FP6367 datasheet(8/9 Pages) FITIPOWER

8 / 9 page

FP6367-1.3-MAR-2013

FP6367

fitipower integrated technology lnc.

Application Information

Inductor Selection

An 8.2

μH to 10μH is recommended for general used.

The value of inductor depends on the operating

frequency.

Higher

frequency

allows

smaller

inductor

and

capacitor

but

increases

internal

switching loss.

Two inductor parameters should be

considered, current rating and DCR.

The inductor

with the lowest DCR is chosen for the highest

efficiency.

The inductor value can be calculated as:

IL: inductor ripple current, which is defined as:

(General Setting)

The inductor should be rated for the maximum output

current (IO-MAX) plus the inductor ripple current ( L)

to avoid saturation.

The maximum inductor current

(IL-MAX) is given by:

Capacitor Selection

The small size of ceramic capacitors makes them

ideal for FP6367 applications.

X5R and X7R types

are

recommended

because

they

retain

their

capacitance over wider voltage and temperature

ranges than other types such as Y5V or Z5U.

μF input capacitor and a

μF output capacitor

are sufficient for most FP6367 applications.

When selecting an output capacitor, consider the

output ripple voltage and the ripple current.

The

ESR of capacitor is a major factor to the output

ripple.

For the best performance, a low ESR output

capacitor is required.

The ripple voltage is given

by:

Output Voltage Programming

The output voltage of FP6367 is set by the resistor

divider according to the following formula:

R1 is the upper resistor of the voltage divider.

For

transient response reason, a small feed-forward

capacitor (CF) is required in parallel to the upper

feedback resistor, and 68pF is recommended.

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 shi ts by an amount equal to (Δ

LOAD •

ESR), where ESR is the effective series resistance

of COUT.

LOAD also begins to charge or discharge

COUT, which generates a feedback error signal.

The regulator loop then acts 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 discharged bypass capacitors are effectively

put in parallel with COUT, causing a rapid drop in

VOUT.

No regulator can deliver enough current to

prevent this problem if the load switch resistance is

low and driven quickly.

The only solution is to limit

the rise time of the switch drive so that the load rise

time is limited to approximately ( 5 •

OAD).

Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle current

limit for superior load, line response, protection of

the internal main switch and synchronous rectifier.

The FP6367 switches at a constant frequency

(1.5MHz) and regulates the output voltage.

During

each cycle the PWM comparator modulates the

power transferred to the load by changing the

inductor peak current based on the feedback error

voltage.

During normal operation, the main switch

is turned on for a certain time to ramp the inductor

current at each rising edge of the internal oscillator,

and switched off when the peak inductor current is

above the error voltage.

When the main switch is

off, the synchronous rectifier will be turned on

immediately and stay on until next cycle starts.

Dropout Operation

The FP6367 allows the main switch to remain on for

more than one switching cycle, and increases the

duty cycle while the input voltage is dropping close

to the output voltage.

When the duty cycle

reaches 100%, the main switch will be held on

continuously to deliver current to the output up to

the MOSFET current limit.

Then the output voltage

will be the input voltage minus the voltage drop

across the main switch and the inductor.

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