LP3958 Datasheet, PDF (14/36 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

LP3958
SNVS423C – JANUARY 2006 – REVISED MARCH 2013
OVERLAPPING MODE
The brightness is controlled using PWM duty cycle based control method as Figure 14 shows.
FRAME
50 Ps / 20 kHz
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KEY1
ON
KEY2
ON
KEY3
ON
Figure 14. Overlapping Mode
Since KEY outputs are on simuneltaneously, the maximum load peak current is:
I_MAX = 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.
FRAME
50 Ps / 20 kHz
KEY1
ON
KEY2
ON
KEY3
ON
SUM
KEY1
33%
KEY2
33%
KEY3
33%
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:
I_AVG=(C_KEY1×I_KEY1+C_KEY2×I_KEY2+C_KEY3×I_KEY3)×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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