DEMO CIRCUIT 825 QUICK START LT1936 GUIDE LT1936 500kHz Step-Down Switching Regulator DESCRIPTION Demonstration circuit 825 is a monolithic step-down DC/DC switching regulator featuring the LT1936. The demo board is optimized for 3.3V output at 1.2A from a 4.5V to 36V input or a 5V, 1.2A output from a 6.8V to 36V input. The output is selectable by placing the JP1 jumper at the 3.3V or 5V position. The wide input range of the LT1936 allows a variety of input sources. The typical sources are automotive batteries, wall adaptors, 5V logic supplies and industrial supplies. The 500kHz switching frequency allows small, low cost inductor and ceramic capacitors, resulting in low, predictable output ripple. The currentmode control topology creates fast transient response and great loop stability. A VC pin is available for customizing external compensation. If smaller size is critical, internal compensation can be used. Higher efficiency is achieved by using a low resistance internal power switch. The gate drive of the switch is boosted to a voltage that is higher than the Vin to ensure saturation of the switch. A charge pump consisting of D2 and C8 on the demo board performs the boost function. The SHDN pin can be used to set the part in micropower shutdown mode, reducing the supply current to less than 2uA. The SHDN pin can also be used to program soft start. In this mode, the SHDN pin is driven through an external RC filter (R6 and C9) to create a voltage ramp at this pin. The soft start function reduces input current surge during start-up. The optional capacitor C6 is to damp overshoot voltage that otherwise might damage the IC when the circuit is plugged into a live supply through long wires. Cycle-bycycle current limit, frequency foldback and thermal shutdown provide protection against shorted outputs. The LT1936 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction with this quick start guide for demo circuit 825. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and PowerPath are trademarks of Linear Technology Corporation. 1 LT1936 PERFORMANCE SUMMARY Specifications are at TA = 25°C SYMBOL Iq Iqs VIN Vin VOUT Vout Iout EFf EFf Vpp Vpp PARAMETER Quiescent Current Quiescent Current in Shutdown Input Voltage to Run Input Voltage to Run Output Voltage Output Voltage Output Current Efficiency Efficiency Output Voltage Ripple Output Voltage Ripple CONDITIONS Iout=0A JP2 on OFF position Vout = 3.3V Vout = 5V JP1 on 3.3V Position JP1 on 5V Position Vin=12V, Vin=12V, Vin=12V, Vin=12V, MIN 4.5 6.8 3.168 4.8 Vout=3.3V, Iout=1.2A Vout=5V, Iout=1.2A Vout=5V, Iout=1.2A Vout=3.3V, Iout=1.2A TYP 1.8 0.1 3.3 5 83 87 15 13 MAX 2.5 2 36 36 3.432 5.2 1.2 UNITS mA uA V V V V A % % mV mV QUICK START PROCEDURE Demonstration circuit 825 is easy to set up to evaluate the performance of the LT1936. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: Note 1. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), an inductor with saturation current above 2.6A should be used. Note 2. When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See Figure 2 for proper scope probe technique. 1. Place jumpers in the following posi- tions: JP1 JP2 3.3V or 5V ON 2. With power off, connect the input power supply to Vin and GND. 3. Turn on the power at the input. Note 3. Make sure that the input voltage does not exceed 36V. During hot-plug, the transient voltage at the VIN pin can exceed this limit even when the source voltage is well below it. See application note 88 for more details. 4. Check for the proper output volt- ages. Note 4. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. Also refer to the Typical Minimum Input Voltage vs. Load Current in the Typical Performance Characteristics section of the datasheet. 5. Once the proper output voltages are established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 6. To evaluate soft start, replace R6 on the demo board with a 15KΩ resistor and measure the inductor current. 7. To use internal compensation, remove R3, C3 and C4, and populate R5. 2 LT1936 Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 3 LT1936 4