QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER LT1765EFE DESCRIPTION Demonstration circuit 476 is a 1.25MHz 3A monolithic step-down DC/DC switching converter using the LT1765EFE. The LT1765 features fast switching speed, a 3A internal power switch, and a wide input voltage range, making it a versatile and powerful IC that fits easily into space-constrained applications. The constant 1.25MHz switching frequency allows for the use of tiny, surface mount external components. The current-mode control topology yields fast transient response and good loop stability, requiring a minimum number of external compensation components and allowing the use of ceramic input and output capacitors. The low RDS(ON) internal power switch (0.09Ω) maintains high efficiencies (as high as 90%) over a wide range of input voltages and loads. Its 15µA shutdown current (activated via the SHDN pin) extends battery life. The wide VIN range of the LT1765 allows step-down configurations from 3V to 25V input. Synchronization of switching frequencies up to 2MHz is possible. 4.7V–25V† input, respectively, covering the common values used in cable modems, handhelds, automotive, and desktop computer applications. The 5V or 3.3V output voltage is jumper selectable. This board is designed for applications that require 2A of load current from a wide input voltage range plus simplicity, small circuit size, and low component count. The use of ceramic capacitors in this circuit not only demonstrates small size and low cost, but the advantage of current-mode control in step-down applications with a simple compensation network and a feedforward capacitor for more rugged stability and excellent transient response. Design files for this circuit board are available. Call the LTC factory. † Higher input voltages may pulse-skip due to minimum on-time restrictions. Compensation component changes may be necessary to optimize pulseskipping during high-temperature, high-voltage conditions and maintain control of switch current. The demonstration circuit is designed to provide either 3.3V at 2A or 5V at 2A output, from a 7V–25V input or Figure 1. Demonstration Circuit 476 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER TYPICAL PERFORMANCE SUMMARY (TA = 25°C) Table 1. Step-Down Converter (VOUT = 5V) Table 2. Step-Down Converter (with VOUT = 3.3V) PARAMETER VALUE PARAMETER VALUE VIN 7V to 25V VIN 4.7V to 18V (up to 25V) VOUT 5.0V VOUT 3.3V IOUT 2A(max) IOUT 2A(max) Efficiency Up to 90% at 1A out and up to 89% at 2A output Efficiency Up to 88% at 1A out and up to 85% at 2A output 90 80 VIN = 24V 75 70 65 60 85 75 VIN = 18V 70 65 60 55 55 50 50 45 VIN = 5V VIN = 8V VIN = 12V 80 EFFICIENCY (%) 85 EFFICIENCY (%) 90 VIN = 8V VIN = 12V VIN = 18V 45 10 1k 100 LOAD CURRENT (mA) 10k Figure 2. Typical Efficiency of DC476A LT1765EFE, VOUT = 5V 10 1k 100 LOAD CURRENT (mA) 10k Figure 3. Typical Efficiency of DC476A LT1765EFE, VOUT = 3.3V QUICK START PROCEDURE Demonstration circuit 476 is easy to set up to evaluate the performance of the LT1765EFE. Refer to Figure 4 for proper measurement equipment setup and follow the procedure below: 1. Connect the 7–25V or 4.7–18V input power supply to the VIN and GND terminals on the board. 2. Connect an ammeter in series with the input supply to measure input current. 6. Connect a voltmeter across the VOUT and GND termi- nal to measure output voltage. 7. After all connections are made, turn on input power and verify that the output voltage is either 5.0V or 3.3V (the output voltage jumper setting). 8. The synchronization and shutdown functions are op- tional and their terminals can be left floating (disconnected) if their functions are not being used. 3. Connect either power resistors or an electronic load to the VOUT and GND terminals on the board. 4. Connect an ammeter in series with output load to measure output current. 5. Connect a voltmeter across the VIN and GND termi- nals to measure input voltage. MINIMUM INPUT VOLTAGE The minimum LT1765EFE operating input voltage is 3.0V. Nevertheless, a DC/DC buck converter must have an input voltage that is greater than the output voltage by a certain margin in order to provide the desired regulated output voltage. Maximum duty cycle, switch onresistance, and inductor and diode DC losses all play a 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER part in determining the minimum input voltage for a selected output over the full current range of the application. For this demonstration board, with up to 2A output current, the minimum input voltages required to maintain regulated output voltages is listed below. Customizable output voltages of 2.5V and 1.8V are very common and are thus additionally listed in the table (see Customizing the Board below). If the input voltage falls below the minimum input voltage listed below, the output voltage can drop accordingly from the programmed output voltage. This mode is called maximum duty cycle drop-out. CUSTOMIZING THE BOARD The components used in this demonstration circuit are optimized for a wide input voltage range. Nevertheless, the bandwidth can be increased for more specific input voltages such as 12V±10% or 5V±10% if desired. Some typical applications are shown in Figure 5 to Figure 11. The adjustable feedback resistors allow the output voltage to be customized. For output voltages below 3.3V, the boost diode should be moved from D2 to D3 to provide the minimum boost voltage required for the internal power switch. Make sure that the boost capacitor (C8) has a voltage rating greater than or equal to the output voltage for applications where the boost diode is placed in D2. However, the boost capacitor must have a voltage rating greater than the input voltage whenever the boost diode is placed in position D3. Table 3. Minimum Input Voltage vs Output Voltage MINIMUM INPUT VOLTAGE VIN (V) OUTPUT VOLTAGE (V) 7 5 4.7 3.3 3.7 2.5 3 1.8 – + + + – – – + + + – – + – LOAD Figure 4. Proper Measurement Equipment Setup 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER 10-14Vin 2.5V @2.5A Vin D3 CMDSH-3 LT1765EFE Vin C2 2.2uF 25V X7R 1206 Boost U1 C8 0.22uF, 16V CDRH5D28-2R5M R1 FB R3 10.7k Gnd Vc C3 4.7nF L1 Vsw Sync /Shdn Ceramic Vout C4 100pF C7 100pF D1 R2 10.0k B320A C5 22uF 6.3V X5R 1206 Ceramic 3.3k Other frequency comp. option... R3=2.4k, C3=10nF, C7=open Figure 5. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 10V–14V, VOUT = 2.5V at 2.5A 12V +/-10% 3.3V @2A Vin LT1765EFE Vin C2 2.2uF 25V X7R 1206 Boost U1 C8 0.22uF D2 CMDSH-3 L1 Vsw CDRH5D28-2R5M Sync /Shdn Ceramic R1 FB R3 17.4k Gnd Vc C3 1000pF Vout C4 100pF C7 62pF D1 R2 10.0k B220A C5 10uF 6.3V X5R 0805 Ceramic 4.7k Figure 6. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 12V±10%, VOUT = 3.3V at 2A 12V +/-10% 5V @2A Vin LT1765EFE Vin C2 2.2uF 25V X7R 1206 Boost U1 C8 0.22uF D2 CMDSH-3 A916CY-2R7M R1 FB R3 2.4k 31.6k Gnd Vc C3 10nF L1 Vsw Sync /Shdn Ceramic C4 100pF Vout D1 B220A C7 62pF R2 10.0k C5 10uF 6.3V X5R 0805 Ceramic Figure 7. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 12V±10%, VOUT = 5V at 2A 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER 14-16V 3.3V @2.5A Vin LT1765EFE Vin C2 2.2uF 25V X7R 1206 Boost U1 C8 0.22uF D2 CMDSH-3 L1 Vsw CDRH5D28-2R5M Sync /Shdn Ceramic R1 FB 17.4k Gnd Vc C3 1800pF R3 Vout C4 100pF C7 62pF D1 R2 10.0k B220A C5 10uF 6.3V X5R 0805 Ceramic 2.4k Figure 8. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 14V–16V, VOUT = 3.3V at 2.5A 3-5.5V 1.8V @2A Vin D3 CMDSH-3 LT1765EFE Vin C2 2.2uF 10V X5R 0805 Boost U1 Vout C8 0.22uF L1 Vsw CDRH4D28-1R2M Sync /Shdn Ceramic R1 FB 10nF R3 4.99k Gnd Vc C3 C4 100pF D1 R2 10.0k B220A C5 10uF 6.3V X5R 0805 Ceramic 1.0k Figure 9. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 3V–5.5V, VOUT = 1.8V at 2A 5-15V 3.3V @2A Vin LT1765EFE Vin C2 2.2uF 25V X7R 1206 Boost U1 C8 0.22uF D2 CMDSH-3 CDRH6D28-3R0M R1 FB R3 3.3k 17.4k Gnd Vc C3 10nF L1 Vsw Sync /Shdn Ceramic C4 100pF Vout D1 B220A C7 62pF R2 10.0k C5 22uF 6.3V X5R 1206 Ceramic Figure 10. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 5V–15V, VOUT = 3.3V at 2A 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER 5V +/-10% 2.5V @2A Vin D3 CMDSH-3 LT1765EFE Vin C2 2.2uF 10V X5R 0805 Boost U1 C8 0.22uF CDRH4D28-1R2M R1 FB R3 2.2k 10.7k Gnd Vc C3 10nF L1 Vsw Sync /Shdn Ceramic Vout C4 100pF D1 B220A R2 10.0k C5 10uF 6.3V X5R 0805 Ceramic Figure 11. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 5V±10%, VOUT = 2.5V at 2A 6