Where L is the inductor value and RESR is the equivalent series resistance (ESR) value of the output

capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated

by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification,

the output voltage ripple can be mated by:

]

×

×

×

×

=

△

IN

OUT

S

OUT

OUT

V

V

C

L

f

V

V

1

8

2

2

In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching

frequency. For simplification, the output ripple can be approximated to:

IN

OUT

S

OUT

OUT

R

V

V

L

f

V

V

×

]

×

×

=

△

1

The characterics of the output capacitor also affect the stability of the regulation system.

The BC3470 can be optimized for a wide range of capacitance and ESR values.

PCB Layout Guide

PCB layout is very important to achieve stable operation. It is highly recommended to duplicate EVB

layout for optimum performance. If change is necessary,please follow these guidelines and take

for reference.

1) Keep the path of switching current short and minimize the loop area formed by Input capacitor,

high-side MOSFET and low-side MOSFET.

2) Bypass ceramic capacitors are suggested to be put close to the Vin Pin.

3) Ensure all feedback connections are short and direct. Place the feedback resistors and

compensation components as close to the chip as possible.

4) VOUT, LX away from sensitive analog areas such as FB.

5) Connect IN, LX, and especially GND respectively to a large copper area to cool the chip to improve

thermal performance and long-term reliability.

ESR

Function Description(Cont.)

2018. 12. 27

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Revision No : 0

FC3470