User's Guide
SNVA675A–August 2012–Revised April 2013
AN-2284 LM34926 Isolated Evaluation Board
1 Introduction
An isolated bias supply is implemented in this evaluation board with LM34926 Constant-On-Time
regulator. LM34926 regulator integrates both the high and low side power switches essential for creating
isolated buck converter.
Board Specifications:
• Input Range: 20V to 100V
• Primary Output Voltage: 10V
• Secondary (Isolated) Output Voltage: 9.5V
• Maximum Load Current (Primary + Secondary): 250mA
• Maximum Power Output: 2.5W
• Nominal Switching Frequency: 750kHz
• Efficiency (FIN = 36V, IOUT2 = 250mA): 77 percent
• Board size: 2 inch x 2 inch
Figure 1. LM34926 Evaluation Board (Top View)
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1
SNVA675A–August 2012–Revised April 2013 AN-2284 LM34926 Isolated Evaluation Board
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+
VOUT2
D1
C4
T1
9.5V
+
+
+
VIN
BST
RON
RTN
SW
VCC
FB
VIN VOUT1
R6
R2
R3
C3
C2
C1
R4
R1
UVLO
C6
LM34926
20V-100V
1
2
3
4
5
6
8
7
(TP7)
+C5
1 F0.1 F
127 NŸ
8.25 NŸ
130 NŸ
(J1)
UVLO U1 R5
(J2)
GND
R8
C7
C8
+
D2
EXP 1 NŸ
7.32 NŸ
1 F
0Ÿ
47 H
0.01 F
1 F
1000 pF
0.1 F
46.4 NŸ
(TP11)
SW
10V
(1:1)
(J3)
IGND
1 F
R10
2 NŸ
R1
VIN (UVLO,rising) = 1.225V x R2+1
¹
·
©
§
VIN(HYS) =IHYSR1
UVLO Threshold and Hysteresis
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2 UVLO Threshold and Hysteresis
The UVLO resistors are selected using the following two equations:
(1)
and
(2)
On this evaluation board R1 = 127kΩand R2 = 8.25kΩ, resulting in UVLO rising threshold at VIN = 20.5V
and a hysteresis of 2.54V.
2.1 Board Connection and Start-up
The input connections are made using TP1 (VIN) and TP2 (GND) terminals. The primary output appears
at TP3 (VOUT1) and TP4 (GND). The secondary (isolated) output is available across TP5 (VOUT2) and
TP6 (IGND). The input voltage should be gradually increased above UVLO set point of 20.5V. Both the
outputs (VOUT1 and VOUT2) should be close to 10V at this point. This board is designed to function with
input voltage range of 20V to 100V. The minimum VIN threshold can be changed by changing the UVLO
resistors R1, R2. VIN should not exceed 100V.
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 250mA, 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.
Figure 2. Complete Evaluation Board Schematic
2AN-2284 LM34926 Isolated Evaluation Board SNVA675A–August 2012–Revised April 2013
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Bill of Materials
3 Bill of Materials
Table 1. Bill of Materials
Item Description Mfg., Part Number Package Value
U1 Sync Switching Texas Instruments, LM34926 WSON-8 100V, 300mA
Regulator
T1 Coupled Inductor, Coilcraft, LPD5030V-473ME 5mm x 5mm 47uH, 0.47A
1500 VDC
Alternate Part Wurth, 750312750 8.26mm x 22uH, 0.76A
6.60mm
D1 Schottky Diode Diodes Inc., DFLS1100-7 Pwr–DI123 100V, 1A
D2 Schottky Diode Diodes Inc., SDM10U45-7 SOD–523 40V, 100mA
C1 Ceramic Capacitor MuRata, GRM32CR72A105KA35L 1210 1uF, 100V, X7R
C2 Ceramic Capacitor TDK, C1608X7R1C103K 0603 0.01uF, 16V, X7R
C3, C4 Ceramic Capacitor TDK, C2012X7R1E105K 0805 1uF, 25V, X7R
C5 Ceramic Capacitor Kemet, C0805C104K1RACTU 0805 0.1uF, 100V, X7R
C6 Ceramic Capacitor TDK, C1608X7R1C105K 0603 1uF, 16V, X7R
C7 Ceramic Capacitor Murata, GRM188R71E102KA01D 0603 1000pF,25V, X7R
C8 Ceramic Capacitor AVX, 0603YC104KAT2A 0603 0.1uF, 16V, X7R
R1 Resistor Vishay/Dale, CRCW0805127KFKEA 0805 127kΩ, 1%
R2 Resistor Vishay/Dale, CRCW08058K25FKEA 0805 8.25kΩ, 1%
R3 Resistor Vishay/Dale, CRCW0805130KFKEA 0805 130kΩ, 1%
R4 Resistor Panasonic, ERJ-3EKF7321V 0603 7.32kΩ, 1%
R5 Resistor Panasonic, ERJ-3EKF1001V 0603 1.0kΩ,1%
R6 Resistor Yageo, RC0603JR-070RL 0603 0Ω
R8 Resistor Panasonic, ERJ-3EKF4642V 0603 46.4kΩ, 1%
R10 Resistor Panasonic, ERJ-6GEYJ202V 0805 2kΩ, 5%
3
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50 100 150 200 250 300
50
60
70
80
90
100
EFFICIENCY (%)
LOAD CURRENT (mA)
VIN=36V VIN=24V VIN=48V
VOUT2=10V, IOUT1=0
Performance Curves
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4 Performance Curves
Figure 3. Efficiency at 750kHz, VOUT1=10V
Figure 4. Steady State Waveform (VIN=48V, IOUT1=0mA, IOUT2= 100mA)
Figure 5. Step Load Response (VIN=48V, IOUT1=0, Step Load on IOUT2=80mA-180mA)
4AN-2284 LM34926 Isolated Evaluation Board SNVA675A–August 2012–Revised April 2013
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