tidu271

Power Supply Design and Isolation
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Figure 56. UVLO and OVLO Hysteresis
Refer to LM5069 datasheet SNVS452D and LM5069EVAL evaluation board SNVA184B for device
operation, design procedure, and recommended PCB layout guidelines.
To protect against transients and ground loops, the field side or the sensor side is galvanically isolated
from the control side. The LM5017 device has been used to generate two supplies, nonisolated 3.3 V and
isolated 5 V, from 24-V DC input. The LM5017 is a synchronous buck (step-down) regulator with
integrated MOSFETs. The isolated Fly-Buck topology uses a smaller transformer for power transfer from
primary and secondary. In the Fly-Buck, the regulation is done on the primary side. Therefore, no optocoupler
or auxiliary winding is needed, resulting in smaller size and a more cost effective solution.
When the circuit is in regulation, the buck switch is turned on each cycle for a time determined by R ON and
V IN according to Equation 43.
T
ON
10 10
10 RON
10 137 K
0.571sec
VIN 24 V
The operating frequency can be calculated with Equation 44:
VOUT
7 V
FSW
511KHz
10 10
10 R ON 10 137 K
The non-isolated output voltage (VCC_NON_ISO) is set by two external resistors (R8 and R10). The
regulated output voltage is calculated in Equation 45:
R8 46.4 K
VCC _NON_ISO 1.225 1 1.255 1 6.91 V
R10
10 K
VCC_NON_ISO is given to TPS71533DCK LDO that generates 3.3 V_NON_ISO and is capable of
delivering 50 mA of output current. 3.3 V_NON_ISO is then used to power-up one EEPROM and two
digital isolators.
Selection of rectifier diode D1
The reverse bias voltage across D1 when the high side buck switch is on:
NS
VD1
VIN _ MAX 1 32 32 V
NP
Therefore, the PIV for the selected diode must be greater than the 32 V.
Rectified output (+VCC_ISO) on the secondary side will be:
N
N
VCC _ISO
S
VCC _NON_ISO VF _ D1 6.91 0.4 V 6.51 V
P
D15 is an optional diode connected between VCC_NON_ISO and VCC regulator output. When
VCC_NON_ISO is > VCC, the VCC is supplied from VCC_NON_ISO, which results in reduced losses in
the VCC regulator inside the IC.
(43)
(44)
(45)
(46)
(47)
44 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014
(PLC)
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