LP3944 datasheet(11/17 Pages) TI1

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VDD

LED 7

LED 0

LP3944

LM2750-5.0

VIN

VOUT

CAP+

CAP-

CFLY

1 PF

2.2 PF

2.7V to 5.5V

2.2 PF

LP3944

www.ti.com

SNVS264 – MAY 2004

Table 9. LED States With Respect To Values in "B1" and "B0" (continued)

Function

Output dims

(DIM0 rate)

Output dims

(DIM1 rate)

Programming Example:

Dim LEDs 0 to 7 at 1 Hz at 25% duty cycle

1. Set PSC0 to achieve DIM0 of 1s

2. Set PWM0 duty cycle to 25%

3. Set PSC1 to achieve DIM1 of 0.2s

4. Set LEDs 0 to 7 to point to DIM0

Step

Description

Set to (Hex)

Set DIM0 = 1s

PSC0

0x09F

1 = (PSC0 + 1)/160

PSC0 = 159

Set duty cycle to 25%

PWM0

0x40

Duty Cycle = PWM0/256

PWM0 = 64

Set DIM1 = 0.2s

PSC1

0x1F

0.2 = (PSC1 + 1)/160

PSC1 = 31

LEDs 0 to 7

LS0, LS1

LS0 = 0xAA

Output = DIM0

LS1 = 0xAA

Reducing IQ When LEDS are Off

In many applications, the LEDs and the LP3944 share the same VDD, as shown in Typical Application Circuit.

When the LEDs are off, the LED pins are at a lower potential than VDD, causing extra supply current (ΔIQ). To

minimize this current, consider keeping the LED pins at a voltage equal to or greater than VDD.

Figure 12. Methods to Reduce IQ When LEDs Are Off

Figure 13. Application Circuit

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