APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family Introduction This application note describes the current limit network calculations for the different DC winding resistances (DCR) of inductors selected for the Skyworks family of high-voltage step-down converters. This family includes the AAT1184, AAT1185, AAT1189, AAT2687, AAT2688, and AAT2689. In order to protect the device from permanent damage during over-current stress or short circuit events, the current is sensed through the output inductor DC winding resistance. An external resistor (R1) and capacitor (C4) network adjusts the pre-set over-current threshold (IPRESET) based on the DCR of the inductor. The over-current offset voltage threshold (VOCP) is internally set to 100mV. The R1-C4 network senses the voltage drop across the inductor and compares it to the over-current offset voltage threshold. As illustrated in Figure 2, in normal operation the differential voltage (VRS – VOS) signal is a triangle waveform and in phase with the inductor current. The over-current controller is triggered when the differential voltage signal from RS to OS exceeds 100mV (nominal). When the over-current is triggered, the switching frequency and the output voltage reduce to limit the current to a user designed value (see Figure 3). The designed current limit threshold (ILIMIT) can be set by the current limit network. The operating frequency returns to the nominal setting when over-current conditions are removed. The current limit network including components (R1, C4, R6, R7, and R8)1 can be calculated and configured according to the DCR of the selected inductor. With the same inductor value, a small footprint inductor has a greater DCR compared to a larger footprint inductor. The inductor DCR variation results in the variation of the pre-set over current threshold (IPRESET). There are three current limit network configurations used to set the design current limit (ILIMIT) for different pre-set over-current thresholds (IPRESET) due to the inductor DCR selection. U1 1 LX LX 16 VOUT 5V/2.5A L1 4.7µH 2 3 LX LX 15 LX RS 8 4 LX 5 LX OS D2 B340A R1 6.34k C7 330pF C4 68nF R8 open R4 44.2k R6 R3 6.34k 499 C8 22µF 9 C9 22µF C10 open AAT1189 C2 0.1µF 6 D1 BAS16 VCC 13 11 COMP 10 GND 12 VL 14 3 C11 C1 100µF open 25V C12 1µF 25V IN EP EN R2 C5 24.3k 220pF R7 649k R5 6.04k C6 56pF C3 2.2µF VIN 6.0V -25.0V + BST FB 7 2 1 EN TDFN34-16 Figure 1: AAT1189 Typical Application Schematic with Current Limit Network (R1, C4, R6, R7, R8). 1. R1, C4, R6, R7, R8 may have different names in AAT1184, AAT1185, AAT2687, AAT2688, and AAT2689 schematics. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 1 APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family Figure 2: AAT1189 (VRS – VOS) Signal and Inductor Current are In Phase. CH2 (Blue): RS signal; CH3 (Pink): OS signal; CHM (Red): (RS-OS) signal; CH4: Inductor Current (1A/div). Figure 3: AAT1189 Short Circuit Test. CH1: Output Voltage (2V/div); CH2: Switching Node (LX); CH4: Inductor Current (1A/div). 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family Current Limit Network Calculation In order to correctly sense the inductor current, the R1-C4 network time constant needs to be matched to the inductor time constant as shown in the following equation: Eq. 1: R1 · C4 = L1 DCR The pre-set over-current threshold with the matching R1-C4 network is the ratio of the over-current offset voltage threshold and the winding resistance (DCR) of the inductor: Eq. 2: IPRESET = VOCP 100mV = DCR DCR Where the over-current offset voltage threshold (VOCP) is internally set at 100mV with 20% tolerance. 1. Setting the Current Limit close to the Pre-set Over-Current (ILIMIT = IPRESET) For example, for the 7447789004, 4.7µH Würth inductor with a typical DCR of 35mΩ is used in the AAT1189. The pre-set over-current threshold is determined as Equation 2: IPRESET = VOCP 100mV = = 2.86A DCR 35mΩ If the current limit threshold is set around 3A, there is no need to use the divider R6 and R7 (Figure 4). The sensing capacitor C4 = 68nF and the sensing resistor R1 can be calculated from Equation 1: R1 = L 4.7µH = = 1.97kΩ; choose R1 = 1.91kΩ DCR · C4 35mΩ · 68nF ILIM LX DCR L1 35mΩ 4.7µH R1 C4 1.91k + - + RS 68nF VL1 V OUT 5V/2.5A - + 100mV OS Figure 4: Setting the Current Limit Close to the Pre-set Over-Current for the AAT1189 (R6 = 0, R7 = Open). Note: The value of R1 should be adjusted from the calculation value to derive the designed current limit due to the variation of over-current offset voltage threshold (VOCP) between parts. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 3 APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family 2. Setting the Current Limit less than the Pre-set Over-Current (ILIMIT < IPRESET) For example, the RCH108NP-4R7M, 4.7μH Sumida inductor with a typical DCR of 11.7mΩ is used with the AAT1189. The pre-set over-current threshold is determined by Equation 2: IPRESET = VOCP 100mV = = 8.5A > ILIMIT = 3A DCR 11.7mΩ If the AAT1189 current limit is set to 3A, the divider (R6 and R7) needs to be added into the network (Figure 5). If the sensing capacitor C4 = 68nF, then the sensing resistor R1 value can be calculated using Equation 1: R1 = L 4.7µH = = 5.9kΩ; choose R1 = 5.9kΩ DCR · C4 11.7mΩ · 68nF I LIM LX DCR L1 12mΩ 4.7µH R1 C4 5.9kΩ + - RS + 100mV OS + VOUT 5V/2.5A 68nF VL1 + VR6 - R6 R7 Figure 5: Setting the Current Limit Less than the Pre-set Over-Current for the AAT1189 (R6 = 5.9k, R7 = 464k). Neglecting the current that sinks into RS and OS terminals of the comparator, the voltage drop on the inductor (VL1) should be identical to the voltage drop across C4, hence: Eq. 3: VL1 + VR6 = 100mV Eq. 4: ILIM · DCR + R6 · VOUT = 100mV R6 + R7 Eq. 5: ILIM · DCR + R6 · R7 · VOUT = 100mV (R6 + R7) · R7 4 To balance the impedance between the two terminals of the comparator, R1 should be equal to the equivalent resistance of R6 and R7 in parallel: Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family Eq. 6: R1 = Substituting Equation 6 into Equation 5 and solving for R7: R7 = VOUT · R1 5V · 5.9kΩ = = 454.5kΩ; choose R7 = 464kΩ VOCP - ILIMIT · DCR 0.1V - 3A · 11.7mΩ Substituting R1 and R7 into Equation 6 and solving for R6: Eq. 7: R6 = R6 · R 7 R6 + R7 R1 · R 7 5.9kΩ · 464kΩ = = 5.9kΩ R7 - R1 464kΩ - 5.9kΩ Note: The value of R1, R6 and R7 should be adjusted from the calculated values to get the designed current limit due to the variation of over-current offset voltage threshold (VOCP) between parts. 3. Setting the Current Limit Greater than the Pre-set Over-Current (ILIMIT> IPRESET) The AAT2687 and AAT2688 can provide up to 4.5A constant output current which is equivalent to a 5.0A peak inductor current. That requires at least 5.5A or above for the designed current limit. If the IHLP-2525CZ-01, 4.7µH Vishay inductor with a maximum DCR of 40mΩ is used, the pre-set over-current threshold is determined by Equation 2: IPRESET = VOCP 100mV = = 2.5A < ILIMIT = 6A DCR 40mΩ If the current limit is set to 6A, R6 and R8 need to be added to the network (see Figure 6). If the sensing capacitor C4 = 56nF, then the sensing resistor R1 can be calculated from Equation 1: R1 = L 4.7µH = = 2.098kΩ; choose R1 = 2.05kΩ DCR · C4 35mΩ · 56nF VL1 + LX DCR L1 40mΩ 4.7µH R1 C4 2.05k + - 56nF - V OUT 5V/2.5A I LIM + VC4 = VR8 RS + 100mV - R8 R6 OS Figure 6: Setting the Current Limit Greater than the Pre-set Over-Current (R6 = 1k, R8 = 2.05k) for the AAT2687 and AAT2688. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012 5 APPLICATION NOTE Setting the Current Limit for the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family Neglecting the current that sinks into the RS and OS terminals of the comparator, the over-current offset voltage threshold is identical to the voltage drop across R8: Eq. 8: VR8 = VC4 = VOCP = 100mV Where: R8 · VL1 R · I · DCR = 8 LIM = VOCP = 100mV R8 + R1 R8 + R1 Eq. 9: VR8 = Solve for R8: Eq. 10: R8 = VOCP · R1 100mV · 2.05kΩ = = 1.47kΩ ILIM · DCR - VOCP 6A · 40mΩ - 100mV To balance the impedance between two terminals of the comparator, R6 should equal the equivalent resistance of R1 and R8 in parallel: Eq. 11: R6 = R1 · R 8 2.05kΩ · 1.47kΩ = = 856Ω R1 + R8 2.05kΩ + 1.47kΩ Note: The value of R1, R6 and R8 should be adjusted from the calculated values to derive the designed current limit due to the variation of over-current offset voltage threshold (VOCP) between parts. Sensing Capacitor Selection and Current Limit Network Layout Considerations The sensing capacitor C4 should be large enough to hold the sensing voltage from noise interference. A typical value of C4 is not smaller than 47nF for adequate filtering. The sensing components C4 and R1 should be connected as close as possible to the RS1 and OS1 pins and can be placed on the bottom side of the layout to avoid noise coupling from the inductor. The trace that connects from RS and OS to the inductor terminals should be kept as short as possible to prevent adding more DCR to the inductor. 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. 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