Analog Integrated Circuit Device Data

Freescale Semiconductor

13

34830

FUNCTIONAL DEVICE OPERATION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

the level-shifter acts simply as a buffer of gain one. The

second reason, is to bring the input signal into the proper

operating range of the output buffer. Shifting the signal up

allows the output buffer to work in its sweet spot. This also

prevents the output devices of the output buffer from going

into saturation.

Since the level-shifter needs to pass the signal without

affecting it, it really is a high-speed amplifier. The current that

biases this block comes from the bandwidth adjust section,

which allows for the power consumption to be decreased if

lower bandwidths are required. Refer to Figure 6.

OUTPUT BUFFER

The output buffer is a high-speed (800MHz open-loop

bandwidth), operational amplifier used in a non-inverting gain

of two configuration through resistive feedback. The amplifier

uses a class AB topology with a rail-to-rail output that

incorporates saturation protection as well as current-limiting.

In this way the 34830 is protected against excessive loads or

short-circuit conditions to both supply and ground and will

resume its normal operation as soon as the short-circuit or

overload condition is removed.

The output buffer also uses PTAT current biasing that

bandwidth can be decreased, resulting in power consumption

savings.

The output buffer has been optimized to drive a standard

video load (150Ω) with up to 5pF of load capacitance, while

meeting all of the specifications listed in the electrical

characteristics table. The output buffer can also support two

standard video loads with a slight relaxation in the

specifications.

SHUTDOWN

The 34830 features an enable input (EN) that allows the

device to be placed in a low-supply-current shutdown state

when not required to pass a video signal. Driving EN high

puts the 34830 in its active mode. Driving EN low puts the

34830 in shutdown. In shutdown, the device has a supply

current of 120nA and its output becomes high impedance.

The shutdown feature makes the 34830 ideal for portable

applications where power consumption is critical.

SETTING KEY CLAMP BIAS

Figure 23. In this configuration.

negative value of the signal at the input to the 34830 is above

50mV. This prevents the internal clamp from turning on. To

bias current that flows through the resistor divider network

The general relationship between input and output voltage

of the channel is given by the formula:

Where the 250mV term is the offset provided by the

internal level shifter. The 100mV term that is added to the

equation represents the worst case errors and offsets that

can be expected from the signal path, due to process and

temperature variations. The DC bias at the output is given by

Figure 23. Key Clamp DC Bias Configuration

SETTING BANDWIDTH

The bandwidth of the 34830 is set through an external

the value of the resistor causes the quiescent current of the

device to decrease, which in turn decreases its bandwidth.

mainly to increase quiescent supply current and thus

9kΩ and 108kΩ. Refer to Figure 5 for a relationship between

34830. To ensure that the channel bandwidth is greater than

the one needed for the application, after taking into account

process and temperature variation, multiply the value of

RFREQ obtained from the graph by 0.6. Use this number as

the value of the external resistor.

It is recommended to place a small capacitor (100pF) in

input, which may disturb the bias conditions of the device.

VCLAMP

×

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RC2

RC1 VCC VCLAMP

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×

VCLAMP

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VOUT 2VIN 250mV

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×

100mV

±

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MC34830

IN

AC coupling

capacitor