CAT37

8

Doc. No. 5005, Rev. R

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.

LED DIMMING WITH DC SIGNAL

Dimming the LEDs can be done by applying a variable

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

µA) is much smaller

than the LED current but much larger than the FB

and 2V, the resistors shown on Figure 3 will set the LED

current between 0mA and 15mA.

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 5kHz to

40kHz PWM signal is used.

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

in Figure 4.

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

15mA or 20mA. 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.

Figure 3. Dimming with a

DC Voltage

CAT37

FB

R1

RINCR

LOGIC

SIGNAL

2N7002

CAT37

FB

R3

18k

PWM

10k

R2

1k

CAT37

FB

R3

18k

VDC

R2

1k

Figure 4. Dimming with a Filtered

PWM Signal

Figure 5. Dimming with a

Logic Signal

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

the NMOS switch is OFF):

R1=

0.095V

R

when the external NMOS switch is turned ON.

0.095V