ISL24021
10
FN6637.1
October 18, 2011
.
Typical Application Circuit
A typical application of the ISL24021 is as a TFT-LCD VCOM driver
(see Figure 27). A VCOM driver maintains the backplane common voltage of a TFT-LCD panel. Maintaining the VCOM voltage at a
steady level is critical to panel performance. The ability of the
ISL24021 to source/sink large peak short circuit currents make
it ideal as a VCOM driver. The ±1A short circuit current capability
combined with a large bandwidth and fast settling time give the
ISL24021 ideal VCOM driver characteristics, and make it a great
choice for TFT-LCD applications.
Power Dissipation
With a 300mA maximum continuous output drive capability, it is
possible to exceed the rated +150°C maximum junction
temperature. It is important to calculate the maximum power
dissipation of the ISL24021 for the application. Proper load
conditions will ensure that the ISL24021 junction temperature
stays within a safe operating region.
The ISL24021 has a built-in thermal protection, which
automatically shuts the output OFF (high impedance) when the
die temperature reaches +165°C. This ensures safe operation
and prevents internal damage to the device. When the die cools
by +15°C the output will automatically turn ON.
The maximum power dissipation allowed in a package is
determined according to Equation
1:where:
TJMAX = Maximum junction temperature
TAMAX = Maximum ambient temperature
θJA = Thermal resistance of the package
PDMAX = Maximum power dissipation in the package
The actual maximum power dissipation of the IC is the total
quiescent supply current, times the total power supply voltage,
plus the power dissipation in the IC caused by the loading
condition.
Sourcing:
Sinking:
VS = Total supply voltage range (VDD - VSS)
IS = Device supply current
VDD = Positive supply voltage
VSS = Negative supply voltage
VOUT = Output voltage
ILOAD = Load current
Device overheating can be avoided by calculating the minimum
resistive load condition, RLOAD, resulting in the highest power
dissipation. To find RLOAD, set the two PDMAX equations equal to
each other and solve for VOUT/ILOAD. Reference the package
power dissipation curve, Figure
28, for further information.
ISL24021
INP
OUT
CSNUBBER
RSNUBBER
VDD
4.7F
0.1F
4.7F
VSS
THERMAL PAD
CONNECTED TO VSS
INN
ZL
FIGURE 25. OUTPUT SNUBBER CIRCUIT
ISL24021
INP
OUT
RSERIES
VDD
4.7F
0.1F
4.7F
VSS
THERMAL PAD
CONNECTED TO VSS
INN
ZL
FIGURE 26. OUTPUT SERIES RESISTOR CIRCUIT
INP
OUT
VDD = 15V
4.7F
0.1F
VSS
NOTE: CSTORAGE WILL VARY
DEPENDING ON THE
APPLICATION
INN
PANEL
ISL24021
TFT-LCD
CAPACITANCE
VCOM
CALIBRATOR/
RESISTOR
LADDER
THERMAL PAD CONNECTED
TO VSS
-
+
CSTORAGE
+
0.1F
FIGURE 27. TYPICAL APPLICATION CIRCUIT: TFT-LCD VCOM
PDMAX
TJMAX TAMAX
–
ΘJA
----------------------------------------
=
(EQ. 1)
PDMAX
VS IS VDD VOUT) ILOAD
×
–
[]
+
×
=
(EQ. 2)
PDMAX
VS IS VOUT VSS) ILOAD
×
–
[]
+
×
=
(EQ. 3)