Application Note 125 February 2010 Using the LTM2881 as an Isolated 5V Power Supply Brian Jadus Features: 2500VRMS Isolated 5V 1W Power With No External Components n ® The LTM 2881 is an isolated RS485 transceiver that guards against large ground-to-ground differentials. An onboard DC/DC converter provides isolated 5V power to the output. Figure 1 shows the LTM2881 configured as a dedicated isolated power supply with the RS485 transceiver disabled, providing a simple powerful general purpose isolated power supply capable of delivering 1W at 5V. Figure 2 shows a logically-controlled switched output by taking advantage of the isolated RS485 driver to control a discrete PMOSFET. The controlling signal DI operates relative to the VL supply, which supports voltages from 1.62V to 5.5V. In both configurations, observation of a VCC2 fault is accomplished by observing a high impedance condition on RO or DOUT . When VCC2 is greater than 2.6V, RO and DOUT are driven to the appropriate logic level based on the inputs A and B and DIN. When VCC2 is less than 2.4V, RO and DOUT are high impedance. n n n n n 5V (LTM2881-5) or 3.3V (LTM2881-3) Input Supply Voltage 5V DC Output Delivers 200mA at 62% Efficiency with a 5V Input (LTM2881-5) or 120mA at 52% Efficiency with a 3.3V Input (LTM2881-3) Zero Current (Typical) Shutdown Mode When ON Pin is Low. Overcurrent and Overtemperature Protection No External Components Required. Decoupling Capacitors are Integrated. Small Package: 15mm × 11.25mm × 2.8mm L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VCC VCC 1.8V TO 5.5V VL RO ON TE DE DI ON RO IOUT B TE DE Z DIN GND2 OFF ON CMOS OUTPUT PWR IOUT REGULATED 5V SWITCHED 5V DI LTM2881 330k Z DIN DOUT GND IRLML6402 B GND2 AN125 F01 Figure 1. LTM2881 Configured as a Dedicated Isolated 5V Power Supply VCC2 A RE LTM2881 DOUT GND VL VCC REGULATED 5V A ISOLATION BARRIER OFF ON PWR VCC2 ISOLATION BARRIER VCC CMOS INPUT AN125 F02 Figure 2. Switched 5V Power with Isolated CMOS Logic Connection to VL Voltage Interface an125f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. AN125-1 Application Note 125 Typical Performance Characteristics TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5V, VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted. IOUT Current vs Temperature; RS485 Disabled: DE = TE = 0, VCC2 = 4.75V VCC Supply Current vs Temperature at IOUT = 100mA on VCC2 350 300 5.2 LTM2881-3 300 LTM2881-5 250 VCC2 VOLTAGE (V) 200 200 LTM2881-5 150 LTM2881-3 150 100 100 0 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 5.0 5.5V 4.9 5.0V 3.3V 4.8 3.0V 4.7 50 50 VCC = 3.0V TO 3.6V, LTM2881-3 VCC = 4.5V TO 5.5V, LTM2881-5 5.1 250 IOUT (mA) ICC CURRENT (mA) VCC2 Output Voltage vs IOUT Load Current 0 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 4.5 AN125 G01 3.6V 4.5V 4.6 0 50 100 200 150 250 IOUT LOAD CURRENT (mA) 300 AN125 G03 AN125 G02 VCC2 Power Efficiency VCC2 Load Step (100mA) VCC2 Noise 70 LTM2881-5 VCC2 100mV/DIV EFFICIENCY (%) 60 10mV/DIV 50 LTM2881-3 IOUT 50mA/DIV 40 100μs/DIV 30 AN125 G05 200μs/DIV AN125 G06 20 10 0 50 100 150 IOUT OUTPUT CURRENT (mA) 200 AN125 G04 an125f AN125-2 Linear Technology Corporation LT 0210 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2010