Data Sheet

ADA4858-3

Rev. B | Page 13 of 20

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 total 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, and 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 supply and the charge pump 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 36 shows the charging cycle when the supply voltage +VS

charges C1 through Φ1 to ground. During this cycle, C1 quickly

charges to reach the +VS voltage. The discharge cycle then begins

with switching Φ1 off and switching Φ2 on, as shown in Figure 37.

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;

however, this frequency changes with different loads and supply

conditions.

+VS

CPO

C2

C1

Φ1

Φ1

a

b

Figure 36. C1 Charging Cycle

+VS

CPO

C2

C1

Φ2

Φ2

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 amplifiers are off. However, if a negative

current is not used, the charge pump 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 that provides access to the negative

supply generated by the charge pump.

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 C2 placed 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 are still

available for external use.

It is recommended to use 1 µF low ESR and low ESL capacitors

for C1 and C2. These capactiors should be placed very close to

the part. C1 should be placed between Pin C1_a and Pin C1_b,

and C2 should be placed between Pin CPO and ground. If the

charge pump ripple at the CPO pin is too high, larger capacitors

(that is, 4.7 µF) can replace the 1 µF at C1 and C2.