ISL59837 Datasheet, PDF (13/14 Pages) Intersil Corporation – 200MHz Single Supply Video Driver With Charge Pump and Power Down

ISL59837
designed to drive three channels; simply deal with each
channel separately as described in this section.
DC-RESTORE
When the ISL59837 is AC-coupled it becomes necessary to
restore the DC reference for the signal. This is accomplished
with a DC-restore system applied between the capacitive
"AC" coupling and the input of the device. Refer to
“ISL59837 + DC-Restore Solution” on page 10.
DISABLE/POWER-DOWN
The ISL59837 can be disabled and its output placed in a
high impedance state. The turn-off time is around 25ns and
the turn-on time is around 200ns. When the PD pin is taken
high, the chip’s total supply current is reduced to 0.1mA
typically, all but eliminating the power used by the part.
Taking the EN pin high powers down the amplifiers, leaving
the charge pump running. Both pins can be controlled by
standard TTL or CMOS signal levels (0.8V V_IL, 2.0V V_IH
relative to GND).
OUTPUT DRIVE CAPABILITY
The ISL59837 does not have internal short-circuit protection
circuitry. A short-circuit current of 80mA sourcing and 150mA
sinking for the output is connected half way between the rails
with a 10Ω resistor. If the output is shorted indefinitely, the
power dissipation could easily increase such that the part will
be destroyed. Maximum reliability is maintained if the output
current never exceeds ±40mA, after which the electro-migration
limit of the process will be exceeded and the part will be
damaged. This limit is set by the design of the internal metal
interconnections.
POWER DISSIPATION
With the high output drive capability of the ISL59837, it is
possible to exceed the +150°C absolute maximum junction
temperature under certain load current conditions.
Therefore, it is important to calculate the maximum junction
temperature for an application to determine if load conditions
or package types need to be modified to assure operation of
the amplifier in a safe operating area.
The maximum power dissipation allowed in a package is
determined according to Equation 1:
PD_MAX
=
T_{----J---M-----A----X-----–-----}T_{----A---M-----A----X--}
θ_JA
(EQ. 1)
Where:
T_JMAX = Maximum junction temperature
T_AMAX = Maximum ambient temperature
θ_JA = Thermal resistance of the package
The maximum power dissipation actually produced by an IC
is the total quiescent supply current times the total power
supply voltage, plus the power in the IC due to the load, or:
for sourcing:
PD_MAX
=
V_S
×
I_SMAX
+
(V_S
–
V_OUTi)
×
-^V----^O----^U----^T----ⁱ
R_Li
(EQ. 2)
for sinking:
PD_MAX = V_S × I_SMAX + (V_OUTi – V_S) × I_LOADi
(EQ. 3)
Where:
V_S = Supply voltage
I_SMAX = Maximum quiescent supply current
V_OUT = Maximum output voltage of the application
R_LOAD = Load resistance tied to ground
I_LOAD = Load current
i = Number of output channels
By setting the two P_DMAX equations equal to each other, we
can solve the output current and R_LOAD to avoid the device
overheat.
Power Supply Bypassing and Printed Circuit
Board Layout
Strip line design techniques are recommended for the input
and output signal traces. As with any high frequency device,
a good printed circuit board layout is necessary for optimum
performance. Lead lengths should be as short as possible.
The power supply pin must be well bypassed to reduce the
risk of oscillation. For normal single supply operation, where
the V_S- pin is connected to the ground plane, a single 4.7µF
tantalum capacitor in parallel with a 0.1µF ceramic capacitor
from V_S+ to GND will suffice. This same capacitor
combination should be placed at each supply pin to ground if
split-internal supplies are to be used. In this case, the V_S-
pin becomes the negative supply rail.
For good AC performance, parasitic capacitance should be
kept to a minimum. Use of wire-wound resistors should be
avoided because of their additional series inductance. Use
of sockets should also be avoided if possible. Sockets add
parasitic inductance and capacitance can result in
compromised performance. Minimizing parasitic capacitance
at the amplifier's inverting input pin is also very important.
13
FN6335.1
March 5, 2007