ADA4858-3

Rev. 0 | Page 13 of 16

THEORY OF OPERATION

OVERVIEW

The ADA4858-3 is a current feedback amplifier designed for

exceptional performance as a triple amplifier with a variable

gain capability. Its specifications make it especially suitable

for SD and HD video applications. The ADA4858-3 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 2 mA when activated.

The ADA4858-3 can be used in applications that require both

ac- and dc-coupled inputs and outputs. The output stage on the

ADA4858-3 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 ADA4858-3 includes ground,

while the output range is limited by the output headroom set by

the voltage drop across the 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.

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 +5 V supply and −2 V for

+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 reach

the dc level if the amplifier is powered down.

CPO

C2

C1

a

b

Figure 36. C1 Charging Cycle

CPO

C2

C1

a

b

Figure 37. C1 Discharging Cycle

The ADA4858-3 specifications make it especially suitable for SD

and HD video applications. It also allows dc-coupled video signals

with its black level set to 0 V and its sync tip at −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, if a negative

current is not used, it is in an idle state. 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.

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 ADA4858-3, the charge pump is not affected and its

output voltage and current is still available for external use.