© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5354 • Rev. 1.0.4

11

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

μs 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

μs,

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

on the maximum

VIN

VOUT that the FAN5354 can provide,

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

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

ns

3

.

333

1

,

t

1

min

f

)

MAX

(

SW

SW

where

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

•

−

•

+

+

•

=

OUT

ON

OUT

IN

OFF

OUT

OUT

)

MAX

(

SW

V

R

I

V

R

I

V

1

ns

45

t

OFF

R

=

L

N

_

DSON

DCR

R

+

ON

R

=

L

P

_

DSON

DCR

R

+

(4)

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

−

•

≈

Δ

SW

OUT

IN

IN

OUT

f

L

V

V

V

V

I

(5)

2

I

I

I

)

PK

(

LIM

)

LOAD

(

MAX

Δ

−

=

(6)

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

is stable with inductances up to 1.2

μH (nominal). The

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

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.

12

I

I

I

2

2

)

DC

(

OUT

RMS

Δ

+

=

(7)

The increased RMS current produces higher losses through

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 470nH on regulator performance.

Table 2. Effects of Increasing the Inductor

Value (from 470nH Recommended) on

Regulator Performance

Transient

Response

Increase

Decrease

Degraded

Inductor Current Rating

The FAN5354’s current limit circuit can allow a peak current

of 5.5A 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.

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.

therefore be increased to reduce output voltage ripple or to

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

•

•

•

Δ

=

Δ

ESR

f

C

8

1

I

V

SW

OUT

OUT

(8)