MAX16818EVKIT+【PDF 20/25 页】KIT EVALUATION FOR MAX16818

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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

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:

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

L MIN =

(13.2 ? 7.8) x 7.8

13 . 2 x 330 k x 0 . 4

= 24 . 2 μ H

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-

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:

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

L MIN =

(15.6 ? 13.2) x 13.2

15 . 6 x 330 k x 0 . 4

= 15 . 3 μ H

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

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:

boost-mode topologies. Discontinuous boost, buck-

boost, and SEPIC topologies are quite different in

regards to component selection.

ILPEAK =

V CL

R S

+

? I L

2

Use the following equations to determine the minimum

inductance value:

Buck regulators:

where R S is the inductor sense resistor and V CL =

0.0282V.

Switching MOSFETs

L MIN =

Boost regulators:

L MIN =

(V INMAX ? V LED ) x V LED

V INMAX x f SW x ? I L

(V LED ? V INMAX ) x V INMAX

V LED x f SW x ? I L

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

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.

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 (CV 2 ) loss, the cross-conduction loss

in the upper MOSFET due to finite rise/fall times, and

the I 2 R loss due to RMS current in the MOSFET

R DS(ON) account for the total losses in the MOSFET.

20

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