FAN5358 Datasheet, PDF (10/13 Pages) Fairchild Semiconductor – 2MHz, 500mA, SC70 Synchronous Buck Regulator

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
PWM-to-PFM transition point, 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
⎟⎟⎠⎞
(1)
The maximum average load current, IMAX(LOAD), is related to
the peak current limit, ILIM(PK) by the ripple current:
I_MAX(LOAD)
= I_LIM(PK)
−
ΔI
2
(2)
The transition between PFM and PWM operation is
determined by the point at which the inductor valley current
crosses zero. The regulator DC current when the inductor
current crosses zero, IDCM, is:
I_DCM
=
ΔI
2
(3)
The FAN5358 is optimized for operation with L=2.2µH. The
inductor should be rated to maintain at least 70% of its value
at ILIM(PK).
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
(4)
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 3 shows the
effects of inductance higher or lower than the recommended
inductor on regulator performance.
Thermal Considerations
The FAN5358 is designed to supply a maximum of 500mA,
at the specified output voltage, with an operating junction
temperature of up to 125°C. Once the power dissipation and
thermal resistance is known, the maximum junction
temperature of the device can be calculated. The power
dissipation by the IC can be calculated from the power
efficiency diagram Figure 5 and subtracting the power
dissipated by the inductor due to its serial resistance (ESR).
The inductor ESR is dependent, not only upon the size and
type of inductor, but also upon the switching frequency,
which depends on the load and VIN. Some inductor
manufacturers provide full information regarding the variation
of the inductor ESR with the switching frequency. This
information can be used to show that, at high switching
frequency (~2 MHz) and maximum load, the power
dissipated by the inductor can exceed the power dissipated
by the IC package itself.
The actual thermal resistance depends upon the thermal
characteristics of the SC-70 surface-mount package and the
surrounding printed circuit board (PCB) copper to which it is
mounted. This can be improved by providing a heat sink of
surrounding copper ground on the PCB. Depending on the size
of the copper area, the resulting θJA can be reduced below
280°C/W. The addition of backside copper with through holes,
stiffeners, and other enhancements can also help reduce
thermal resistance. The heat contributed by the dissipation of
other devices, particularly the inductor, located nearby, must be
included in the design considerations. Once the limiting
parameters are determined, the design can be modified to
ensure that the device remains within specified operating
conditions even if the maximum load is applied permanently.
In short circuit VOUT-to-GND condition, the FAN5358 is fully
protected and the power dissipated is internally reduced
below 100mW. Overload conditions at minimum VIN should
be considered as worst case, when it is possible for the
device to enter a thermal cycling loop in which the circuit
enters a shutdown condition, cools, re-enables, and again
overheats and shuts down repeatedly due to an unmanaged
fault condition. The diagram in Figure 20 was determined
experimentally, using the recommended two-layer PCB in
still air, to be used as a thermal guide.
90
Area Where Thermal Protection May Trigger
85
80
75
70
Safe Operating Area
65
for 500mA Load
60
55
2.7
2.9
3.1
3.3
3.5
Input Voltage (V)
Figure 20. Maximum Ambient Temperature vs.
Input Voltage at 500mA
© 2009 Fairchild Semiconductor Corporation
FAN5358 • Rev. 1.0.1
10
www.fairchildsemi.com