MAX16818_15 datasheet(20/25 Pages) MAXIM

Part #	MAX16818

Description	1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

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Manufacturer	MAXIM [Maxim Integrated Products]

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nous MOSFET keeps the power dissipation to a minimum,

especially when the input voltage is large when compared

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

frequencies 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 worsens 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

VIN > +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:

INMAX

LED

MIN

INMAX

SW

L

(V

V

) x V

L

V

x f

x I

−

=

∆

Boost regulators:

LED

INMAX

MIN

LED

SW

L

(V

V

) x V

L

V

x f

x I

−

=

∆

where VLED is the total voltage across the LED string.

As a first approximation choose the ripple current, ∆IL,

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

voltage ripple requirement. Conversely, lower ripple

current increases the inductance value, but allows the

output capacitor to reduce in size. This trade-

off can be altered once standard inductance and

capacitance values are chosen. Choose inductors from the

standardsurface-mountinductorseriesavailablefromvarious

manufacturers.

For example, for a buck regulator and 2 LEDs in series,

calculate the minimum inductance at VIN(MAX) = 13.2V,

VLED = 7.8V, ∆IL = 400mA, and fSW = 330kHz:

Buck regulators:

MIN

(13.2 7.8) x 7.8

L

24.2 H

13.2 x 330k x 0.4

−

=

µ

For a boost regulator with four LEDs in series, calculate

the minimum inductance at VIN(MAX) = 13.2V, VLED =

15.6V, ∆IL =400mA, and fSW = 330kHz:

Boost regulators:

MIN

(15.6 13.2) x 13.2

L

15.3 H

15.6 x 330k x 0.4

−

=

µ

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:

CL

L

ILPEAK

S

VI

R2

∆

=

+

where RS is the inductor sense resistor and VCL =

0.0282V.

Switching MOSFETs

When choosing a MOSFET for voltage regulators,

consider the total gate charge, RDS(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 performance.

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, VCC, and VDD 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 RDS(ON) account for the total losses in the

MOSFET.

www.maximintegrated.com

Maxim Integrated │ 20

MAX16818

1.5MHz, 30A High-Efficiency, LED Driver

with Rapid LED Current Pulsing

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MAX16818 Datasheet(PDF) 20 Page - Maxim Integrated Products

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