Rainbow Electronics MAX8848Z - Manual (Page 10)

High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
MAX8848Y/MAX8848Z
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Detailed Description
The MAX8848Y/MAX8848Z have an inverting charge
pump and seven current regulators capable of 24mA
each to drive up to 7 white LEDs. The current regulators
are matched to within 1% (typ) providing uniform white
LED brightness for LCD backlight applications. To maxi-
mize efficiency, the current regulators operate with as
little as 0.15V voltage drop.
Individual white LED current regulators conduct current
to GND or NEG to extend usable battery life. In the case
of mismatched forward voltage of white LEDs, only the
white LEDs requiring higher voltage are switched to pull
current to NEG instead of GND, further raising efficiency
and reducing battery current drain.
Current Regulator Switchover
When V
IN
is higher than the LED forward voltage plus the
150mV dropout voltage of the current regulator, the LED
current returns through GND. If this condition is satis-
fied for all active white LEDs, the charge pump remains
inactive. When the input voltage drops so that the cur-
rent regulator voltage (V
LED_ ) cannot be maintained for
any of the individual white LEDs, the inverting charge
pump activates and generates a voltage on NEG that is
no greater than 5V below V
IN . For any current regulator
that is detected at the switchover threshold voltage of
150mV (typ, V
IN falling), internal circuitry switches that
current regulator’s return path from GND to NEG to pro-
vide enough voltage across that regulator to overcome
dropout. When V
LED_
rises to 250mV (typ), the return
of that current regulator is switched back from NEG to
GND. Each current regulator is independently monitored
to detect when switchover is required. Since the LED
current is switched for only the individual LED current
regulators requiring higher voltage, power consumption
is minimized.
Enable and Dimming Control Input
(ENA, ENB)
ENA and ENB inputs have dual functions: LED on/off
control and PWM or serial-pulse dimming control. See
Table 1 for details. For the MAX8848Y, ENA functions as
on/off control and serial-pulse dimming control for LED1–
LED5. ENB functions as on/off control and serial-pulse
dimming control for LED6 and LED7. For the MAX8848Z,
only ENA functions as on/off control as well as PWM dim-
ming control for LED1–LED7.
PWM Dimming Control (MAX8848Z)
When V IN is above its undervoltage lockout threshold,
UVLO, apply a PWM signal to ENA to set the corre-
sponding WLED current (see Table 1) that is proportional
to the signal duty cycle (0% duty cycle corresponds to
zero LED current and 100% duty cycle corresponds to
full LED current). The allowed PWM frequency range is
from 200Hz to 200kHz. If PWM dimming control is not
required, ENA works as a simple on/off control.
Serial-Pulse Dimming Control (MAX8848Y)
The MAX8848Y uses ENA/ENB as a serial-pulse control
interface to program the intensity of LED1–LED7. When
the LEDs are enabled by driving ENA/ENB high, the
MAX8848Y ramps LED current to 24mA. Subsequent
pulses on ENA/ENB reduce the LED current from 24mA
to 0.1mA in 31 steps. After the current reaches 0.1mA,
the next pulse restores the current to 24mA. See Table 2
Table 1. ENA and ENB Enable and Dimming Control
PART ENA ENB
MAX8848Y LED1–LED5 enable and serial-pulse dimming control
LED6 and LED7 enable and serial-pulse
dimming control
MAX8848Z LED1–LED7 enable and PWM dimming control —