FAN53541 Datasheet, PDF (11/15 Pages) Fairchild Semiconductor – 2.4 MHz, 5 A TinyBuck Synchronous Buck Regulator

PGOOD Pin
The PGOOD pin is an open-drain that indicates that the IC is
in regulation when its state is open. PGOOD pulls LOW
under the following conditions:
 The IC has operated in cycle-by-cycle current limit for
eight consecutive PWM cycles;
 The circuit is disabled, either after a fault occurs or when
EN is LOW; or
 The IC is performing a soft-start.
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 155°C with a
20°C hysteresis.
Minimum Off-Time Effect on Switching
Frequency
t_OFF(MIN) is 45 ns, which constrains the maximum V_OUT/V_IN
that the FAN53541 can provide, while still maintaining a fixed
switching frequency in PWM Mode. Regulation is maintained
even though the regulator is unable to provide sufficient
duty-cycle and operate at 2.4 MHz.
Switching frequency is the lower of 2.4 MHz or:
f_SW (MHz)  22.2  _{}1
V_OUT  I_OUT  R_OFF
V_IN  I_OUT  (R_OFF  R_ON )

(4)
where:
I_OUT = load current, in A;
R_ON = R_{DS(ON)_P} + DCR_L, in Ohms; and
R_OFF = R_{DS(ON)_N} + DCR_L, in Ohms.
A result of <0 MHz indicates 100% duty cycle operation.
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, output
voltage ripple, transient response, and efficiency.
The ripple current (∆I) of the regulator is:
I 
V_OUT
V_IN


V_IN
L
 V_OUT
 f_SW

(5)
The maximum average load current, I_MAX(LOAD), is related to
the peak current limit, I_LIM(PK), by the ripple current as:
I_MAX(LOAD)

I_LIM(PK)

I
2
(6)
The FAN53541 is optimized for operation with L=470 nH, but
is stable with inductances up to 1.2 H (nominal). The
inductor should be rated to maintain at least 80% of its value
at I_LIM(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

I²
12
(7)
The increased RMS current produces higher losses through
the R_DS(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 on regulator performance of higher
inductance than the recommended 470 nH.
Table 3. Inductor Value and Regulator
Performance
I_MAX(LOAD)
Increase
∆V_OUT (Eq.(8))
Decrease
Transient Response
Degraded
Inductor Current Rating
The FAN53541’s current-limit circuit can allow a peak current
of about 8.8 A 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 FAN53541 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.
© 2013 Fairchild Semiconductor Corporation
FAN53541 • Rev. 1.0.2
11
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