MAX17127ETP+【PDF 19/22 页】IC WLED DVR SIX STRING 20TQFN

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Six-String WLED Driver with Integrated

Step-Up Converter

V OUT(OVP) OVP_TH × (1 +

= V )

The voltage rating and temperature characteristics of the

output capacitor must also be considered.

Rectifier Diode Selection

The MAX17127’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommended

for most applications because of their fast recovery time

and low forward voltage. The diode should be rated to

handle the output voltage and the peak switch current.

Make sure that the diode’s peak current rating is at least

I PEAK calculated in the Inductor Selection section and

that its breakdown voltage exceeds the output voltage.

Overvoltage-Protection Determination

The overvoltage-protection circuit ensures the circuit

safe operation; therefore, the controller should limit the

output voltage within the ratings of all MOSFET, diode,

and output capacitor components, while providing suf-

ficient output voltage for LED current regulation. The

OVP pin is connected to the center tap of a resistive

voltage-divider (R1 and R2 in Figure 1) from the high-

voltage output. When the controller detects the OVP pin

voltage reaching the threshold V OVP_TH , typically 1.25V,

overvoltage protection is activated. Hence, the step-up

converter output overvoltage-protection point is:

R1

R2

V OUT(OVP) depends on how many LEDs are used for

each string and V OUT(OVP) = 1.25V x V OUT , generally

and where V OUT is the LED’s operating voltage for each

string.

In Figure 1, the output OVP voltage is set to:

tion can be tolerated on C IN if IN is decoupled from C IN

using an RC lowpass filter.

LED Selection and Bias

The series/parallel configuration of the LED load and

the full-scale bias current have a significant effect on

regulator performance. LED characteristics vary sig-

nificantly from manufacturer to manufacturer. Consult

the respective LED data sheets to determine the range

of output voltages for a given brightness and LED cur-

rent. In general, brightness increases as a function of

bias current. This suggests that the number of LEDs

could be decreased if higher bias current is chosen;

however, high current increases LED temperature and

reduces operating life. Improvements in LED technology

are resulting in devices with lower forward voltage while

increasing the bias current and light output.

LED manufacturers specify LED color at a given LED

current. With lower LED current, the color of the emit-

ted light tends to shift toward the blue range of the

spectrum. A blue bias is often acceptable for business

applications, but not for high-image-quality applications

such as DVD players. Direct-DPWM dimming is a viable

solution for reducing power dissipation while maintaining

LED color integrity. Careful attention should be paid to

switching noise to avoid other display-quality problems.

Using fewer LEDs in a string improves step-up converter

efficiency, and lowers breakdown voltage requirements

of the external MOSFET and diode. The minimum num-

ber of LEDs in series should always be greater than

maximum input voltage. If the diode voltage drop is

lower than maximum input voltage, the voltage drop

across the current-sense inputs (FB_) increases and

V OUT(OVP) = 1.25V × (1 +

2.21M ?

71.5k ?

) = 39.71V

causes excess heating in the IC. Between 8 and 12

LEDs in series are ideal for input voltages up to 20V.

Input Capacitor Selection

The input capacitor (C IN ) filters the current peaks drawn

from the input supply and reduces noise injection into

the IC. A 4.7 F F ceramic capacitor is used in the typical

operating circuit (Figure 1) because of the high source

impedance seen in typical lab setups. Actual applica-

tions usually have much lower source impedance since

the step-up regulator often runs directly from the output

of another regulated supply. In some applications, C IN

can be reduced below the values used in the typical

operating circuit. Ensure a low-noise supply at IN by

using adequate C IN . Alternatively, greater voltage varia-

Applications Information

LED V FB_ Variation

The forward voltage of each white LED may vary up

to 25% from part to part and the accumulated voltage

difference in each string equates to additional power

loss within the IC. For the best efficiency, the voltage

difference between strings should be minimized. The

difference between lowest voltage string and highest

voltage string should be less than 8V (typ). Otherwise,

the internal LED short-protection circuit disables the high

FB_ voltage string.

______________________________________________________________________________________

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