QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 610 DUAL OUTPUT ISOLATED POWER SUPPLY LT3804, LT3781, and LTC1693-1 DESCRIPTION Demonstration circuit 610 is a dual output high effi- output feedback amplifier to achieve high regulation ac- ciency isolated DC/DC power supply with 36V to 72V curacy. The complete design is mounted within a stan- input range. It has two outputs: 3.3V at 15A max and dard half brick PCB area with about half inch height. 1.8V at 15A max. The LT3804, a secondary side dual Design files for this circuit board are available. Call output controller, is used to tightly regulate both out- the LTC factory. puts. True differential Kelvin sensing is used for each Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Minimum Input Voltage 36V Maximum Input Voltage 72V V V = 36V to 72V, I = 0A to 15A, 3.31V 2% OUT1 IN OUT1 I = 0A to 15A OUT2 V V = 36V to 72V, I = 0A to 15A, 1.81V 2% OUT2 IN OUT1 I = 0A to 15A OUT2 Typical Output Ripple V V = 48V, I = 15A (20MHz BW) 30mV OUT1 IN OUT1 PP Typical Output Ripple V V = 48V, I = 15A (20MHz BW) 15mV OUT2 IN OUT2 PP Line 0.1% Output Regulation Load 0.2% Nominal Switching Frequency 230kHz V = 48V, I = I = 15A IN OUT1 OUT2 Efficiency 85% Typical V = 48V, I = I = 15A. With output current sense IN OUT1 OUT2 87% Typical resistor R55=R49=0 ohm Power Good (high) Normal operation (V = 3.3V and V = 1.8V) 9.3V Typical OUT1 OUT2 Logic Low Voltage-Off 0.6V MAX On/Off Control Logic High Voltage-On 1V MIN Isolation Voltage Basic Insulation 1500VDC 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 610 DUAL OUTPUT ISOLATED POWER SUPPLY OPERATING PRINCIPLES Demonstration circuit 610 features the LT3804, LT3781 The LT3804 provides output current limit function for and LTC1693-1 controllers. The basic power stage to- both 1.8V and 3.3V outputs. The current limits for 3.3V pology is a two-switch forward converter with synchro- and 1.8V outputs are estimated to be 50mV/R55 and nous rectification. The primary side controller uses an 50mV/R49, respectively. LT3781, a current mode two-switch forward controller A planar transformer PA0191 by Pulse Engineering is with built-in MOSFET drivers. On the secondary side, the employed as the power transformer in this design. This LT3804 is used to provide the voltage feedback for the transformer is constructed on a PQ20 core with nine 3.3V output (Vo1). The output from the built-in opto- turns of primary windings, two turns of secondary wind- coupler driver is fed into an opto-coupler (MOC207) and ings and seven turns of auxiliary windings for the then transferred to LT3781 on the primary side to com- LT3781 bias supply. Si7892DP MOSFETs are selected for plete the 3.3V regulation. An LTC1693-1 high speed dual the secondary side due to their low Rds (on), 30V V DSS N-channel MOSFET driver provides the gate driving for rating and the compact and thermally enhanced Power- the synchronous MOSFETs at the 3.3V output stage. The Pak SO-8 package. LTC1693-1 drivers input signals come from SG and BG The switching frequency of the circuit is about 230KHz. outputs of the LT3781 through two small gate drive 1500V input to output isolation is provided. Additional transformers (T2 and T3). features of this design include primary side on/off con- The LT3804 also precisely regulates the 1.8V output trol, input over voltage protection, under voltage lockout, (Vo2) by further reducing and controlling the duty cycle soft start and board over temperature shutdown. of the switching waveform from the power transformer Without airflow, the hottest spot on the demo board has (T1) secondary winding. In fact, the 1.8V circuit is a a temperature rise about 75C at 48Vin and full loads. special synchronous buck converter whose input is a Airflow is required at heavy load with elevated tempera- pulsed waveform instead of a DC voltage. ture operation. True differential remote sensing is provided for both outputs to achieve high regulation accuracies. Power good indicator PGOOD, will be high only if both outputs are within +/-10% of their nominal values. QUICK START PROCEDURE Demonstration circuit 610 is easy to set up to evaluate 1. With power off, connect the input power supply to the performance of the LT3804, LT3781, and LTC1693-1. +Vin (36V-72V) and Vin (input return). Refer to Figure 1 for proper measurement equipment 2. The on/off control pin should be left open for normal setup and follow the procedure below: operation. (Connecting the on/off pin to the Vin will NOTE: When measuring the input or output voltage rip- turn off the converter.) ple, care must be taken to avoid a long ground lead on 3. Connect the 3.3V load (Load 1 in Figure 1) between the oscilloscope probe. Measure the input or output +Vo1 and Vo RTN connect the 1.8V load (Load 2 in voltage ripple by touching the probe tip directly across Figure 1) between +Vo2 and Vo RTN. (Initial loads: no the Vin or Vout and GND terminals. See Figure 2 for load.) proper scope probe technique. 2