NCP5021 データシートの表示（PDF） - ON Semiconductor
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NCP5021 Datasheet PDF : 18 Pages
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NCP5021
HORUS : Iout %= F(Step)
100
Iout %
10
1
0.1
−0.1
10
20.1
30.2
STEP
Figure 9. Typical ILED Programming Curve
ILEDREG[0..7] I−LED PEAK CURRENT
B7
B6
B5
step
0
0
0
RESET
0
0
0
B4
ILED16
0
B3
ILED8
0
B2
ILED4
0
B1
ILED2
0
B0
ILED1
0
Bits [B7:B5] : Reserved for future use
Bits [B4:B0] : ILED peak current setup
Inductor Peak Current
The peak current flowing into the inductor during normal
application is set up by the external resistor RPCA
connected between the IPK pin and ground.
The maximum IPK current is given by the internal current
mirror ratio (multiplier − kk −) typically equal to 9700. In
other word, to get a 750 mA maximum peak current in the
inductor, the reference current shall be 750 mA/9700 =
79 mA. This current is used to calculate the resistor
connected between the IPK pin and GND: RREF = 1.144 /
79e−6 = 14.4 kW.
The concept depicted in the ILED peak current
paragraph applies as well and cares must be observed to
avoid any voltage source connection to the IPK pin.
Gradual Dimming
The purpose of that function is to gradually Increase or
Decrease the brightness of the backlight / keyboard LED
upon command from the external MCU. The function is
activated and controlled by means of the I2C protocol.
The period (either upward or downward) is equal to the
time defined for each step, multiplied by the number of
steps. The number of step is derived from the value
associated with the target current.
To operate such a function, the MCU will provide three
information:
1. The target current level (either upward or
downward)
2. The time per step
3. The Upward or Downward mode of operation
When a new gradual dimming sequence is requested, the
output current increases, according to the logarithmic
curve, from the existing start value to the end value. The
end current value is defined by the contain of the Upward
or Downward register, the width of each step is defined by
the third register, the number of step being in the 1 to 32
range ($00 to $1F). In the event of software error, the
system checks that neither the maximum output current
(25 mA), nor the zero level are forced out of their
respective bounds. Similarly, software errors shall not
force NCP5021 into an uncontrolled mode of operation.
The dimming is built with 32 steps and the time delay
encoded into the third byte of the I2C transaction.
When the gradual dimming is not requested (register
selection = $01), the output current is straightforwardly set
up to the level defined by the contain of the related register
upon acknowledge of the output current byte.
The gradual dimming sequence must be set up before a
new output current data byte is send to NCP5021. At this
point, the brightness sequence takes place when the new
data byte is acknowledged by the internal I2C decoder.
Since the six registers are loaded on independent byte flow
associated to the I2C address, any parameter of the
NCP5021 chip can be updated ahead of the next function.
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