MCP19035_13 datasheet(30/44 Pages) MICROCHIP

30 / 44 page

MCP19035

DS22326B-page 30

 2012-2013 Microchip Technology Inc.

• For double layer boards, a single ground plane

(usually the bottom) is recommended.

• Use short, wide traces for the MOSFET’s gate

drive connection (LDRV and HDRV signals).

• Place the main MOSFET (control/high-side MOS-

FET) as close as possible to the input capacitors.

• Minimize the connections between MOSFETs, the

inductor and the MCP19035 case (PHASE node).

Place this node over a ground plane to minimize

the radiated noise.

• Place the compensation network components

near the MCP19035 case and connect these

components to a low noise ground (signal

ground).

• Locate the VIN decoupling capacitor close to the

MCP19035 case.

• Locate the Bootstrap Circuit capacitor close to the

MCP19035 case.

• Minimize the area of high frequency current loops.

Figure 6-1 helps the PCB designer to identify the main

high frequency current paths for the Synchronous Buck

Converter.

FIGURE 6-1:

High Frequency Current Paths.

All these currents contain high-frequency components

and can produce EMI. Minimizing the area of these

loops will reduce the radiated noise.

The Reverse Recovery of the SR MOSFET Body Diode

current is an important source of noise and EMI. This

current, although very short (less than 10 ns), can

easily reach a few hundred amps, especially when

using low ESR capacitors for input bypass and very fast

MOSFETs for switching transistors. If this current

passes through a path that has a high inductance, it will

produce an intense voltage ringing.

For noise sensitive applications (for example, RF

applications) the excessive voltage ringing in the

PHASE node produced by Reverse Recovery of SR

MOSFET Body Diode can be reduced by placing a low-

value resistor in series with the bootstrap capacitor.

This resistor will slow down the high-side MOSFET

during low-to-high transition, reducing the slew rate of

the SW node signal. The recommended value for this

resistor is between 2.2Ω and 10Ω, and should be

determined by lab measurements. The penalty of

including this resistor is an efficiency reduction. It

should, however, be no more than 0.5%.

CIN

COUT

MCP19035

HDRV

LDRV

PHASE

IIN

ISR

IHDRV

ILDRV

IRR

GND

Where:

IIN = Input Converter current

ISR = Current through the Synchronous Rectifier (SR) MOSFET

IHDRV and ILDRV = MOSFET drivers’ currents

IRR = Current produced by the Reverse Recovery of the SR MOSFET body diode

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