MAX16821A/MAX16821B/MAX16821C
Inductor Selection
The switching frequency, peak inductor current, and
allowable ripple at the output determine the value and
size of the inductor. Selecting higher switching frequen-
cies reduces inductance requirements, but at the cost
of efficiency. The charge/discharge cycle of the gate
and drain capacitance in the switching MOSFETs cre-
ate switching losses worsening at higher input volt-
ages, since switching losses are proportional to the
square of the input voltage. The MAX16821A/
MAX16821B/MAX16821C operate up to 1.5MHz.
Choose inductors from the standard high-current, sur-
face-mount inductor series available from various manu-
facturers. Particular applications may require
custom-made inductors. Use high-frequency core mate-
rial for custom inductors. High ΔI
L
causes large peak-to-
peak flux excursion increasing the core losses at higher
frequencies. The high-frequency operation coupled with
high ΔI
L
reduces the required minimum inductance and
makes the use of planar inductors possible.
The following discussion is for buck or continuous
boost-mode topologies. Discontinuous boost, buck-
boost, and SEPIC topologies are quite different in
regards to component selection. Use the following
equations to determine the minimum inductance value:
Buck regulators:
Boost regulators:
where V
LED
is the total voltage across the LED string.
The average current-mode control feature of the
MAX16821A/MAX16821B/MAX16821C limits the maxi-
mum peak inductor current and prevents the inductor
from saturating. Choose an inductor with a saturating
current greater than the worst-case peak inductor cur-
rent. Use the following equation to determine the worst-
case current in the average current-mode control loop.
where R
S
is the sense resistor and V
CL
= 0.030V. For
the buck converter, the sense current is the inductor
current and for the boost converter, the sense current is
the input current.
Switching MOSFETs
When choosing a MOSFET for voltage regulators, con-
sider the total gate charge, R
DS(ON)
, power dissipation,
and package thermal impedance. The product of the
MOSFET gate charge and on-resistance is a figure of
merit, with a lower number signifying better perfor-
mance. Choose MOSFETs optimized for high-frequen-
cy switching applications. The average current from the
MAX16821A/MAX16821B/MAX16821C gate-drive out-
put is proportional to the total capacitance it drives
from DH and DL. The power dissipated in the
MAX16821A/MAX16821B/MAX16821C is proportional
to the input voltage and the average drive current. The
gate charge and drain capacitance losses (CV
2
), the
cross-conduction loss in the upper MOSFET due to
finite rise/fall time, and the I
2
R loss due to RMS current
in the MOSFET R
DS(ON)
account for the total losses in
the MOSFET. Estimate the power loss (PD
MOS_
) in the
high-side and low-side MOSFETs using the following
equations:
where Q
G
, R
DS(ON
), t
R
, and t
F
are the upper-switching
MOSFET’s total gate charge, on-resistance, rise time,
and fall time, respectively.
For the buck regulator, D is the duty cycle, I
VALLEY
=
(I
OUT
- ΔI
L
/ 2) and I
PK
= (I
OUT
+ ΔI
L
/ 2).
Input Capacitors
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input
capacitor. The switching frequency, peak inductor cur-
rent, and the allowable peak-to-peak voltage ripple
reflected back to the source dictate the capacitance
requirement. The input ripple is comprised of ΔV
Q
(caused by the capacitor discharge) and ΔV
ESR
(caused by the ESR of the capacitor).
