SP7656EN2-LSLASHTR datasheet(7/10 Pages) EXAR

7 / 10 page

11/07/08 SP7656 PowerBlox



Inductor Selection

Select the Inductor for inductance L and

saturation current

ISAT. Make sure to select an

inductor with

ISAT higher than the programmed

overcurrent level.

The inductance can be calculated from:













































Irip

Vin

Vout

Vin

where:

VIN is converter input voltage

VOUT is converter output voltage

f is the switching frequency (300kHz)

IRIP is inductor peak-to-peak current ripple

(nominally set to 30% of IOUT)

Keep in mind that a higher

inductor ripple

current results in a smaller inductance – and

smaller inductor. A smaller inductor has the

advantages of small size, low DC equivalent

resistance DCR, high saturation current and

allows the use of a lower output capacitance

to meet a given step load transient. A higher

inductor

ripple

current

level

also

has

disadvantages. It increases the output voltage

ripple and increases the current at which

converter enters Discontinuous Conduction

Mode. The output current at which converter

enters DCM is ½ of

IRIP. Note that a negative

current step load that drives the converter into

DCM will result in a large output voltage

transient. Therefore the lowest current for a

step load should be larger than ½ of

IRIP.

Restriction on high duty cycle operation

The SP7656 is optimized to provide superior

performance for low duty cycle applications.

For applications with output voltages below

9V, the device will operate normally at the

expected 600kHz switching frequency for

conversions with less than 50% duty cycle.

For applications with output voltages below 9V

and greater than 50% duty cycle, the device

will enter into a pulse skipping mode. This is

due to the FB voltage to internal ramp voltage

ratio of the device, and is an intended

behavior from an architecture optimized for

superior performance in low duty cycle

conversion applications and results in a small

increase in output ripple voltage for any given

circuit. For output voltages above 9V, the

device will operate at a constant 600kHz

switching frequency across the specified duty

cycle range up to 100%.

Layout Suggestions

Place the input capacitor(s) as close

as possible to the 7656 IC.

ii)

Create a pad under the IC that

connects the power pad (pin 9) to the

inductor. Duplicate this pad through

the pcb layers if present, and on the

bottom side of the PCB. Use multiple

vias to connect these layers to aid in

heat dissipation. Do not oversize this

pad - since the LX node is subjected

to very high dv/dt voltages, the stray

capacitance formed between these

islands and the surrounding circuitry

will tend to couple switching noise

iii)

Connect the Schottky diode cathode

as close as possible to the LX node

and inductor input side. Connect the

anode to a large diameter trace or a

copper area that connects the input

ground to the output ground.

iv)

The output capacitors should be

placed as close to the load as

possible. Use short wide copper

regions to connect output capacitors

to load to avoid inductance and

resistances.

Keep other sensitive circuits and

traces away from the LX node in

particular and away from the power

supply completely if possible.

For more detail on the SP7656 layout see the

SP7656EVB (Evaluation Board) Manual

available on our web site. Each layer is shown

in detail as well as a complete bill of materials

and

performance

characterization.

Cin

Cout

Lout

7656

Diode

SP7656EN2-L/TR датащи(PDF) 7 Page - Exar Corporation

SP7656EN2-L/TR датащи(HTML) 7 Page - Exar Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10

датащи скачать

ссылки URL