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The MAX1578/MAX1579 provide four regulated outputs
to meet all the voltage requirements for small activematrix
TFT-LCD displays in handheld devices where
minimum external components and high efficiency are
required. Each device consists of three advanced
charge pumps for LCD bias power and a step-up converter
for driving up to 8 series white LEDs for backlighting.
The input voltage range is from 2.7V to 5.5V.
The charge pumps provide fixed +5V, +15V, and -10V
for the LCD bias circuits. No external diodes are needed.
A high-efficiency, fractional (1.5x/2x) charge pump
followed by a low-dropout linear regulator provides +5V
to power the source driver. Automatic mode changing
achieves the highest conversion efficiency. Two multistage,
high-voltage charge pumps generate +15V and
-10V to provide VON and VOFF, respectively. Utilizing a
unique clocking scheme and internal drivers, these
charge pumps eliminate parasitic charge-current glitches
and reduce maximum input current, resulting in low
electromagnetic emissions. The outputs are sequenced
during startup and shutdown. In shutdown, the outputs
are discharged to zero.
The high-efficiency inductor step-up converter drives
up to 8 white LEDs in series with a constant current to
provide backlighting. The series connection allows the
LED currents to be identical for uniform brightness and
minimizes the number of traces to the LEDs. The
MAX1578 regulates constant LED current over the
entire temperature range. The MAX1579 features a temperature
derating function to avoid overdriving the
white LEDs during high ambient temperatures, enabling
higher drive current below +42°C.
The MAX1578/MAX1579 are available in space-saving
24-lead 4mm x 4mm thin QFN packages.
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