ISL97646 Просмотр технического описания (PDF) - Intersil
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ISL97646 Datasheet PDF : 15 Pages
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ISL97646
Low to high transition is determined primarily by the switch
resistance and the external capacitive load. High to low
transition is more complex. Take the case where the block is
already enabled (VDPM is H). When VFLK is H, pin CE is
grounded. On the falling edge of VFLK, a current is passed
into pin CE, to charge an external capacitor to 1.2V. This
creates a delay, equal to CE*4200. At this point, the output
begins to pull down from VGH to VDD1. The slew current is
equal to 300/(RE + 5000)*Load Capacitance.
VDPM
0
VFLK
0
VGH_M
0
SLOPE CONTROLLED
BY RE AND LOAD
CAPACITANCE
VGH
VDD_1
DELAY TIME
CONTROLLED BY CE
FIGURE 17. GATE PULSE MODULATOR TIMING DIAGRAM
Start-Up Sequence
Figure 18 shows a detailed start-up sequence waveform.
VIN
0
ENABLE
0
VIN THRESHOLD
VDPM
0
AVDD
0
LDO OUTPUT
VGH_M
FIGURE 18. START-UP SEQUENCE
When VIN exceeds 2.5V and ENABLE reaches the VIH
threshold value, Boost converter and LDO start-up, and gate
pulse modulator circuit output holds until VDPM goes to
high. Note that there is a DC path in the boost converter from
the input to the output through the inductor and diode, hence
the input voltage will be seen at output with a forward voltage
drop of diode before the part is enabled. If this voltage is not
desired, the following circuit can be inserted between input
and inductor to disconnect the DC path when the part is
disabled.
TO INDUCTOR
INPUT
ENABLE
FIGURE 19. CIRCUIT TO DISCONNECT THE DC PATH OF
BOOST CONVERTER
VCOM Amplifier
The VCOM amplifier is designed to control the voltage on the
back plate of an LCD display. This plate is capacitively
coupled to the pixel drive voltage which alternately cycles
positive and negative at the line rate for the display. Thus,
the amplifier must be capable of sourcing and sinking
capacitive pulses of current, which can occasionally be quite
large (a few 100mA for typical applications).
The ISL97646 VCOM amplifier's output current is limited to
400mA. This limit level, which is roughly the same for
sourcing and sinking, is included to maintain reliable
operation of the part. It does not necessarily prevent a large
temperature rise if the current is maintained. (In this case the
whole chip may be shut down by the thermal trip to protect
functionality). If the display occasionally demands current
pulses higher than this limit, the reservoir capacitor will
provide the excess and the amplifier will top the reservoir
capacitor back up once the pulse has stopped. This will
happen on the µs time scale in practical systems and for
pulses 2 or 3 times the current limit, the VCOM voltage will
have settled again before the next line is processed.
Fault Protection
ISL97646 provides the overall fault protections including
overcurrent protection and over-temperature protection.
An internal temperature sensor continuously monitors the
die temperature. In the event that die temperature exceeds
the thermal trip point, the device will shut down and disable
itself. The upper and lower trip points are typically set to
+140°C and +100°C, respectively.
13
FN6265.1
December 14, 2007
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