TPS7A39
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ZHCSGP0A –JULY 2017–REVISED SEPTEMBER 2017
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Switcher Choice
This design incorporates a push-pull driver for center-tapped transformers that takes a single-ended supply and
converts the supply to an isolated split rail design. The SN6505B provides a simple small-form factor isolated
supply. The input voltage of the SN6505B can vary from 2.25 V to 5 V, which allows for use with a wide range of
input supplies. The output voltage can be adjusted through the turns ratio of the transformer. Based on the
choice of the transformer this design can be used to create output voltages from ±3.3 V to ±15 V. In this design
the SN6505B was paired with the 750315371 center-tapped transformer from Wurth Electronics™. This
transformer has a turns ratio of 1:1.1 and an isolation rating of 2500 VRMS (the total system isolation was never
tested).
8.2.1.2.2 Full Bridge Rectifier With Center-Tapped Transformer
To create the isolated supply, the SN6505B uses a center-tapped transformer. A full bridge rectifier and
capacitors are required to regulate the signal before reaching the LDO because of the alternating nature of the
input signal. TI recommends having a fast switching and low forward voltage diode to improve efficiency because
of how fast the SN6505 switches; Schottky diodes work well. 图 73 shows the switching nodes of the SN6505 D1
and D2 and also shows where the transformer connects to the full bridge rectifier TP1 and TP2. 图 73 shows the
switching waveforms across the rectifier diodes.
n
V
OUT+
= n·V
IN
V
IN
V
OUT-
= n·V
IN
图 72. Bridge Rectifier With Center-Tapped Secondary Enables Bipolar Outputs
8.2.1.2.3 Total Solution Efficiency
公式 12 shows how the efficiency of the system can be measured by taking the output power and dividing by the
input power. IOUTP = |IOUTN| = IOUT / 2 because this system has two output rails to simplify the efficiency
measurement. When the necessary parameters are measured, and by using 公式 12, the overall system
efficiency can be plotted as in 图 74. 图 74 shows the overall system efficiency for this design, at the maximum
output current of 100 mA (IOUTP = 50 mA, IOUTN = –50 mA) the efficiency of the system is 85%.
η = (IOUTP × VOUTP + IOUTN × VOUTN) / (IIN × VIN)
(12)
8.2.1.2.4 Feedback Resistor Selection
公式 13 and 公式 14 calculate the values of the feedback resistors.
VOUTP = VFBP × (1 + R1P / R2P
)
(13)
(14)
VOUTN = VBUF × (–R1N / R2N
)
For this design the recommended 10-kΩ resistors are used for R2P and R2N. R1P and R1N can be calculated by
substituting R2P and R2N into 公式 15 and 公式 16 because R2P and R2N are already selected
R1P = [(VOUTP / VFBP) – 1] × R2P = [(5 V / 1.188 V) – 1] × 10 kΩ = 32.2 kΩ
R1N = –VOUTN × R2N / VBUF = –(–5 V) × 10 kΩ / 1.19 V = 42 kΩ
(15)
(16)
After solving for 公式 15 and 公式 16, the closest one percent resistors are selected, R1N = 42.2 kΩ and R1P
32.4 kΩ.
=
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