Keywords: synchoronous buck converter, set top box, LCD TV power supply, step-down controller, auxiliary power
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REFERENCE DESIGN 4365  INCLUDES: Tested Circuit Schematic BOM Test Data 

Reference Design Using the MAX15026 for an Auxiliary Power Supply for LCD TVs or Set-Top Boxes

By:  Venkata Nandam
Apr 15, 2009

Abstract: This application note shows a reference design using the MAX15026 low-cost, high-efficiency synchronous buck converter in an auxiliary power supply for LCD TVs or set-top boxes. This reference design generates a 5V, 5A power supply from a wide, 6.5V to 24V input voltage range.

Key specifications for the reference design are listed below, along with a detailed schematic (Figure 1) and the bill of materials (Table 1) needed for this application.

Specifications

  • Input Voltage Range: 6.5V to 24V
  • Output Voltage: 5V
  • Output Current: 5A
  • Output Voltage Ripple (pk-pk): 50mV
  • Switching Frequency of Converter: 300kHz
  • Overcurrent Protection Limit: 6A
  • Efficiency: > 90% for Normal Operation
Figure 1. Schematic of the MAX15026 synchronous buck converter for a 300kHz switching frequency.
Figure 1. Schematic of the MAX15026 synchronous buck converter for a 300kHz switching frequency.

Table 1. Bill of Materials
Designator Value Description Part Footprint Manufacturer Quantity
C1 220µF, 35V Capacitor EEVFK1V221P 8mm x 10.2mm Panasonic 1
C2, C3 4.7µF, 50V Capacitor GRM31CR71H475KA12L 1206 Murata 2
C4 0.1µF, 50V Capacitor GRM188R71H104KA93D 0603 Murata 1
C5 4.7µF, 6.3V Capacitor GRM188R60J475KE19D 0603 Murata 1
C6 2.2µF, 6.3V Capacitor GRM188R60J225KE19D 0603 Murata 1
C7 1µF, 50V Capacitor GRM21BR71H105KA12L 0805 Murata 1
C8 0.47µF, 16V Capacitor GRM188R71C474KA88D 0603 Murata 1
C9, C16 1200pF, 50V Capacitor GRM1885C1H122JA01D 0603 Murata 2
C10, C11 47µF, 10V Capacitor GRM32ER71A476KE15L 1210 Murata 2
C12, C13, C14 22µF, 10V Capacitor GRM31CR71A226KE15L 1206 Murata 3
C15 560pF, 50V Capacitor GRM1885C1H561JA01D 0603 Murata 1
C17 22pF, 50V Capacitor GRM1885C1H220JA01D 0603 Murata 1
R1 10Ω Resistor SMD 1% Resistor 0603 Vishay 1
R2 10kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R3, R6 2.2Ω Resistor SMD 1% Resistor 0603 Vishay 2
R4 15kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R5 56.2kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R7 3.57Ω Resistor SMD 1% Resistor 1206 Vishay 1
R8 102kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R9 13.7kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R10 2.74kΩ Resistor SMD 1% Resistor 0603 Vishay 1
R11 47.5kΩ Resistor SMD 1% Resistor 0603 Vishay 1
L1 10µH ±20%, 5.5A Inductor SLF12575T-100M5R4-PF 12.5mm x 12.5mm x 7.5mm TDK 1
N1 30V, 6.3A, 35.5mΩ—Q1;
30V, 8.6A, 16.5mΩ—Q2		 Dual MOSFET FDS6982AS 8-SO Fairchild 1
U1 PWM Controller Low-Cost, DC-DC Synchronous Buck Controller with a 4.5V to 28V Operating Range MAX15026BETD+ 14-TDFN-EP Maxim 1

Performance Data

Figure 2. Steady-state output voltage, input voltage, and switching-node voltage at VIN = 24V, VOUT = 5V. Ch1: VOUT; Ch2: VIN; Ch3: LX Node Voltage
Figure 2. Steady-state output voltage, input voltage, and switching-node voltage at VIN = 24V, VOUT = 5V.
Ch1: VOUT; Ch2: VIN; Ch3: Lx Node Voltage


Figure 3. Enable, soft-start, soft-stop and PGOOD sequence waveforms. Ch1: Enable; Ch2: VOUT; Ch3: PGOOD
Figure 3. Enable, soft-start, soft-stop and PGOOD sequence waveforms.
Ch1: Enable; Ch2: VOUT; Ch3: PGOOD


Figure 4. Output and input ripples at VIN = 24V, VOUT = 5V and IOUT = 5A. Ch1: Output Voltage Ripple; Ch2: Input Voltage Ripple
Figure 4. Output and input ripples at VIN = 24V, VOUT = 5V and IOUT = 5A.
Ch1: Output Voltage Ripple; Ch2: Input Voltage Ripple


Figure 5. Converter load transient response with a 2A step change in load current. Ch1: Output Voltage Dip; Ch2: Load-Step Change (1A/div)
Figure 5. Converter load transient response with a 2A step change in load current.
Ch1: Output Voltage Dip; Ch2: Load-Step Change (1A/div)

Test Conditions

VIN = 12V, VOUT = 5V
Load Current Step Change: 1A to 3A
Load Current Slew Rate: 2A/µs

Figure 6. Hiccup-mode overcurrent protection at 7A load current. Ch1: Output Voltage; Ch2: Gate Pulses
Figure 6.Hiccup-mode overcurrent protection with output shorted.
CH1: Output Voltage; CH2: Gate Pulses


Figure 7. Efficiency vs. load current plots.
Figure 7. Efficiency vs. load current plots.

Related Parts
MAX15026 Low-Cost, Small, 4.5V to 28V Wide Operating Range, DC-DC Synchronous Buck Controller Free Samples  


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APP 4365: Apr 15, 2009
REFERENCE DESIGN 4365, AN4365, AN 4365, APP4365, Appnote4365, Appnote 4365

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