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ADM8843
AUTOMATIC GAIN CONTROL
The automatic gain control block controls the operation of the
charge pump by selecting the appropriate gain for the charge
pump. Doing so maintains sufficient drive for the LED anodes
at the highest power efficiency over a 2.6 V to 5.5 V input
supply range. The charge pump switching thresholds are
described in Table 5.
Table 5. Charge Pump Switching Thresholds
Gain Threshold (V)
1.5× to 2× 3.33
2× to 1.5× 3.36
1× to 1.5× 4.77
Applying a digital PWM signal to one or both digital input
control pins, CTRL1 and CTRL2, adjusts the brightness of the
sub and/or main displays. The ADM8843’s four white LEDs are
organized into two groups, main display (FB1 to FB3) and sub
display (FB4); refer to the Theory of Operation section.
The brightness of the ADM8843’s main and sub displays can be
controlled together or separately. This is achieved by applying a
digital PWM signal to both the CTRL1 and CTRL2 pins. The duty
cycle of the applied digital PWM signal determines the brightness
of the main and sub displays together. Varying the duty cycle of
the applied PWM signal varies the brightness of the main and
sub displays from 0% to 100%.
1.5× to 1×
4.81
By holding CTRL1 low and applying a digital PWM signal to
CTRL2, the sub display is turned off and the main display is
BRIGHTNESS CONTROL WITH A
DIGITAL PWM SIGNAL
PWM brightness control provides the widest brightness control
method by pulsing the white LEDs on and off using one or both
digital input control pins, CTRL1 and CTRL2. PWM brightness
control also removes any chromaticity shifts associated with
changing the white LED current, because the LEDs operate at
either zero current or full current (set by R SET ).
The digital PWM signal applied with a frequency of 100 Hz to
200 kHz turns the current control sinks on and off using CTRL1
and/or CTRL2. The average current through the LEDs changes
with the PWM signal duty cycle. If the PWM frequency is much
less than 100 Hz, flicker may be seen in the LEDs. For the
ADM8843, zero duty cycle turns off the LEDs, and a 50% duty
cycle results in an average LED current, I LED , that is half the pro-
grammed LED current. For example, if R SET is set to program
20 mA/LED, a 50% duty cycle results in an average I LED of
10 mA/LED, which is half the programmed LED current.
turned on. Then the brightness of the main display is determined
by the duty cycle of the applied digital PWM signal.
By applying a digital PWM signal to CTRL1 and holding CTRL2
low, the sub display is turned on and the main display is turned
off. Then the brightness of the sub display is determined by the
duty cycle of the applied digital PWM signal.
By applying a digital PWM signal to CTRL1 and holding
CTRL2 high, the sub display is turned on and the main display
is turned on. Then the brightness of the sub display is determined
by the duty cycle of the applied digital PWM signal. The bright-
ness of the main display is set to the maximum (maximum is set
by R SET ).
By holding CTRL1 high and applying a digital PWM signal to
CTRL2, the sub and main displays are turned on. Then the
brightness of the main display is determined by the duty cycle of
the applied digital PWM signal. The brightness of the sub
display is set to the maximum (maximum is set by R SET ).
C1
1 μ F
C2
1 μ F
When CTRL1 and CTRL2 are low, the LED current control
sinks shutdown. Shutdown of the charge pump is delayed by
3.4V I
IN
V CC
ADM8845
V OUT
C3
2.2 μ F
15 ms. This timeout period, t CP , allows the ADM8843 to
determine if a digital PWM signal is present on CTRL1 and
CTRL2, or if the user has selected a full chip shutdown (see
PWM INPUT
OR
HIGH/LOW
CTRL1
Figure 20).
PWM INPUT
OR
HIGH/LOW
CTRL2
FB1
FB2
FB3
FB4
If digital PWM brightness control of the LEDs is not required, a
constant Logic Level 1 (V CC ) or Logic Level 0 (GND) must be
applied.
I SET
R SET
Figure 19. Digital PWM Brightness Control Application Diagram
The four white LED in the ADM8843 are arranged into two
groups, sub and main. It is possible to configure the four LEDs
as in Table 6. Refer also to Figure 20.
Rev. C | Page 10 of 16
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