MAX16818_09 Datasheet, PDF (20/25 Pages) Maxim Integrated Products – 1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

1.5MHz, 30A High-Efficiency, LED Driver
with Rapid LED Current Pulsing
nous MOSFET keeps the power dissipation to a minimum,
especially when the input voltage is large when com-
pared to the voltage on the LED string. It is important to
keep the current-sense resistor, R1, inside the LC loop,
so that ripple current is available. To regulate the LED
current, R2 creates a voltage that the differential amplifier
compares to 0.6V. If power dissipation is a problem in R2,
add a noninverting amplifier and reduce the value of the
sense resistor accordingly.
Inductor Selection
The switching frequencies, peak inductor current, and
allowable ripple at the output determine the value and
size of the inductor. Selecting higher switching frequen-
cies reduces the inductance requirement, but at the
cost of lower efficiency. The charge/discharge cycle of
the gate and drain capacitances in the switching
MOSFETs create switching losses. The situation wors-
ens at higher input voltages, since switching losses are
proportional to the square of the input voltage. The
MAX16818 can operate up to 1.5MHz, however for
V_IN > +12V, use lower switching frequencies to limit the
switching losses.
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:
L_MIN
=
(V_INMAX − V_LED) x V_LED
V_INMAX x f_SW x ∆I_L
Boost regulators:
L_MIN
=
(V_LED − V_INMAX) x V_INMAX
V_LED x f_SW x ∆I_L
where V_LED is the total voltage across the LED string.
As a first approximation choose the ripple current, ∆I_L,
equal to approximately 40% of the output current.
Higher ripple current allows for smaller inductors, but it
also increases the output capacitance for a given volt-
age ripple requirement. Conversely, lower ripple cur-
rent increases the inductance value, but allows the
output capacitor to reduce in size. This trade-off can be
altered once standard inductance and capacitance val-
ues are chosen. Choose inductors from the standard
surface-mount inductor series available from various
manufacturers.
For example, for a buck regulator and 2 LEDs in series,
calculate the minimum inductance at V_IN(MAX) = 13.2V,
V_LED = 7.8V, ∆I_L = 400mA, and f_SW = 330kHz:
Buck regulators:
L_MIN =
(13.2 − 7.8) x 7.8
13.2 x 330k x 0.4
= 24.2µH
For a boost regulator with four LEDs in series, calculate
the minimum inductance at V_IN(MAX) = 13.2V, V_LED =
15.6V, ∆I_L =400mA, and f_SW = 330kHz:
Boost regulators:
L_MIN
=
(15.6 − 13.2) x 13.2
15.6 x 330k x 0.4
= 15.3µH
The average-current-mode control feature of the
MAX16818 limits the maximum peak inductor current
and prevents the inductor from saturating. Choose an
inductor with a saturating current greater than the
worst-case peak inductor current. Use the following
equation to determine the worst-case inductor current:
ILPEAK
=
V_CL
R_S
+
∆I_L
2
where R_S is the inductor sense resistor and V_CL =
0.0282V.
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-frequency
switching applications.
The average current from the MAX16818 gate-drive
output is proportional to the total capacitance it drives
at DH and DL. The power dissipated in the MAX16818
is proportional to the input voltage and the average
drive current. See the IN, V_CC, and V_DD section to
determine the maximum total gate charge allowed from
the combined driver outputs. The gate-charge and
drain-capacitance (CV²) loss, the cross-conduction loss
in the upper MOSFET due to finite rise/fall times, and
the I²R loss due to RMS current in the MOSFET
R_DS(ON) account for the total losses in the MOSFET.
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