9.3 Power Supply Recommendations

The LM5013 buck converter is designed to operate from a wide input voltage range between 6 V and 100 V. In

addition, the input supply must be capable of delivering the required input current to the fully-loaded regulator.

Use Equation 19 to estimate the average input current.

K

˜

˜

IN

OUT

OUT

IN

V

I

V

I

(19)

where

• η is the efficiency.

If the converter is connected to an input supply through long wires or PCB traces with a large impedance,

take special care to achieve stable performance. The parasitic inductance and resistance of the input cables

can have an adverse effect on converter operation. The parasitic inductance in combination with the low-ESR

ceramic input capacitors form an underdamped resonant circuit. This circuit can cause overvoltage transients

at VIN each time the input supply is cycled ON and OFF. The parasitic resistance causes the input voltage to

dip during a load transient. If the converter is operating close to the minimum input voltage, this dip can cause

false UVLO fault triggering and a system reset, in addition to potential stability issues. The circuit can be damped

with a "parallel damping network." For example, a 22-μF damping capacitor in series with a 1.4-Ω resistor

connected to the VIN node, creates a parallel damped network, providing sufficient damping for a 8.2-μH input

filter inductor and 4.4-μF ceramic input capacitance. Damping is not only needed for an input EMC filter, but also

when the application utilizes a power harness, which can present a large input loop inductance. For example,

two cables (one for VIN and one for GND), each one meter (approximately three feet) long with approximately

1-mm diameter (18 AWG), placed 1 cm (approximately 0.4 inch) apart forms a rectangular loop resulting in about

1.2 µH of inductance. The Input Filter Design for Switching Power Supplies application report provides more

detail on this topic.

An EMI input filter is often used in front of the regulator that, unless carefully designed, can lead to instability as

well as some of the effects mentioned above. The Simple Success with Conducted EMI for DC-DC Converters

application report provides helpful suggestions when designing an input filter for any switching regulator.

9.4 Layout

9.4.1 Layout Guidelines

PCB layout is a critical portion of good power supply design. There are several paths that conduct high slew-rate

currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise and EMI or

degrade the power supply performance.

• To help eliminate these problems, bypass the VIN pin to GND with a low-ESR ceramic bypass capacitor with

a high-quality dielectric. Place CIN as close as possible to the VIN and GND pins. Grounding for both the

input and output capacitors must consist of localized top-side planes that connect to the GND pin and GND

PAD.

• Minimize the loop area formed by the input capacitor connections to the VIN and GND pins.

• Place the inductor and Schottky diode close to the SW pin. Minimize the area of the SW trace or plane to

prevent excessive capacitive coupling.

• Place the Schottky diode anode pin in close proximity to input capacitor ground or return.

• Tie the GND pin directly to the power pad under the device and to a heat-sinking PCB ground plane.

• Use a ground plane in one of the middle layers as a noise shielding and heat dissipation path.

• Have a single-point ground connection to the plane. Route the ground connections for the feedback, soft

start, and enable components to the ground plane, which prevents any switched or load currents from flowing

in analog ground traces. If not properly handled, poor grounding results in degraded load regulation or erratic

output voltage ripple behavior.

• Make VIN, VOUT, and ground bus connections as wide as possible, which reduces any voltage drops on the

input or output paths of the converter and maximizes efficiency.

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