HT7937 datasheet(5/13 Pages) HOLTEK

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HT7937

Rev 1.10

January 18, 2010

Component Selection

· Setting the LED Current

The step-up converter regulates the LED current by

regulating the voltage across the current sense resis-

tor, RFB. To ensure the generation of accurate LED

currents, it is recommended that a precision resistor is

used for RFB. The voltage across the sense resistor is

regulated to the internal reference voltage of 95mV.

The LED current is calculated using the following

equation:

LED

Where

VFB = Feedback voltage is 95mV

· Inductor Selection

The selected inductor must have a saturation current

greater than the maximum peak current of the step-up

converter. A recommended value of inductor for 3 to 6

white LED applications is 4.7

mHto22mH. For good ef-

ficiency the inductor should have low core loss and

low DCR (DC Resistance).

The peak inductor current is calculated using the

following equation:

Where

VO = Output Voltage

Vi = Input Voltage

IO = Output Current

h = Efficiency

L = Inductance

FS = Switching Frequency is 1.2MHz

· Diode Selection

The diode must have a rating greater than the output

voltage and output current. In switching applications

both forward voltage drop and diode capacitance are

important considerations.

A Schottky diode is used due to its low forward voltage

drop and fast switching speeds which improves effi-

ciency. The Schottky diode has average current of IO,

and a peak current which is the same as the inductor

¢s

peak current and a voltage rating at least 1.5 times the

output voltage. A recommended Schottky diode type

is the 1N5819.

· Capacitor Selection

A low ESR (Equivalent Series Resistance) capacitor

should be used for the input/output capacitors to mini-

mize ripples. Both X5R and X7R types are suitable

due to their wider voltage and temperature ranges. In-

put and output ceramic capacitors of 1

mF are recom-

mended for HT7937 applications.

The output ripple voltage is calculated as the following

equation:

Where

IL(PEAK) = Peak Inductor Current

CO(ESR) = Output Capacitor

¢s Equivalent Series

Resistance

IO = Output Current

VO = Output Voltage

Vi = Input Voltage

CO = Output Capacitance

FS = Switching Frequency is 1.2MHz

Layout Considerations

Circuit board layout is a very important consideration for

switching regulators if they are to function properly. Poor

circuit layout may result in related noise problems.

In order to minimise EMI and switching noise, please fol-

low the guidelines below:

· All tracks should be as wide as possible.

· The input and output capacitors, C1 and C2, should

be placed close to the VIN, VOUT and GND pins.

· The Schottky diode, D1, and inductor, L1, must all be

placed close to the SW pin.

· Feedback resistor, R1, must be placed close to the FB

and GND pins.

· A full ground plane is always helpful for better EMI

performance.

A recommended PCB layout with component locations

is shown below.

Top Layer

Bottom Layer

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