MAX16818EVKIT+【PDF 22/25 页】KIT EVALUATION FOR MAX16818

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1.5MHz, 30A High-Efficiency, LED Driver

with Rapid LED Current Pulsing

C IN = OUT

Buck:

I x D(1 ? D)

? V Q x f SW

where I OUT is the output current of the converter. For

example, at V IN = 13.2V, V LED = 7.8V, I OUT = 1A, ? I L =

0.4A, and f SW = 330kHz, the ESR and input capaci-

tance are calculated for the input peak-to-peak ripple of

100mV or less yielding an ESR and capacitance value

of 25m ? and 10μF.

For boost regulator designs, the input-capacitor current

waveform is dominated by the inductor, a triangle wave

a magnitude of ? I L. For simplicity’s sake, the current

waveform can be approximated by a square wave with

a magnitude that is half that of the triangle wave.

Calculate the input capacitance and ESR required for a

specified ripple using the following equation:

Current Limit

In addition to the average current limit, the MAX16818

also has hiccup current limit. The hiccup current limit is

set to 10% below the average current limit to ensure that

the circuit goes in hiccup mode during continuous out-

put short circuit. Connecting a resistor from LIM to

ground increases the hiccup current limit, while shorting

LIM to ground disables the hiccup current-limit circuit.

Average Current Limit

The average-current-mode control technique of the

MAX16818 accurately limits the maximum output current.

The MAX16818 senses the voltage across the sense

resistor and limit the peak inductor current (I L-PK )

accordingly. The on-cycle terminates when the current-

sense voltage reaches 25.5mV (min). Use the following

equation to calculate the maximum current-sense resis-

tor value:

ESR IN =

? V ESR

? I L

R S =

0 . 0255

I OUT

? I L

PD R =

Boost:

C IN =

x D

2

? V Q x f SW

0 . 75 x 10 ? 3

R S

where PD R is the dissipation in the series resistors.

Select a 5% lower value of R S to compensate for any

Duty cycle, D, for a boost regulator is equal to (V OUT -

V IN ) / V OUT. As an example, at V IN = 13.2V, V LED =

15.6V, I OUT = 1A, ? I L = 0.4A, and f SW = 330kHz, the

ESR and input capacitance are calculated for the input

peak-to-peak ripple of 100mV or less yielding an ESR

and capacitance value of 250m ? and 1μF, respectively.

Output Capacitor

For buck converters, the inductor always connects to

the load, so the inductance controls the ripple current.

The output capacitance shunts a fraction of this ripple

current and the LED string absorbs the rest. The

capacitor reactance (which includes the capacitance

and ESR) and the dynamic impedance of the LED

diode string form a conductance divider that splits the

ripple current between the LEDs and the capacitor. In

many cases, the capacitor is very large as compared to

the ESR, and this divider reduces to the ESR and the

LED resistance.

Boost converters place a harsher requirement on the

output capacitors as they must sustain the full load dur-

ing the on-time of the MOSFET and are replenished

during the off-time. The ripple current in this case is the

full load current, and the holdup time is equal to the

duty cycle times the switching period.

parasitics associated with the PCB. Also, select a non-

inductive resistor with the appropriate power rating.

Hiccup Current Limit

The hiccup current-limit value is always 10% lower than

the average current-limit threshold, when LIM is left

unconnected. Connect a resistor from LIM to SGND to

increase the hiccup current-limit value from 90% to

100% of the average current-limit value. The average

current-limit architecture accurately limits the average

output current to its current-limit threshold. If the hiccup

current limit is programmed to be equal or above the

average current-limit value, the output current does not

reach the point where the hiccup current limit can trig-

ger. Program the hiccup current limit at least 5% below

the average current limit to ensure that the hiccup cur-

rent-limit circuit triggers during overload. See the

Hiccup Current Limit vs. R EXT graph in the Typical

Operating Characteristics.

22

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