MP4420 – 36V, 2A SYNCHRONOUS STEP-DOWN CONVERTER

MP4420 Rev.1.0

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18

8/5/2014

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BST Resistor and External BST Diode

A 20ohm resistor in series with BST capacitor is

recommended to reduce the SW spike voltage.

Higher resistance is better for SW spike

reduction, but will compromise the efficiency on

the other hand.

An external BST diode can enhance the

efficiency of the regulator when the duty cycle is

high (>65%). A power supply between 2.5V and

5V can be used to power the external bootstrap

diode and VCC or VOUT is the good choice of

this power supply in the circuit as shown in

Figure 6.

L

3

5

BST

SW

External BST diode

1N4148

Figure 6: Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is IN4148,

and the BST capacitor value is 0.1µF to 1μF.

PCB Layout

PCB layout, especially the input capacitor and

VCC capacitor placement, is very important to

achieve stable operation. For the best results,

follow these guidelines:

1) Place the ceramics input capacitor as close to

IN and GND pins as possible, especially the

small package size (0603) input bypass capacitor.

Keep the connection of input capacitor and IN pin

as short and wide as possible.

2) Place the VCC capacitor to VCC pin and GND

pin as close as possible. Make the trace length of

VCC pin-VCC capacitor anode-VCC capacitor

cathode-chip GND pin as short as possible.

3) Use large ground plane directly connect to

GND pin. Add vias near the GND pin if bottom

layer is ground plane.

4) Route SW, BST away from sensitive analog

areas such as FB.

5) Place the T-type feedback resistor close to

chip to ensure the trace which connects to FB pin

as the short as possible.

Top Layer

Bottom Layer

Figure 7: Recommended PCB Layout