Power Inductor 1μH 3.3A 83mΩ SMD Samsung Table 4: Evaluation Board BOM List. Copyright 2012 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. ( Skyworks ) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conflicts, incompatibilities, or other difficulties arising from any future changes. No license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. Skyworks assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of Skyworks products, information or materials, except as may be provided in Skyworks Terms and Conditions of Sale. THE MATERIALS, PRODUCTS AND INFORMATION ARE PROVIDED AS IS WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHERWISE, INCLUDING FITNESS FOR A PARTICULAR PURPOSE OR USE, MERCHANTABILITY, PERFORMANCE, QUALITY OR NON-INFRINGEMENT OF ANY INTELLECTUAL PROPERTY RIGHT; ALL SUCH WARRANTIES ARE HEREBY EXPRESSLY DISCLAIMED. SKYWORKS DOES NOT WARRANT THE ACCURACY OR COMPLETENESS OF THE INFORMATION, TEXT, GRAPHICS OR OTHER ITEMS CONTAINED WITHIN THESE MATERIALS. SKYWORKS SHALL NOT BE LIABLE FOR ANY DAMAGES, IN- CLUDING BUT NOT LIMITED TO ANY SPECIAL, INDIRECT, INCIDENTAL, STATUTORY, OR CONSEQUENTIAL DAMAGES, INCLUDING WITHOUT LIMITATION, LOST REVENUES OR LOST PROFITS THAT MAY RESULT FROM THE USE OF THE MATERIALS OR INFORMATION, WHETHER OR NOT THE RECIPIENT OF MATERIALS HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. Skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, or other equipment in which the failure of the Skyworks products could lead to personal injury, death, physical or environmental damage. Skyworks customers using or selling Skyworks products for use in such applications do so at their own risk and agree to fully indemnify Skyworks for any damages resulting from such improper use or sale. Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product design, or damage to any equipment resulting from the use of Skyworks products outside of stated published specifications or parameters. Skyworks, the Skyworks symbol, and Breakthrough Simplicity are trademarks or registered trademarks of Skyworks Solutions, Inc., in the United States and other countries. Third-party brands and names are for identification purposes only, and are the property of their respective owners. Additional information, including relevant terms and conditions, posted at are incorporated by reference. 13