MAX8678
White LED Charge Pump with
1.1W Audio Amplifier
8 _______________________________________________________________________________________
LED Charge Pump
The charge pump drives up to four white LEDs with
regulated constant current for displaying backlight
applications. By utilizing an adaptive charge pump and
extremely low-dropout current regulators, it achieves
high efficiency over the full 1-cell lithium ion (Li+) bat-
tery input-voltage range. Fixed frequency of 1MHz
switching allows for tiny external components. The reg-
ulation scheme is optimized to ensure low EMI and low
input ripple.
Adaptive Charge-Pump Modes
When the supply voltage is sufficient to drive the LEDs,
the charge pump is turned off to minimize the input
supply current and the LED currents are linearly regu-
lated. When the supply voltage is insufficient to drive
the LEDs at the set current, the charge pump is
enabled, creating a negative voltage at NEG and allow-
ing the LED_ outputs to pull below ground to maintain
the set LED current.
Low LED Current Levels
The MAX8678 internally generates a PWM signal to
obtain higher resolution at lower currents. As the I
LED
setting is below 6.4mA, the IC adjusts I
LED
DC current,
and the duty cycle is controlled by the PWM signal. The
frequency of the PWM dimming signal is set at 1kHz
with a minimum duty cycle of 1/16 to avoid the LED
flicking effect to human eyes. Table 1 shows the current
level and the corresponding duty cycle.
Soft-Start and Shutdown
The MAX8678 LED driver features a low-power shut-
down mode for reduced current consumption. Hold
ENLED low for 3.2ms to enter shutdown mode. This dis-
ables the charge pump and the LED current drivers.
When initially powering the MAX8678, or when starting
up from shutdown mode, a soft-start feature prevents
input current overshoot. See the startup waveforms in
the
Typical Operating Characteristics
section.
Serial-Pulse Dimming Control (ENLED)
The MAX8678 includes a serial-pulse logic interface for
on/off and dimming of the backlight. The dimming
range is pseudo-logarithmic from 24mA to 0.1mA in 31
steps. Driving ENLED high turns on the IC and sets the
internal register to 24.0mA. To dim the MAX8678, pulse
ENLED low (500ns to 500µs pulse width). Each rising
edge sets the LED current setting as shown in Table 2.
Once the desired setting is reached, hold ENLED high
for at least 3.2ms to set the internal register and the
LED current changes. To set a new current level, repeat
the previous sequence from the beginning.
To turn off the LEDs, hold ENLED low for at least 3.2ms.
Figure 2 shows a timing diagram for ENLED.
If dimming control is not required, ENLED works as a
simple on/off logic control. Drive ENLED high for at
least 3.2ms to enable the LEDs, or drive ENLED low for
at least 3.2ms for shutdown. The LED driver operates at
100% brightness under or OFF under these conditions.
