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FAN53526 • Rev. 3.1 www.onsemi.com
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FAN53526 — 3.0 A, 2.4 MHz, Digitally Programmable TinyBuck® Regulator
FAN53526 — 3.0 A, 2.4 MHz, Digitally Programmable TinyBuck® Regulator
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:
SW
OUTIN
IN
OUT fL VV
V
V
ΔI
The maximum average load current, IMAX(LOAD), is related to
the peak current limit, ILIM(PK), by the ripple current such that:
2I
II )PK(LIM)LOAD(MAX
The FAN53526 is optimized for operation with L=330 nH, but
is stable with inductances up to 1.0 H (nominal). The
inductor should be rated to maintain at least 80% of its value
at ILIM(PK). Failure to do so decreases 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:
The increased RMS current produces higher losses through
the RDS(ON) of the IC MOSFETs and 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 8. Effects of Inductor Value (from 330 nH
Recommended) on Regulator Performance
Inductor Current Rating
The current-limit circuit can allow substantial peak currents to
flow through L1 under worst-case conditions. If it is possible
for the load to draw such currents, the inductor should be
capable of sustaining the current or failing in a safe manner.
For space-constrained applications, a lower current rating for
L1 can be used. The FAN53526 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. Refer to
Table 2 for the recommended inductors.
Output Capacitor and VOUT Ripple
If space is at a premium, 0603 capacitors may be used.
Increasing COUT has negligible effect on loop stability and
can be increased to reduce output voltage ripple or to
improve transient response. Output voltage ripple, ∆VOUT, is
calculated by:
OUTSW
2
OUTSW
LOUT Cf8 1
D1D2 ESRCf
IV
where COUT is the effective output capacitance.
The capacitance of COUT decreases at higher output voltages,
which results in higher ∆VOUT. Equation (6) is only valid for
CCM operation, which occurs in PWM Mode.
The FAN53526 can be used with either 2 x 22 µF (0603) or 2
x 47 µF (0603) output capacitor configuration. If a tighter ripple
and transient specification is need from the FAN53526, then
the 2 x 47 µF is recommended.
The lowest ∆VOUT is obtained when the IC is in PWM Mode
and, therefore, operating at 2.4 MHz. In PFM Mode, fSW is
reduced, causing ∆VOUT to increase.
ESL Effects
The Equivalent Series Inductance (ESL) 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 ESL and the output inductor (LOUT). The square-
wave component due to the ESL can be estimated as:
1L
ESL
VV COUT
IN)SQ(OUT
A good practice to minimize this ripple is to use multiple output
capacitors to achieve the desired COUT value. For example, to
obtain COUT=20 F, a single 22 F 0805 would produce twice
the square wave ripple as two x 10 F 0805.
To minimize ESL, try to use capacitors with the lowest ratio of
length to width. 0805 s have lower ESL than 1206 s. If low
output ripple is a chief concern, some vendors produce 0508
capacitors with ultra-low ESL. Placing additional small-value
capacitors near the load also reduces the high-frequency
ripple components.
Input Capacitor
The ceramic input capacitors should be placed as close as
possible between the VIN and PGND pins 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 under-damped ringing that can occur between the
inductance of the power source leads and CIN.
The effective CIN capacitance value decreases as VIN
increases due to DC bias effects. This has no significant
impact on regulator performance.