FAN5354 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – 3MHz, 3A Synchronous Buck Regulator

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
1200μs 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 50μs,
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 45ns. 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
f_SW
=
min
⎜⎛
⎜⎝
1
t_SW(MAX)
,
1
333.3ns
⎟⎞
⎟⎠
where
t _{SW (MAX)}
=
45ns •
⎜⎜⎝⎛¹⁺
V_OUT + I_OUT • R_OFF
V_IN − I_OUT • R_ON − V_OUT
⎟⎟⎠⎞
(4)
R_OFF= R_{DSON_ N} + DCR_L
R_ON = R_{DSON_ P} + DCR_L
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:
ΔI ≈
V_OUT
V_IN
•
⎜⎜⎝⎛
V_IN
L
− V_OUT
• f_SW
⎟⎟⎠⎞
(5)
The maximum average load current, IMAX(LOAD) is related to
the peak current limit, ILIM(PK), by the ripple current as:
I_MAX(LOAD)
=
I_LIM(PK)
−
ΔI
2
(6)
The FAN5354 is optimized for operation with L=470nH, but
is stable with inductances up to 1.2μH (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.
I_RMS =
I_OUT(DC)2
+
ΔI²
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 470nH on regulator performance.
Table 2. Effects of Increasing the Inductor
Value (from 470nH Recommended) on
Regulator Performance
I_MAX(LOAD)
Increase
∆V_OUT (EQ. 8)
Decrease
Transient
Response
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.
Output Capacitor and V_OUT 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:
ΔV_OUT
=
ΔI
•
⎜⎜⎝⎛
8
•
1
C_OUT
•
f_SW
+ 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. 1.0.4
11
www.fairchildsemi.com