POWER-DOWN MODE

VCC

PD

VCC-

To Internal Bias

Circuitry Control

50 k

Ω

10

100

1000

100 k

1 M

10 M

f Frequency - Hz

OUTPUT IMPEDANCE (SHUTDOWN)

vs

FREQUENCY

100 M

1 G

VCC = ±5 V

Rf = R(g) = 500 Ω

THS4150

THS4151

SLOS321G – MAY 2000 – REVISED MARCH 2009........................................................................................................................................................... www.ti.com

The power-down mode is used when power saving is required. The power-down terminal (PD) found on the

THS415x is an active low terminal. If it is left as a no-connect terminal, the device will always stay on due to an

internal 50 k

Ω resistor to VCC. The threshold voltage for this terminal is approximately 1.4 V above VCC–. This

means that if the PD terminal is 1.4 V above VCC–, the device is active. If the PD terminal is less than 1.4 V

above VCC–, the device is off. For example, if VCC– = –5 V, then the device is on when PD reaches 3.6 V, (–5 V +

1.4 V = –3.6 V). By the same calculation, the device is off below –3.6 V. It is recommended to pull the terminal to

VCC– in order to turn the device off. Figure 42 shows the simplified version of the power-down circuit. While in the

power-down state, the amplifier goes into a high-impedance state. The amplifier output impedance is typically

greater than 1 M

Ω in the power-down state.

Figure 42. Simplified Power-Down Circuit

Due to the similarity of the standard inverting amplifier configuration, the output impedance appears to be very

low while in the power-down state. This is because the feedback resistor (Rf) and the gain resistor (R(g)) are still

connected to the circuit. Therefore, a current path is allowed between the input of the amplifier and the output of

the amplifier. An example of the closed-loop output impedance is shown in Figure 43.

Figure 43.

20

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