Highly efficient DC-DC converters with footprint equivalent to LDO

Efficiency is becoming increasingly important, and in the face of rising energy prices and growing concerns about global warming, designing high-efficiency "green" circuits will effectively reduce operating costs and environmental impact. In cost-conscious applications, LDO regulators are commonly used to generate 1.5V/3A output from a 3.3V input. Since LDOs need to dissipate a lot of power (5.4W), they must be packaged in TO220 or D2PAC. To reduce power consumption, switching regulators are beginning to replace LDO regulators in low-end server designs. However, existing LDO designs have very limited board space, and in most cases, this space is too crowded for a complete switching regulator. For example, two 1206 ceramic capacitors and a D2PAC packaged LDO regulator occupy an area of ​​approximately 18mm x 14mm. It is very difficult to fit a switching regulator into such a small area, especially when an external MOSFET is required. The MAX8505 is a high-efficiency, PWM step-down switching regulator with integrated high-side and low-side MOSFETs in a 16-pin, QSOP package (5mm x 6.5mm). The 1MHz switching frequency allows the use of small inductors and ceramic capacitors for the input and output filters, and a very small footprint. Figure 1 shows the complete MAX8505 application circuit. The MAX8505 accepts a 5V or 3.3V input and provides a 1.5V/3A output. Typical transient current peaks are 5.6A. The main circuit components can be mounted in an area of ​​16mm x 14mm, which can even achieve a smaller footprint than LDO regulators. The total power consumption of the device is only 0.67W, which can save almost 5W of power compared to LDOs. Table 1 provides the corresponding feedback resistor values ​​for 0.9V, 1.5V, 2.0V, and 2.5V outputs. Figure 1. MAX8505 step-down regulator with internal switch Table 1. Feedback resistors at specific voltages

VOUT	R3	R2
0.9V	10kΩ	1.24kΩ
1.5V	10kΩ	8.66kΩ
2.0V	10kΩ	15kΩ
2.5V	10.5kΩ	22.1kΩ

Figure 2 shows the top layer of the MAX8505 PCB layout with the D2PAC packaged device on the right for reference. The top layer of the PCB contains the bulk of the components: the MAX8505, L1, C1, C2, and C4. Seven additional small (0603 package) components are placed on the bottom layer of the board; placement of these components is less critical.


Figure 2. Photo of the MAX8505 switching regulator with the D2PAC packaged device on the right for comparison.

Table 2 compares the LM317, MAX8505, and a typical controller IC with an external MOSFET in terms of power dissipation, footprint, and external component count. The MAX8505 requires the smallest board area and provides the highest efficiency. The step load transient response is shown in Figure 3. The output ripple voltage is less than 10mV, see Figure 4. Table 3 shows the bill of materials for the MAX8505. Table 2. Comparison of the LM317 LDO, MAX8505, and a typical controller IC with an external MOSFET

Parameters	LDO (LM317)	MAX8505	Typical Controller IC
Efficiency (%)	45.5	87	76
Power Dissipation (W)	5.4	0.67	1.42W
Total Components	5	12	11*
Area (mm x mm)	18 x 14, Single Side	16 x 14, Double Side	–
Conditions: 3.3V to 1.5V at 3A, design for adjustable output
* 11 components are needed to achieve less than 10mV output ripple voltage.

Figure 3. Step load transient response: from 0.5A to 2A, then back to 0.5A


Figure 4. Lx switching waveform and output ripple voltage

Table 3. MAX8505 bill of materials

Designation	QTY	Description	Part Number
C1	1	Input capacitor, 1206	10µF/6.3V
C2	1	Output capacitor, 1210	47µF/6.3V
C3	1	Decoupling capacitor, 0603	0.1µF/6.3V
C4	1	Snubber capacitor, 0603	3300pF/50V
C5	1	Compensation capacitor, 0603	220pF/50V
C6	1	Decoupling capacitor, 0603	0.22µF/6.3V
R1	1	Compensation resistor, 0603	51.1kΩ, 1%
R2	1	Voltage set resistor, 0603	See Table 1
R3	1	Voltage set resistor, 0603	See Table 1
L1	1	2.2µF/7A SMD inductor, 6.3mm x 6.2mm x 2.5mm	Toko
D1	1	Small signal diode	1N4148 or CMDSH-3
IC1	1	3A internal switch regulator	MAX8505