Data Sheet

ADA4870

APPLICATIONS INFORMATION

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Rev. B | 19 of 24

ON, INITIAL POWER-UP, AND SHORT-CIRCUIT

After initial power-up, the ON pin must be pulled low to ensure

that the amplifier is turned on. Subsequently, floating the ON pin en-

ables the short-circuit protection feature while the amplifier remains

on. While ON is held low, the short-circuit protection feature is

disabled.

The ADA4870 short-circuit protection current limit of 1.2 A is a

typical specification designed to protect the device under nominal

conditions and varies over temperature and over supply voltage

configuration. For applications relying on the short-circuit protection

current limit threshold value, characterize this threshold in the target

configuration.

When a short-circuit condition is detected, the amplifier is disabled,

the supply current drops to about 5 mA, and the TFL pin outputs

a dc voltage of ~300 mV. To turn the amplifier back on after a

short-circuit event, follow the sequence for initial power-up.

Pulling the ON pin high disables the amplifier and causes the

supply current to drop to about 5 mA, as if a short-circuit condition

had been detected.

The impedance at the ON pin is ~20 kΩ. Lay out the PCB trace

leading to ON to avoid noise coupling into it and triggering a false

event. A 1 nF capacitor between ON and VEE is recommended to

help shunt noise away from ON.

THERMAL PROTECTION

In addition to short-circuit protection, the ADA4870 is also protected

against excessive die temperatures.

During normal operation, the TFL pin outputs a dc voltage (refer-

enced to VEE) ranging from 1.5 V to 1.9 V that is relative to

die temperature. The voltage on TFL changes at approximately

−3 mV/°C and can be used to indicate approximate increases in

die temperature. When the die temperature exceeds approximately

140°C, the amplifier switches to an off state, dropping the supply

current to approximately 5 mA, and TFL continues to report a

voltage relative to die temperature. When the die temperature

returns to an acceptable level, the amplifier automatically resumes

normal operation.

SHUTDOWN (SD)

The ADA4870 is equipped with a power saving shutdown feature.

Pulling SD low places the amplifier in a shutdown state, reducing

quiescent current to approximately 750 µA. When turning the ampli-

fier back on from the shutdown state, pull the SD pin high and then

pull the ON pin low. Following this sequence ensures power-on.

Afterwards, the ON pin can be floated to enable short-circuit protec-

tion.

Pull SD high or low; do not leave SD floating.

FEEDBACK RESISTOR SELECTION

The feedback resistor value has a direct impact on the stability

and closed-loop bandwidth of current feedback amplifiers. Table 6

provides a guideline for the selection of feedback resistors for some

common gain configurations.

Table 6. Recommended RF Values

Closed-Loop Gain

(V/V)

RF (Ω)

RG (Ω)

CL (pF)

RS (Ω)

+1

2000

Open

300

5

−1

1210

1210

300

5

+2

1500

1500

300

5

−2

1210

604

300

5

+5

1210

301

300

5

+10

1210

133

300

5

CAPACITIVE LOAD DRIVING

When driving a capacitive load (CL), the amplifier output resistance

and the load capacitance form a pole in the transfer function of

the amplifier. This additional pole reduces phase margin at higher

frequencies and, if left uncompensated, can result in excessive

peaking and instability. Placing a small series resistor (RS) between

the amplifier output and CL (as shown in Figure 69) allows the

ADA4870 to drive capacitive loads beyond 1 μF. Figure 70 shows

the series resistor value vs. capacitive load for a maximum of 1 dB

peaking in the circuit of Figure 69. For large capacitive loads, RS

values of less than 0.3 Ω are not recommended.

Figure 71 shows the small signal bandwidth (SSBW) vs. CL with

corresponding RS values from Figure 70.

Figure 69. Circuit for Capacitive Load Drive

Figure 70. RS vs. CL for Maximum 1 dB Peaking for Circuit from Figure 69