Datasheet LT1795 (Analog Devices) - 9

LT1795
U U W U APPLICATIO S I FOR ATIO
ture until the device begins thermal shutdown gives a ing 0.5A current peaks into the load, a 1Ω power supply good indication of how much margin there is in the impedance will cause a droop of 0.5V, reducing the thermal design. available output swing by that amount. Surface mount tantalum and ceramic capacitors make excellent low ESR For surface mount devices, heat sinking is accomplished bypass elements when placed close to the chip. For by using the heat spreading capabilities of the PC board frequencies above 100kHz, use 1µF and 100nF ceramic and its copper traces. For the TSSOP package, power is capacitors. If significant power must be delivered below dissipated through the exposed heatsink. For the SO 100kHz, capacitive reactance becomes the limiting factor. package, power is dissipated from the package primarily Larger ceramic or tantalum capacitors, such as 4.7µF, are through the V – pins (4 to 7 and 14 to 17). These pins recommended in place of the 1µF unit mentioned above. should have a good thermal connection to a copper plane, either by direct contact or by plated through holes. The Inadequate bypassing is evidenced by reduced output copper plane may be an internal or external layer. The swing and “distorted” clipping effects when the output is thermal resistance, junction-to-ambient will depend on driven to the rails. If this is observed, check the supply pins the total copper area connected to the device. For example, of the device for ripple directly related to the output the thermal resistance of the LT1795 connected to a 2 × 2 waveform. Significant supply modulation indicates poor inch, double sided 2 oz copper plane is 40°C/W. bypassing.
Capacitance on the Inverting Input CALCULATING JUNCTION TEMPERATURE
Current feedback amplifiers require resistive feedback The junction temperature can be calculated from the from the output to the inverting input for stable operation. equation: Take care to minimize the stray capacitance between the TJ = (PD)(θJA) + TA output and the inverting input. Capacitance on the invert- where ing input to ground will cause peaking in the frequency response (and overshoot in the transient response), but it TJ = Junction Temperature does not degrade the stability of the amplifier. TA = Ambient Temperature
Feedback Resistor Selection
PD = Device Dissipation The optimum value for the feedback resistors is a function θJA = Thermal Resistance (Junction-to-Ambient) of the operating conditions of the device, the load imped- ance and the desired flatness of response. The Typical AC
Differential Input Signal Swing
Performance tables give the values which result in less The differential input swing is limited to about ±5V by an than 1dB of peaking for various resistive loads and oper- ESD protection device connected between the inputs. In ating conditions. If this level of flatness is not required, a normal operation, the differential voltage between the higher bandwidth can be obtained by use of a lower input pins is small, so this clamp has no effect. However, feedback resistor. in the shutdown mode, the differential swing can be the For resistive loads, the COMP pin should be left open (see same as the input swing. The clamp voltage will then set Capacitive Loads section). the maximum allowable input voltage.
Capacitive Loads POWER SUPPLY BYPASSING
The LT1795 includes an optional compensation network To obtain the maximum output and the minimum distor- for driving capacitive loads. This network eliminates most tion from the LT1795, the power supply rails should be of the output stage peaking associated with capacitive well bypassed. For example, with the output stage supply- loads, allowing the frequency response to be flattened. 1795fa 9