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
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®
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.
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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
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