ON

ON

ON

FRAME

50 Ps / 20 kHz

KEY1

KEY2

KEY3

SUM

KEY1

33%

KEY2

33%

KEY3

33%

ON

ON

ON

FRAME

50 Ps / 20 kHz

KEY1

KEY2

KEY3

LP3958

SNVS423C – JANUARY 2006 – REVISED MARCH 2013

www.ti.com

OVERLAPPING MODE

The brightness is controlled using PWM duty cycle based control method as Figure 14 shows.

Figure 14. Overlapping Mode

Since KEY outputs are on simuneltaneously, the maximum load peak current is:

IMAX = I(KEY1)MAX + I(KEY2)MAX + I(KEY3)MAX

(1)

NON-OVERLAPPING MODE

The timing diagram shows the splitted KEY1, KEY2 and KEY3 and brightness control effect to splitted parts. Full

brightness is used in the diagram. If for example ½ brightness is used, the frame is still 50µs, but all LED outputs’

ON time is 50% shorter and at the last 25µs all LED outputs are OFF.

Figure 15. Non-overlapping Mode

The non-overlapping mode has 8-programmed balance ratios. Since the KEY1, KEY2 and KEY3 are split in to

non-overlapping slots the output current through the keypad LED can be calculated by following equation:

IAVG=(CKEY1×IKEY1+CKEY2×IKEY2+CKEY3×IKEY3)×B

where

•

C = Balance [%] (see Table 3)

•

B = Brightness [%] (see Table 2)

(2)

LED ON/OFF CONTROL WITH KEYPAD CONTROL REGISTER

Each LED output can be set ON by writing the corresponding bit high in the control register. K1SW controls

KEY1, K2SW controls KEY2 and K3SW controls KEY3 output. Note that EN_KEYP bit must be high and

KEYP_PWM bit low. In this mode, the KEYPAD register does not have any effect. CC_SW bit in control register

defines the LED output mode.

14

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