BL8521 datasheet(4/8 Pages) BELLING

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BL8521

5.5V, 1.4MHz, 3A Synchronous Buck Converter

APPLICATION INFORMATION

The basic BL8521 application circuit is shown in

Typical Application Circuit. External component

selection is determined by the maximum load

current and begins with the selection of the

inductor value and operating frequency followed

by CIN and COUT.

Output Voltage Programming

The output voltage is set by an external resistive

divider according to the following equation:

VOUT =VREF×(1+R1/R2),

where VREF equals to 0.8V typical.

RT Pin Resistor Selection to set Frequency

The resistor connected between RT pin and Gnd

is used to set the oscillation frequency of BL8521.

The relation between RT resistor and frequency is

shown below:

Inductor Selection

For a given input and output voltage, the inductor

value and operating frequency determine the

ripple current. The ripple current

IL increases

with

higher

VIN and

decreases

with

higher

inductance.

ΔI= [VOUT/( f×L)] ×[1- VOUT/VIN]

Having a lower ripple current reduces the ESR

losses in the output capacitors and the output

voltage ripple. Highest efficiency operation is

achieved at low frequency with small ripple

current. This, however, requires a large inductor.

A reasonable starting point for selecting the ripple

current is

I = 0.4(IMAX). The largest ripple current

occurs at the highest VIN. To guarantee that the

ripple current stays below a specified maximum,

the inductor value should be chosen according to

the following equation :

L=[VOUT/ f

× ΔIL(MAX)] ×[ 1- VOUT /VIN(MAX)]

Inductor Core Selection

Once the value for L is known, the type of inductor

must

selected.

High

efficiency

converters

generally cannot afford the core loss found in low

cost powdered iron cores, forcing the use of more

expensive ferrite or mollypermalloy cores. Actual

core loss is independent of core size for a fixed

inductor value but it is very dependent on the

inductance selected. As the inductance increases,

core losses decrease. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase.

Ferrite designs have very low core losses and are

preferred at high switching frequencies, so design

goals

can

concentrate

copper

loss

and

preventing saturation. Ferrite core material saturates

“hard”, which means that inductance collapses

abruptly when the peak design current is exceeded.

This results in an abrupt increase in inductor ripple

current and consequent output voltage ripple.

Do not allow the core to saturate!

Different core materials and shapes will change the

size/ current and price/current relationship of an

inductor. Toroid or shielded pot cores in ferrite or

permalloy materials are small and don't radiate

energy but generally cost more than powdered iron

core inductors with similar characteristics. The

choice of which style inductor to use mainly depends

on the price vs. size requirements and any radiated

field/EMI requirements.

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the top MOSFET.

To prevent large ripple voltage, a low ESR input

capacitor sized for the maximum RMS current

should be used.

Several capacitors may also be paralleled to meet

size or height requirements in the design.

The selection of COUT is determined by the effective

series resistance (ESR) that is required to minimize

voltage ripple and load step transients, as well as

the amount of bulk capacitance that is necessary to

ensure that the control loop is stable. Loop stability

can be checked by viewing the load transient

response as described in a later section.

Multiple capacitors placed in parallel may be needed

to meet the ESR and RMS current handling

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