ADA4859-3 Analog Devices, ADA4859-3 Datasheet - Page 12

• Manufacturer Part Number: ADA4859-3
• Description: Single-Supply, Fixed G = 2, High Speed, Video Amplifier with Charge Pump
• Manufacturer: Analog Devices

Specifications of ADA4859-3

• -3db Bandwidth: 265MHz
• Slew Rate: 740V/µs
• Vos: n/a
• Ib: 700nA
• # Opamps Per Pkg: 3
• Input Noise (nv/rthz): 17nV/rtHz
• Vcc-vee: 3V to 5.5V
• Isy Per Amplifier: n/a
• Packages: CSP

ADA4859-3
THEORY OF OPERATION
OVERVIEW
The ADA4859-3 is a fixed gain of two, current feedback amplifier
designed for exceptional performance as a triple video amplifier. Its
specifications make it especially suitable for SD and HD video
applications. The ADA4859-3 provides HD video output on a
single supply as low as 3.0 V while only consuming 13 mA per
amplifier. It also features a power-down pin (PD) that reduces
the quiescent current to 4 mA when activated.
The ADA4859-3 can be used in applications that require both
ac- and dc-coupled inputs and outputs. The output stage on the
ADA4859-3 is capable of driving 2 V p-p video signals into two
doubly terminated video loads (150 Ω each) on a single 5 V supply.
The input range of the ADA4859-3 includes ground, whereas
the output range is limited by the output headroom set by the
voltage drop across two diodes from each rail, which occurs
1.2 V from the positive and negative supply rails.
CHARGE PUMP OPERATION
The on-board charge pump creates a negative supply for the
amplifier. It provides different negative voltages depending on
the power supply voltage. For a +5 V supply, the negative supply
generated is equal to −3 V with 150 mA of output supply current,
and for a +3.3 V supply, the negative supply is equal to −2 V
with 45 mA of output supply current.
Figure 30 shows the charging cycle when the supply voltage,
+V
quickly charges to reach the +V
begins with switching Φ
Figure 31. When C1 = C2, the charge in C1 is divided between
the two capacitors and slowly increases the voltage in C2 until it
reaches a predetermined voltage (−3 V for the +5 V supply and
−2 V
for the +3.3 V supply). The typical charge pump charging and
discharging frequency is 550 kHz with a 150 Ω load and no input
signal. This frequency changes with the load current, and it can
get much slower if the amplifier is powered down and no external
current is used.
S
, charges C1 through Φ
1
off and switching Φ
1
to ground. During this cycle, C1
S
voltage. The discharge cycle then
2
on, as shown in
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The ADA4859-3 specifications make it especially suitable for SD
and HD video applications. It also allows dc-coupled video signal
with its black level set to 0 V and its sync tip down to −300 mV
for YPbPr video.
The charge pump is always on, even when the power-down pin
(PD) is enabled and the amplifier is off. However, it would be in
an idle state if the negative current were not used. Each amplifier
needs −6.3 mA of current, which totals −19 mA for all three
amplifiers. This means additional negative current may be available
by the charge pump for external use. Pin 4 (CPO) is the charge
pump output, which provides access to the negative supply
generated by the charge pump. Placing a 1 μF charge capacitor at
the CPO pin is essential to hold the charge and regulate the ripple.
If the negative supply is used to power another device in the
system, it is only possible for the 5 V supply operation. In the
3.3 V supply operation, the charge pump output current is very
limited. The capacitor at the CPO pin, which regulates the ripple
of the negative voltage, can be used as a coupling capacitor for the
external device. However, the charge pump current should be
limited to a maximum of 50 mA for external use. When powering
down the ADA4859-3, the charge pump is not affected and its
output voltage and current remain available for external use.
CPO
CPO
+V
+V
S
S
Figure 31. Discharging Cycle
C2
C2
Figure 30. Charging Cycle
Φ
Φ
2
1
C1
C1
b
a
b
a
Φ
Φ
1
2