FAN5362 Datasheet, PDF (10/14 Pages) Fairchild Semiconductor – 3MHz, 500mA / 750mA Synchronous Buck Regulator

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
tOFF(MIN) is 35ns. This imposes constraints on the maximum
V_{OUT that the FAN5362 can provide, or the maximum}
V_IN
output voltage it can provide at low VOUT while maintaining a
fixed switching frequency in PWM mode.
When VIN is high, fixed switching is maintained as long as
V_OUT
V_IN
≤ 1− t_OFF(MIN) • f_SW
≈ 0.7 .
The switching frequency drops when the regulator cannot
provide sufficient duty cycle at 3MHz to maintain regulation.
This occurs when VIN is below 3.3V at nominal load currents.
The calculation for switching frequency is given by:
f_SW
=
min
⎜⎛
⎜⎝
t
1
SW(MAX)
,
3MHz
⎟⎞
⎟⎠
(4)
where:
t _SW(MAX)
=
35ns • _{⎜⎜⎝⎛}1+
V_OUT + I_OUT • R_OFF
V_IN − I_OUT • R_ON − V_OUT
⎟⎟⎠⎞
(5)
where:
= 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
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
⎟⎟⎠⎞
(6)
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
(7)
The FAN5362 is optimized for operation with L=1μH, but is
stable with inductances up to 1.5μH (nominal) and down to
470nH. 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 that 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 the core and skin effect losses.
I_RMS =
I_OUT(DC)2
+
ΔI²
12
(8)
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 and higher DCR.
Inductor Current Rating
The FAN5362’s current limit circuit can allow a peak current
of 1.25A 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.
Output Capacitor
While 4.7μF capacitors are available in 0402 package size,
0603 capacitors are recommended due to the severe DC
voltage bias degradation in capacitance value that the
0402 exhibits.
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 _{⎟⎟⎠⎞}
(9)
If values greater than 24μF of COUT are used, the regulator
may fail to start. See the sections on Enable and Soft Start
for more information.
Input Capacitor
The 2.2μF ceramic input capacitor should be placed as close
as possible to the VIN pin and GND to minimize the parasitic
inductance. If a long wire is used to bring power to the IC,
additional “bulk” capacitance (electrolytic or tantalum) should
be placed between CIN and the power source lead to reduce
ringing that can occur between the inductance of the power
source leads and CIN.
© 2009 Fairchild Semiconductor Corporation
FAN5362 • Rev. 1.0.1
10
www.fairchildsemi.com