AT1731A
Preliminary Product Information
DC-DC Power IC for TFT Panel
Figure 3 Charge-Pump Block Diagram
Shutdown
The AT1731A shuts down to reduce the supply current to 1uA when SHDN is low. In this
mode, the internal reference , error amplifier , comparators, and biasing circuitry turn off
while the N-channel MOSFET is turned off. The boost converter’s output is connected to Vin
normally. Do not leave SHDN pin floating. A logic-level transition on SHDN clears the fault
latch.
Power-Up Sequencing
The AT1731A goes through start-up sequence after power-up or exiting shutdown. First, the
reference power-up, then the main DC-DC step-up converter powers up with soft-start enable.
Once the main DC-DC step-up converter reaches regulation , the negative charge pump turns
on. When the negative charge pump output voltage reaches approximately 90% of its
nominal value (VFBN<110mV) , then the positive charge pump starts up. Finally, when the
positive output voltage reaches 100% of its nominal value (VFBP>1.1V) .The power-up
sequence is completed, see figure 4.
Vin
Shotdown signal
Nominal Regulator Value
0V
Step-up Vout
Nominal
Regulator 90%
Positive Charge-Pump Vout
0V
Vmain
0V
Nominal Regulator 100%
0V
Negative charge-Pump Vout
Figure 4 Power Sequence
Soft-Start
Soft-start allows a gradual increase of the internal current-limit level for the main step-up
converter during power-up to reduce input surge currents. As the internal 2uA current source
charges the external soft-start capacitor, the peak N-MOS current is limited by the voltage on
the capacitor. For the dual charge pumps , soft-start is achieved by in-turn controlling the
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2/8/2006 REV:1.1
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