Abstract: Switching regulators can replace LDOs for generating 1.5V/3A output from a 3.3V input. Board space in existing LDO designs is typically limited, often too small for a switching regulator. The MAX8505 switching regulator with MOSFETs is packaged in a 16-pin QSOP and fits an LDO footprint.
In cost-sensitive applications, LDOs are the common choice for generating 1.5V/3A output from a 3.3V input. Due to the high power dissipation (5.4W) in the LDO, either a TO220 or D2PAC package must typically be selected. To reduce power dissipation, switching regulators have begun to replace LDOs in low-end server designs. However, board space in existing LDO designs is often limited and, in most cases, too small for a complete switching regulator. A D2PAC LDO regulator with two 1206 ceramic capacitors, for example, occupies about 18mm x 14mm. Fitting a switching regulator into such a small area is not an easy task, particularly if external MOSFETs are required.
The MAX8505 is a high-frequency PWM step-down switching regulator with both high- and low-side MOSFETs in a 16-pin QSOP package (5mm x 6.5mm) . The 1MHz switching frequency allows use of a small inductor and ceramic capacitors for both input and output filtering, thus resulting in a very small footprint. Figure 1 shows the complete MAX8505 circuit. It accepts either a 5V or 3.3V input, and generates a 1.5V at 3A output. Typical peak transient current is 5.6A. The major circuit components fit into a 16mm x 14mm footprint, achieving an even smaller footprint than LDOs. With a total dissipation of only 0.67W, furthermore, it removes nearly the entire power dissipation of the LDO. Table 1 shows feedback resistor values for 0.9V, 1.5V, 2.0V, and 2.5V outputs.
Table 1. Resistor Values at Specific Voltage Outputs
Vout
R3
R2
0.9V
10k
1.24k
1.5V
10k
8.66k
2.0V
10k
15k
2.5V
10.5k
22.1k
Figure 2 shows the top layer of the MAX8505 PCB layout with a D2PAC package positioned on the right for reference. The PCB top layer contains the largest components: the MAX8505, L1, C1, C2, and C4. Seven additional small (0603 package) components are placed on the opposite side of the board, but their placement is less critical.
Figure 2. Picture of the MAX8505 switching regulator with a D2PAC device on the side for comparison.
Table 2 compares power dissipation, footprint, and total external component count of the LM317, the MAX8505, and a typical controller with external MOSFETs. The MAX8505 requires the smallest board space and provides the highest efficiency. Step-load transient response is shown in Figure 3. Output ripple voltage, shown in Figure 4, is less than 10mV. Table 3 presents the bill of materials for the MAX8505.
Table 2. Comparison of the LM317 LDO, MAX8505, and One Typical Controller IC with External MOSFETs.
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 X14,
Single Side
16 X14,
Double Side
?
Conditions: 3.3V to 1.5V@3A, design for adjustable output
* 11 components are needed to achieve less than 10mV output ripple voltage.
Figure 3. Step load-transient response: 0.5A to 2A back to 0.5A.
Figure 4. LX switching waveform and the output ripple voltage.
Table 3. MAX8505 Bill of Materials
Designation
QTY
Description
Part Number
C1
1
Input capacitor, 1206
10uF/6.3V
C2
1
Output capacitor, 1210
47uF/6.3V
C3
1
Decoupling capacitor, 0603
0.1uF/6.3V
C4
1
Snubber capacitor, 0603
3300pF/50V
C5
1
Compensation capacitor, 0603
220pF/50V
C6
1
Decoupling capacitor, 0603
0.22uF/6.3V
R1
1
Compensation resistor, 0603
51.1k, 1%
R2
1
Voltage set resistor, 0603
See Table 1
R3
1
Voltage set resistor, 0603
See Table 1
L1
1
2.2uF/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
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