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User's Guide

SNVA633A–March 2012–Revised April 2013

AN-2202 LM34927 Integrated Secondary Side Bias

Regulator for Isolated DC-DC Converters

1 Introduction

An isolated bias supply is implemented in this evaluation board with LM34927 Constant-On-Time

regulator. The LM34927 regulator integrates both the high and low side power switches essential for

creating isolated buck converter.

The board specifications are as follows:

• Input Range: 20 V to 100 V

• Primary Output Voltage: 10 V

• Secondary (Isolated) Output Voltage: 9.5 V

• Maximum Load Current (Primary + Secondary): 300 mA

• Maximum Power Output: 3 W

• Nominal Switching Frequency: 750 kHz

• Efficiency (FIN = 48 V, IOUT2 = 300 mA): 76 percent

• Board size: 2 inch x 2 inch

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SNVA633A–March 2012–Revised April 2013 AN-2202 LM34927 Integrated Secondary Side Bias Regulator for Isolated

DC-DC Converters

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Summary of content (9 pages)

PAGE 1
User's Guide SNVA633A – March 2012 – Revised April 2013 AN-2202 LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters 1 Introduction An isolated bias supply is implemented in this evaluation board with LM34927 Constant-On-Time regulator. The LM34927 regulator integrates both the high and low side power switches essential for creating isolated buck converter.
PAGE 2
UVLO Threshold and Hysteresis www.ti.com Figure 1. LM34927 Evaluation Board (Top View) 2 UVLO Threshold and Hysteresis The UVLO resistors are selected using the following two equations: VIN(HYS) = IHYSR1 (1) and VIN (UVLO,rising) = 1.225V x § R1 © R2 +1· ¹ (2) On this evaluation board R1 = 127 kΩ and R2 = 8.25 kΩ, resulting in UVLO rising threshold at VIN = 20.5 V and a hysteresis of 2.54 V.
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Board Connection And Start-Up www.ti.com The magnetics in this design is optimized for solution size, and therefore limits the output power. The total load at the output should not exceed 300 mA otherwise the coupled inductor will saturate/overheat which can destroy both the coupled inductor and the regulator IC U1. If a sustained over-current situation is to be tolerated, a coupled inductor with higher saturation and rms ratings should be used. The board schematic is shown in Figure 2. D1 VOUT2 9.
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Bill of Materials 4 www.ti.com Bill of Materials Table 1. Bill of Materials Item Description Mfg., Part Number Package Value U1 Sync Switching Regulator Texas Instruments, LM34927 LLP-8 100V, 0.6A T1 Coupled Inductor, 1500 VDC Coilcraft, LPD5030V-333ME 5mm x 5mm 33uH, 0.47A Alternate Part Wurth, 750312750 8.26mm x 6.60mm 22uH, 0.76A Schottky Diode Diodes Inc., DFLS1100-7 Pwr–DI123 100V, 1A D2 Schottky Diode Diodes Inc.
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Performance Curves www.ti.com 5 Performance Curves Figure 3. Efficiency at 750 kHz, VOUT1=10V Figure 4. Steady State Waveform (VIN=48V, IOUT1= 100mA, IOUT2= 200mA) Figure 5.
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PC Board Layout 6 www.ti.com PC Board Layout Figure 6.
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PC Board Layout www.ti.com Figure 7.
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PC Board Layout www.ti.com Figure 8.
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IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.