FAN5353 データシートの表示（PDF） - ON Semiconductor

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FAN5353

3 MHz, 3 A Synchronous Buck Regulator

ON Semiconductor 

Minimum Off-Time Effect on Switching

Frequency

tON(MIN) and tOFF(MIN) are both 45 ns. This imposes constraints

on the maximum VOUT that the FAN5353 can provide,

VIN

while still maintaining a fixed switching frequency in PWM

mode. While regulation is unaffected, the switching

frequency drops when the regulator cannot provide sufficient

duty cycle at 3 MHz to maintain regulation.

The calculation for switching frequency is given as:

fSW

=

min





1

t SW (MAX)

,

1

333.3ns





(4)

where:

t SW(MAX)

=

45ns • 1+

VOUT + IOUT • ROFF

VIN − IOUT • RON − VOUT



ROFF = RDSON _ N + DCRL

RON = RDSON _ P + DCRL

Applications Information

Selecting the Inductor

The output inductor must meet both the required inductance

and the energy handling capability of the application. The

inductor value affects the average current limit, the output

voltage ripple, and the efficiency.

The ripple current (∆I) of the regulator is:

∆I ≈

VOUT

VIN

•



VIN

L

− VOUT

• fSW



(5)

The maximum average load current, IMAX(LOAD) is related to

the peak current limit, ILIM(PK)by the ripple current as:

IMAX(LOAD)

=

ILIM(PK )

−

∆I

2

(6)

The FAN5353 is optimized for operation with L=470 nH, but

is stable with inductances up to 1.2 µH (nominal). The

inductor should be rated to maintain at least 80% of its value

at ILIM(PK). Failure to do so lowers the amount of DC current

the IC can deliver.

Efficiency is affected by the inductor DCR and inductance

value. Decreasing the inductor value for a given physical

size typically decreases the DCR; but since ∆I increases, the

RMS current increases, as do core and skin effect losses.

IRMS =

IOUT(DC) 2

+

∆I2

12

(7)

The increased RMS current produces higher losses through

the RDS(ON) of the IC MOSFETs as well as the inductor ESR.

Increasing the inductor value produces lower RMS currents,

but degrades transient response. For a given physical

inductor size, increased inductance usually results in an

inductor with lower saturation current.

shows the effects of inductance higher or lower than the

recommended 470 nH on regulator performance.

Table 2. Effects of Increasing the Inductor

Value (from 470nH recommended value) on

Regulator Performance

IMAX(LOAD)

Increase

∆VOUT (EQ. 8)

Decrease

Transient

Response

Degraded

Inductor Current Rating

The FAN5353’s current limit circuit can allow a peak current

of 5.5 A to flow through L1 under worst-case conditions. If it

is possible for the load to draw that much continuous current,

the inductor should be capable of sustaining that current or

failing in a safe manner.

For space-constrained applications, a lower current rating for

L1 can be used. The FAN5353 may still protect these

inductors in the event of a short circuit, but may not be able

to protect the inductor from failure if the load is able to draw

higher currents than the DC rating of the inductor.

Output Capacitor

Table 1 suggests 0805 capacitors, but 0603 capacitors may

be used if space is at a premium. Due to voltage effects, the

0603 capacitors have a lower in-circuit capacitance than the

0805 package, which can degrade transient response and

output ripple.

Increasing COUT has no effect on loop stability and can

therefore be increased to reduce output voltage ripple or to

improve transient response. Output voltage ripple, ∆VOUT, is:

∆VOUT

=

∆I

•



8

•

1

COUT

•

fSW

+ ESR

(8)

where COUT is the effective output capacitance. The

capacitance of COUT decreases at higher output voltages,

which results in higher ∆VOUT .

If COUT is greater than 100 µF, the regulator may fail to start

under load.

If an inductor value greater than 1.0 µH is used, at least

30 µF of COUT should be used to ensure stability.

ESL Effects

The ESL (Equivalent Series Inductance) of the output

capacitor network should be kept low to minimize the square

wave component of output ripple that results from the

division ratio COUT’s ESL and the output inductor (LOUT). The

square wave component due to ESL can be estimated as:

∆VOUT(SQ)

≈

VIN

• ESLCOUT

L1

(9)

A good practice to minimize this ripple is to use multiple

output capacitors to achieve the desired COUT value. For

example, to obtain COUT = 20 µF, a single 22 µF 0805 would

produce twice the square wave ripple of 2 x 10 µF 0805.

© 2009 Fairchild Semiconductor Corporation

FAN5353 • Rev. 1.0.3

10

www.fairchildsemi.com

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