ISL97644 Просмотр технического описания (PDF) - Renesas Electronics
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ISL97644 Datasheet PDF : 15 Pages
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ISL97644
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.
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
VDPM
0
FIGURE 19. CIRCUIT TO DISCONNECT THE DC PATH OF
BOOST CONVERTER
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
VDPM
0
VIN THRESHOLD
AVDD
0
LDO OUTPUT
VGH_M
FIGURE 18. START-UP SEQUENCE
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 ISL97644 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
ISL97644 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.
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
FN9228 Rev 1.00
December 14, 2007
Page 13 of 15
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