BL8521 datasheet(5/8 Pages) BELLING

5 / 8 page

REV2.0

Page5

BL8521

5.5V, 1.4MHz, 3A Synchronous Buck Converter

requirements.

Dry

tantalum,

special

polymer,

aluminum electrolytic and ceramic capacitors are all

available

surface

mount

packages.

Special

polymer capacitors offer very low ESR but have

lower

capacitance

density

than

other

types.

Tantalum capacitors have the highest capacitance

density but it is important to only use types that have

been surge tested for use in switching power

supplies. Aluminum electrolytic capacitors have

significantly higher ESR but can be used in cost-

sensitive applications provided that consideration is

given to ripple current ratings and long term

reliability. Ceramic capacitors have excellent low

ESR characteristics but can have a high voltage

coefficient and audible piezoelectric effects. The

high Q of ceramic capacitors with trace inductance

can also lead to significant ringing.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are

now becoming available in smaller case sizes. Their

high ripple current, high voltage rating and low ESR

make them ideal for switching regulator applications.

However, care must be taken when these capacitors

are used at the input and output. When a ceramic

capacitor is used at the input and the power is

supplied by a wall adapter through long wires, a load

step at the output can induce ringing at the input, VIN.

At best, this ringing can couple to the output and be

mistaken as loop instability. At worst, a sudden

inrush of current through the long wires can

potentially cause a voltage spike at VIN large enough

to damage the part.

Checking Transient Response

The regulator loop response can be checked by

looking at the load transient response. Switching

regulators take several cycles to respond to a step

in load current. When a load step occurs, VOUT

immediately shifts by an amount equal to

ILOAD(ESR),

where ESR is the effective series resistance of COUT.

ILOAD also begins to charge or discharge COUT

generating a feedback error signal used by the

regulator to return VOUT to its steady-state value.

During this recovery time, VOUT can be monitored for

overshoot or ringing that would indicate a stability

problem. The COMP pin external components and

output capacitor shown in Typical Application Circuit

will

provide

adequate

compensation

for

most

applications.

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%.

It is often useful to analyze individual losses to

determine what is limiting the efficiency and which

change would produce the most improvement.

Efficiency can be expressed as :

Efficiency = 100%

(L1+ L2+ L3+ ...) where L1, L2,

etc. are the individual losses as a percentage of

input power. Although all dissipative elements in the

circuit produce losses, two main sources usually

account for most of the losses: VDD quiescent

current and I

2R losses.

The VDD quiescent current loss dominates the

efficiency loss at very low load currents whereas the

2R loss dominates the efficiency loss at medium to

high load current. In a typical efficiency plot, the

efficiency curve at very low load currents can be

misleading since the actual power lost is of no

consequence.

1. The VDD quiescent current is due to two

components: the DC bias current as given in the

electrical characteristics and the internal main switch

and synchronous switch gate charge currents. The

gate charge current results from switching the gate

capacitance of the internal power MOSFET switches.

Each time the gate is switched from high to low to

high again, a packet of charge

Q moves from VDD

to ground. The resulting

Q/t is the current out of

VDD that is typically larger than the DC bias current.

In continuous mode, IGATECHG = f(QT+QB) where QT

and QB are the gate charges of the internal top and

bottom switches.

Both the DC bias and gate charge losses are

proportional to VDD and thus their effects will be

more pronounced at higher supply voltages.

2. I

2R losses are calculated from the resistances of

the internal switches, RSW and external inductor RL.

In continuous mode the average output current

flowing through inductor L is “chopped” between the

main switch and the synchronous switch. Thus, the

series resistance looking into the LX pin is a function

of both top and bottom MOSFET RDS(ON) and the

duty cycle (D) as follows :

RSW = RDS(ON)TOP x D + RDS(ON)BOT x (1"D)

The RDS(ON) for both the top and bottom MOSFETs

can be obtained from the Typical Performance

Characteristics curves. Thus, to obtain I

2R losses,

simply add RSW to RL and multiply the result by the

square of the average output current. Other losses

including CIN and COUT ESR dissipative losses and

BL8521 Datasheet(PDF) 5 Page - SHANGHAI BELLING CO., LTD.

BL8521 Datasheet(HTML) 5 Page - SHANGHAI BELLING CO., LTD.

Html Pages

1 2 3 4 5 6 7 8

Datasheet Download

Link URL