Rainbow-electronics MAX1817 Bedienungsanleitung PDF herunterladen (Seite 8)

MAX1817

the output voltage rises above the undervoltage thresh-

old, the control circuitry is enabled, allowing proper

regulation of the output voltage.

LCD Step-Up Converter

The MAX1817’s LCD step-up converter generates an

LCD bias voltage up to 28V by use of a 500mA, 1.1Ω

internal N-channel switching MOSFET (Figure 1). The

LCD step-up converter control circuitry is powered from

the main step-up converter output (OUT), so the voltage

at OUT must be above the OUT undervoltage lockout

voltage for the LCD step-up converter to operate.

During startup, the MAX1817 extends the minimum off-

time to 5µs for V

FBLCD

voltages <0.9V, limiting initial

surge current. The LCD step-up converter features an

independent shutdown control, ONLCD.

The LCD step-up converter features a minimum-off-

time, current-limited control scheme. A pair of one-

shots that set a minimum off-time and a maximum on-

time governs the duty cycle. The switching frequency

can be up to 500kHz and depends upon the load, and

input and output voltages.

Compact, High-Efficiency, Dual-Output

Step-Up DC-DC Converter

8 _______________________________________________________________________________________

OFF

ONLCD

SHUTDOWN

LOGIC

MAIN

UNDERVOLTAGE

LOCKOUT

DUAL-MODE FEEDBACK

BIAS

LCD

STARTUP

ERROR

COMPARATOR

MAIN

CURRENT

LIMIT

MAIN

ZERO-

CROSSING

DETECTOR

CONTROL

LOGIC

MAIN

1.25V

AGND

GND

OUT

FBLCD

LXLCD

LCD

MAIN

GND

SHUTDOWN

LOGIC

LCD

ERROR

COMPARATOR

LCD

1.25V

AGND

CURRENT

LIMIT

LCD

CONTROL

LOGIC

LCD

MAX1817

75mV

Figure 1. MAX1817 Simplified Functional Diagram

1 2 3 4 5 6 7 8 9 10 11 12

Step-Up DC-DC Converter 1

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