FAN5354 データシートの表示（PDF） - ON Semiconductor

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FAN5354

3 MHz, 3 A Synchronous Buck Regulator

ON Semiconductor 

Current Limiting

A heavy load or short circuit on the output causes the current

in the inductor to increase until a maximum current threshold

is reached in the high-side switch. Upon reaching this point,

the high-side switch turns off, preventing high currents from

causing damage. 16 consecutive PWM cycles in current limit

cause the regulator to shut down and stay off for about

1200 s before attempting a restart.

In the event of a short circuit, the soft-start circuit attempts to

restart and produces an over-current fault after about 50 s,

which results in a duty cycle of less than 10%, providing

current into a short circuit.

Thermal Shutdown

When the die temperature increases, due to a high load

condition and/or a high ambient temperature, the output

switching is disabled until the temperature on the die has

fallen sufficiently. The junction temperature at which the

thermal shutdown activates is nominally 150°C with a

20°C hysteresis.

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 FAN5354 can provide,

VIN

while still maintaining a fixed switching frequency in PWM

mode. While regulation is unaffected, the switching

frequency will drop when the regulator cannot provide

sufficient duty cycle at 3 MHz to maintain regulation.

The calculation for switching frequency is given below

fSW



min





1

tSW(MAX)

,

1

333.3ns





where

t SW(MAX)



45ns  1

VOUT  IOUT  ROFF

VIN  IOUT  RON  VOUT



(4)

= ROFF RDSON _N  DCRL

RON = RDSON _ P  DCRL

Application 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  VOUT

L  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 FAN5354 is optimized for operation with L=470 nH, but

is stable with inductances up to 1.2 H (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.

Table 2 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 470 nH Recommended) on

Regulator Performance

IMAX(LOAD)

Increase

∆VOUT (EQ. 8)

Decrease

Transient Response

Degraded

Inductor Current Rating

The FAN5354’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 FAN5354 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 and VOUT Ripple

Note: 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 .

© 2009 Fairchild Semiconductor Corporation

FAN5354 • Rev. 1.0.7

11

www.fairchildsemi.com

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