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