LM34902EVM/NOPB

User's Guide SNVU156A – July 2012 – Revised May 2013 AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module 1 Introduction With the advent of new handset, tablet, and notebook devices in smaller and smaller footprints, engineers are facing the challenging task of designing next-generation systems that fit into these smaller form factors. Also, consumers are placing more value on the battery life of their portable devices by seeking longer operation time between charges. One of the standards that has become a “must have” for these devices is the USB port that enables a user to connect various accessories to their device. However, power transfer from a host device to a downstream accessory needs to be carefully managed to avoid inrush current and/or voltage transient events upon connection. The accessory typically sends a handshake signal to indicate that it is ready to accept downstream power transfer. Only then should the device’s CPU or system controller authorize current flow. Well-defined overload protection is also mandated to reliably manage capacitive loads or accessory fault modes. With nominal 5 nA shutdown and 47 µA quiescent supply currents, the LM34902/4 current-limited power switch is ideally suited for such applications. Offered in a tiny 6-bump 1.2 mm x 0.8 mm micro SMD package with 0.6 mm height and 0.4 mm pin pitch, the LM34902 and LM34904 are rated for 300 mA and 500 mA continuous load currents, respectively. An input voltage range of 2.8V to 5.3V is provided to cater to 3V and 5V systems. With a low ON resistance, the high-side p-channel MOSFET switch power dissipation is minimized, translating into lower temperature increase—a critical feature in space constrained applications—and longer battery life. Using an integrated sense resistor, the LM34902/4 provides accurate brickwall current limit, protecting the input supply even when the ACC_PWR output is hard-shorted to ground (GND). To assure reliability, thermal shutdown protection is provided if the junction temperature exceeds 135°C (LM34904) or 170°C (LM34902). 2 Evaluation Module Features • • • • 3 Input voltage operating range: 2.8V to 5.3V. Ambient temperature range: -40°C to +85°C. Rated load current: 300 mA (LM34902), 500 mA (LM34904). PCB size: 1.7” × 1.7” (43 mm × 43 mm). Evaluation Module Operation As the pinouts and footprints of the LM34902 and LM34904 are identical, the solution presented in this user's guide enables the user to evaluate both power switches. This commonality provides flexibility if the load current specifications evolve during the design process. The schematic in Figure 1 demonstrates how the LM34902/4 switch connects to a device CPU (or an ASIC with embedded µC core) and an accessory load. Presented also is the LM34902/4 truth table operation (Table 1). When the accessory is connected, it asserts an active-low accessory detect (ACC_DET) control input. The CPU can then selectively turn on (or off) the switch using the ENABLE input, providing power to the accessory (or conserving power when needed). Both the ENABLE and ACC_DET inputs are 1.8V logic compatible. Both inputs need to be asserted to turn on the MOSFET switch (ACC_DET first and then ENABLE). Note that the power switch is not bidirectional: the input voltage must therefore be higher than the output voltage under all conditions. All trademarks are the property of their respective owners. SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated 1 Evaluation Module Operation www.ti.com VCC: 2.8V to 5.3V 200k CPU GND POK A1 B1 C1 LM34902/4 VCC A2 B2 C2 ACC_PWR 200k Accessory ENABLE ACC_DET Accessory Detect Figure 1. Typical System Schematic Table 1. Power Switch Truth Table Input 2 Output ACC_DET Current Limit Detected TJ Limit Exceeded 0 x No x 1 No 0 to 1 0 0 to 1 0 x x 0 x ENABLE (1) (1) 2.8V < VCC < 5.3V PFET Switch Status POK No Yes Open Open Drain No Yes Open Open Drain No No Yes On Grounded Yes No Yes Current Limited Grounded x Yes 2.2V < VCC < 5.3V Open Open Drain x No 2.2V < VCC < 2.8V Open Open Drain x = stands for "don't care". AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback Evaluation Module Schematic www.ti.com 4 Evaluation Module Schematic The schematic of the evaluation module is shown in Figure 2. Note that ENABLE and ACC_DET default to their off-states if left open-circuited in this implementation. An active-low open-drain output (POK) provides an indication of switch status to the CPU. As shown in Figure 2, the module provides pullup resistors to VCC for POK and ACC_DET and a pulldown resistor to GND for ENABLE. VCC C1 C2 ACC_PWR R1 GND B1 POK C1 LM34902/4 A1 C3 A2 R2 B2 C2 ACC_DET R4 ENABLE Figure 2. Evaluation Module Schematic 5 Evaluation Module Bill of Materials Designator Value Description C1, C2 22 µF Ceramic, 22µF, X5R, 10V, 10%, 0805 Taiyo Yuden LMK212BJ226MG-T 2 C3 4.7 µF Ceramic, 4.7µF, X5R, 25V, 10%, 0805 Murata GRM21BR61E475KA12L 1 VCC, ACC_PWR Test Point, TH, Miniature, Red Keystone Electronics 5000 2 ENABLE, ACC_DET, POK Test Point, TH, Miniature, White Keystone Electronics 5002 3 GND Test Point, TH, Miniature, Black Keystone Electronics 5001 1 Resistor, 220k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603220KFKEA 3 500mA Current Limited Power Switch Texas Instruments LM34904 1 R1, R2, R4 U1 220k SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback Manufacturer Part Number AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated Qty 3 Evaluation Module Component Selection 6 www.ti.com Evaluation Module Component Selection Ceramic input and output capacitors are positioned close to the load switch. Specifically, two 22 µF bypass capacitors from VCC to GND minimize input voltage drop during a high current startup event or momentary output short circuit condition. A 1 µF capacitor is usually adequate even though larger capacitance will further reduce the voltage drop. A 4.7 µF decoupling capacitor from ACC_PWR to GND alleviates any negative swing of ACC_PWR induced by parasitic inductance in the load lines as the load current ramps to zero during a turn-off event. Of course, all traces short be kept as short as possible with VCC, ACC_PWR and GND power bus traces kept wide to minimize conduction drop and assist with thermal spreading. When the output is short-circuited, the input voltage appears across the switch, and the instantaneous power dissipation can be quite high. If the fault condition is not removed, the die temperature may increase to the level where the thermal shutdown protection activates. If the faulted short-circuit condition persists and the device engages thermal shutdown, it latches off. ENABLE can be cycled to initiate a restart. 7 Evaluation Module Power Up Procedure Step 1: Apply an input voltage between 2.8V and 5.3V (VCC to GND). Step 2: Connect ACC_DET to GND. Step 3: Connect ENABLE to VCC. The load switch turns on – measure voltage from ACC_PWR to GND. Step 4: Apply a load current up to the rated current of the device – check voltage from ACC_PWR to GND. Step 5: Apply and remove an overload or short circuit – confirm recovery from this condition. Step 6: Cycle ENABLE high and low to assess turn on and turn off wavefroms, respectively. 8 Evaluation Module Performance Characteristics Using the LM34904 with ACC_DET and POK pullup resistors connected to a pullup voltage source of 1.8V, various performance characteristics of the evaluation module are presented in Figure 3 through Figure 9. Figure 3. ENABLE Turn On, VCC = 2.8V, IOUT = 0.5A (Resistive Load) 4 Figure 4. ENABLE Turn On, VCC = 5.3V, IOUT = 0.5A (Resistive Load) AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback Evaluation Module Performance Characteristics www.ti.com Figure 5. ENABLE Turn Off, VCC = 2.8V, IOUT = 0.5A (Resistive Load) Figure 6. ENABLE Turn Off, VCC = 5.3V, IOUT = 0.5A (Resistive Load) Figure 7. ENABLE Turn On Into An Overload Condition (4Ω Resisitve Load), VCC = 5.3V Figure 8. Output Transient Response When Cycling Into and Out Of Current Limit (Electronic Load Output), VCC = 5.3V Figure 9. ENABLE and ACC_DET Cycled Independently, VCC = 5.3V SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated 5 PCB Layout 9 www.ti.com PCB Layout The LM34902/4 evaluation module uses a two-layer FR4 PCB with terminal connections provided for VCC, ACC_PWR, GND, ENABLE, ACC_DET and POK. The top and bottom side PCB layouts are shown in Figure 10 and Figure 11, respectively. Figure 10. PCB Top Layer 6 AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback PCB Layout www.ti.com Figure 11. PCB Bottom Layer SNVU156A – July 2012 – Revised May 2013 Submit Documentation Feedback AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module Copyright © 2012–2013, Texas Instruments Incorporated 7 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. 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