FAN5353 Datasheet, PDF (10/13 Pages) Fairchild Semiconductor – 3MHz, 3A Synchronous Buck Regulator

Minimum Off-Time Effect on Switching
Frequency
tON(MIN) and tOFF(MIN) are both 45ns. This imposes constraints
on the maximum ^VOUT that the FAN5353 can provide, while
VIN
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
3MHz to maintain regulation.
The calculation for switching frequency is given as:
f_SW
=
min
⎜⎛
⎜⎝
1
t_{SW (MAX)}
,
1
333.3ns
⎟⎞
⎟⎠
(4)
where:
t _{SW (MAX)}
=
45ns • _{⎜⎜⎝⎛}1+
V_OUT + I_OUT • R _OFF
V_IN − I_OUT • R _ON − V_OUT
⎟⎟⎠⎞
R_OFF = R_{DSON _ N} + DCR_L
R_ON = R_{DSON _ P} + DCR_L
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:
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 FAN5353 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.
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 value) on
Regulator Performance
I_MAX(LOAD)
Increase
∆V_OUT (EQ. 8)
Decrease
Transient
Response
Degraded
Inductor Current Rating
The FAN5353’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 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
Note:
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 .
If COUT is greater than 100μF, the regulator may fail to start
under load.
If an inductor value greater than 1.0μH is used, at least 30μF
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:
ΔV_OUT(SQ)
≈
V_IN
• ^ESLCOUT
L1
(9)
A good practice to minimize this ripple is to use multiple
output capacitors to achieve the desired COUT value. For
© 2009 Fairchild Semiconductor Corporation
FAN5353 • Rev. 1.0.2
10
www.fairchildsemi.com