ISL97645 Просмотр технического описания (PDF) - Renesas Electronics
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ISL97645 Datasheet PDF : 14 Pages
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ISL97645
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.
the following circuit can be inserted between input and inductor
to disconnect the DC path when the part is disabled.
TO INDUCTOR
INPUT
ENABLE
VDPM
0
VFLK
0
VGH_M
0
SLOPE CONTROLLED
BY RE AND LOAD
CAPACITANCE
VGH
VDD_1
DELAY TIME
CONTROLLED BY CE
FIGURE 15. GATE PULSE MODULATOR TIMING DIAGRAM
Start-Up Sequence
Figure 16 shows a detailed start-up sequence waveform.
VIN
0
ENABLE
0
VDPM
0
VIN THRESHOLD
AVDD
VGH_M
0
FIGURE 16. START-UP SEQUENCE
When VIN exceeds 2.5V and ENABLE reaches the VIH
threshold value, Boost converter starts 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,
FIGURE 17. 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 ISL97645 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
ISL97645 provides the overall fault protections including over
current 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.
FN9263 Rev 1.00
December 14, 2007
Page 12 of 14
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