CAT37 데이터 시트보기 (PDF) - ON Semiconductor

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CAT37

CMOS White LED Driver Boost Converter

ON Semiconductor 

CAT37

LED Dimming with DC Signal

Dimming the LEDs can be done by applying a variable

DC voltage as shown on Figure 14. As the VDC increase the

voltage across R1 decreases and therefore lower the LED

current. The resistors R2 and R3 must be large enough so

that their current (tens of mA) is much smaller than the LED

current but much larger than the FB leakage current (IFB).

When adjusting VDC between 0 V and 2 V, the resistors

shown on Figure 14 will set the LED current between 0 mA

and 15 mA.

LED Dimming with PWM Signal

PWM brightness control provides the widest dimming

range (greater than 20:1). By turning the LEDs ON and OFF

using the control signal, the LEDs operate at either zero or

full current, but their average current changes with the PWM

signal duty cycle. Typically, a 5 kHz to 40 kHz PWM signal

is used.

A filtered PWM signal is used to dim the LEDs, as shown

in Figure 15.

In addition to providing the widest dimming range, PWM

brightness control also ensures the “purest” white LED color

over the entire dimming range. The true color of a white

LED changes with operating current, and is the “purest”

white at a specific forward current, usually 15 mA or

20 mA. If the LED current is less than or more than this

value, the emitted light becomes more blue. Applications

involving color LCDs can find the blue tint objectionable.

LED Dimming with a Logic Signal

For applications that need to adjust the LED brightness in

discrete steps, a logic signal can be used as shown in

Figure 16. R1 sets the minimum LED current value (when

the NMOS switch is OFF):

R1

+

0.095 V

ILED(MIN)

RINCR determines how much LED current increases when

the external NMOS switch is turned ON.

0.095 V

RINCR + ILED(INCREASE)

PCB Layout Guidelines

The CAT37 is a high−frequency switching regulator

where proper PCB board layout and component placement

can minimize noise and radiation and increase efficiency. To

maximize efficiency, the CAT37 design has fast switch rise

and fall times. To prevent radiation and high frequency

resonance problems minimize the length and area of all

traces connected to the SW pin and use a ground plane under

the switching regulator.

The SW pin, schottky diode and capacitor C2 signal path

should be kept as short as possible. The ground connection

for the R1 resistor should be tied directly to the GND pin and

not be shared with other components.

CAT37

FB

R3

180 kW

VDC

R2

10 kW

R1

6.3 W

10 kW

PWM

CAT37

FB

R3

R2

180 kW 10 kW

0.1 mF

R1

Logic

6.3 W Signal

CAT37

FB

RINCR

2N7002

R1

6.3 W

Figure 14. Dimming with a

DC Voltage

Figure 15. Dimming with a

Filtered PWM Signal

Figure 16. Dimming with a

Logic Signal

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