Datasheet

0 3 6 9 12 15

100

105

110

115

120

125

MAX. AMBIENT TEMERATURE (°C)

OUTPUT CURRENT (A)

LM21215

SNVS625E –FEBRUARY 2011–REVISED MARCH 2013

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Figure 37. Maximum Ambient Temperature vs. Output Current (0 LFM)

PCB LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance

of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss

in the traces. These can send erroneous signals to the DC-DC converter resulting in poor regulation or instability.

Good layout can be implemented by following a few simple design rules.

1. Minimize area of switched current loops. In a buck regulator there are two loops where currents are switched

at high slew rates. The first loop starts from the input capacitor, to the regulator PVIN pin, to the regulator SW

pin, to the inductor then out to the output capacitor and load. The second loop starts from the output capacitor

ground, to the regulator GND pins, to the inductor and then out to the load (see Figure 38). To minimize both

loop areas, the input capacitor should be placed as close as possible to the VIN pin. Grounding for both the input

and output capacitor should be close. Ideally, a ground plane should be placed on the top layer that connects the

PGND pins, the exposed pad (EP) of the device, and the ground connections of the input and output capacitors

in a small area near pin 10 and 11 of the device. The inductor should be placed as close as possible to the SW

pin and output capacitor.

2. Minimize the copper area of the switch node. The six SW pins should be routed on a single top plane to the

pad of the inductor. The inductor should be placed as close as possible to the switch pins of the device with a

wide trace to minimize conductive losses. The inductor can be placed on the bottom side of the PCB relative to

the LM21215, but care must be taken to not allow any coupling of the magnetic field of the inductor into the

sensitive feedback or compensation traces.

3. Have a solid ground plane between PGND, the EP and the input and output cap. ground connections. The

ground connections for the AGND, compensation, feedback, and soft-start components should be physically

isolated (located near pin 1 and 20) from the power ground plane but a separate ground connection is not

necessary. If not properly handled, poor grounding can result in degraded load regulation or erratic switching

behavior.

4. Carefully route the connection from the VOUT signal to the compensation network. This node is high

impedance and can be susceptible to noise coupling. The trace should be routed away from the SW pin and

inductor to avoid contaminating the feedback signal with switch noise.

5. Make input and output bus connections as wide as possible. This reduces any voltage drops on the input or

output of the converter and can improve efficiency. Voltage accuracy at the load is important so make sure

feedback voltage sense is made at the load. Doing so will correct for voltage drops at the load and provide the

best output accuracy.

Product Folder Links: LM21215