DC474A Analog Devices

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474 4A, 500KHZ STEP DOWN SWITCHING REGULATOR LT1374CFE DESCRIPTION Demonstration circuit 474 is a step- current between 700mA and 2 amps. down buck regulator using the Full load efficiency is 87%. The LT1374CFE 500kHz-switching regulator. maximum allowed die temperature is The high switching frequency allows 125C. Figure 2 shows die temperature the use of small inductors and filter vs. output load current with V of IN components, making this circuit ideal 12V. At full load the die temperature for space conscious systems. rise is 70C indicating that the demo is suitable for ambient temperatures of up to 125C 70C = 55C. 0.95 80 0.9 60 40 0.85 20 0 0.8 01 2 3 4 012 34 5 Load (Amps) Load Current Figure 2. Die Temperature vs Output Figure 1. Efficiency, 10V V , 5V V IN OUT Current, V = 12V IN Figure 1 shows efficiency versus load for a typical application. The best efficiency is realized with a load QUICK START PROCEDURE Refer to Figure 3 for proper measure- 3. Select desired output voltage using ment equipment setup and follow the the voltage selecting jumper procedure below: NOTE: Choose 3.3V operation by mov- 1. Connect input power to the Vin and ing the voltage selection jumper to GND terminals. V should be be- the 3.3V position. Demonstration IN tween 5V and 25V. The power source circuit 474 is optimized for 5V op- should be capable of delivering eration, and when operating at 3.3V 25W. out, high V and light load some IN instability may arise. Increasing NOTE: 5V output operation requires the output capacitance removes this a minimum V of 6V IN instability. Use a 330F, 4V, low 2. Connect the output load to the Vout ESR cap such as the Poscap or and GND terminals. Oscon. 1 Efficiency Temperature (Degrees C)QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474 4A, 500KHZ STEP DOWN SWITCHING REGULATOR 4. To turn the circuit off, ground the This is especially true at high cur- terminal labeled SHDN rent. Needless to say any efficiency calculation should be made using demo 5. To test the synchronization feature circuit terminal voltages as opposed of the demo board apply a sync sig- to load or power source terminal nal to the sync terminal and note voltages. the shift in the operating fre- quency. The Sync signal should have Ripple Voltage Measurements an amplitude greater than 2.2V, The ceramic capacitors used in this duty cycle of between 10% and 90% demo board have almost no ESR and are and a frequency of 580kHz1000kHz extremely effective filters. In order to correctly measure output ripple MEASUREMENT NOTES voltage correct technique is impor- tant. The scope ground lead and Voltage measurements sheath should be removed and the out- The correct way to measure voltage is put ripple measured by placing the at the terminal of the demo board. scope probe directly across the out- Attempts to measure output voltage at put Vout and GND pins as illustrated the load will result in a too low in Figure 4. output voltage due to wiring loss. + + + 5V TO 25V LOAD SUPPLY 25W Figure 3. Proper Measurement Equipment Setup 2 + + +