ADA4859-3

Rev. 0 | Page 12 of 16

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,

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.

CPO

C2

C1

a

b

Figure 30. Charging Cycle

CPO

C2

C1

a

b

Figure 31. Discharging Cycle

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.