Datasheet

use of a TGM-H281NF transformer from Halo
®
Electronics. The transformer secondary side is connected to voltage
doublers that rectify the AC outputs into DC outputs. Two MAX1659 LDOs regulate the voltages to +12V and -12V,
respectively. The Zener diodes (D3 and D6) protect the LDOs by keeping their input voltages below 16.1V.
The input power can be from the J1 Pmod-compatible connector or from an external power supply connected to the
EXT_+3.3V and DGND connectors. To change the output voltages of this reference design, simply change the
feedback resistors (R2–R5) of the LDOs (U2 and U3).
The output voltage of the MAX1659 LDOs is set by the following equations:
V
OUT
= V
SET
× (1 + R2/R3) for U2
V
OUT
= V
SET
× (1 + R4/R5) for U3
Where V
SET
= 1.21V.
For example, for the ±15V outputs application, change R2 and R4 to 187kΩ, and change R3 and R5 to 16.2kΩ.
This circuit can also be configured for asymmetrical applications.
In applications sensitive to output voltage ripple, a lowpass LC pi filter can be added in front of the LDO input.
The isolation transformer in this design has an isolation voltage of 1500V
RMS
. It is recognized by UL 60950 and EN
60950 and falls into the "functional" insulation class.
Quick Start
Required equipment:
Lakewood (MAXREFDES7#) board
3.3V 1A power supply
Two digital voltmeters
Procedure
The Lakewood board is fully assembled and tested. Follow the steps below to verify board operation.
1. Place the shunt on jumper JU1 to the 1–2 position.
2. Connect the positive terminal of the power supply to the EXT_+3.3V connector.
3. Connect the negative terminal of the power supply to the DGND connector.
4. Connect the positive terminal of the first voltmeter to the +12V connector.
5. Connect the negative terminal of the first voltmeter to the GND1 connector.
6. Connect the positive terminal of the second voltmeter to the -12V connector.
7. Connect the negative terminal of the second voltmeter to the GND2 connector.
8. Turn on the power supply.
9. Use the respective voltmeters to measure the corresponding positive and negative output voltages.
Lab Measurements
The Lakewood design was tested for two pairs of output voltage rails: ±12V and ±15V. Other voltage rails can be
achieved by modifying the resistor values of R2, R3, R4, and R5. When set for ±12V outputs, the circuit can deliver
a maximum load current of approximately 90mA for each of the two rails simultaneously. When set for ±15V
outputs, the circuit can deliver a maximum load current of approximately 40mA for each of the two rails
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