100V 30A 450W Rail to Rail Operational Amplifier

PAD157 – Rev B

DESCRIPTION

The PAD157 rail to rail operational amplifier is constructed with surface mount components to provide a cost-effective solution for many industrial applications where it is important to obtain a maximum output signal with limited supply voltages. The PAD157 is an upgrade to the discontinued PAD127 and needs no balancing resistors for its parallel output MOSFETs. Otherwise, the PAD157 is a form, fit and function replacement for the PAD127. User selectable external compensation tailors the amplifier’s response to the application requirements. Four-wire programmable current limit is built-in, but the PAD157 is compatible with the external PAD125 current limit accessory module as well. The PAD157 also features a substrate temperature reporting analog output and over-temp shutdown. The amplifier circuitry is built on an insulated metal substrate mounted to an integral heat sink and fan assembly. No BeO is used in the PAD157. 

A NEW CONCEPT

A critical task in any power amplifier application is cooling the amplifier. Until now component amplifier manufacturers often treated this task as an after-thought, left for the user to figure out. At Power Amp Design the best heat sink and fan combination is chosen at the start and becomes an integral part of the overall amplifier design. The result is the most compact and volumetric efficient design combination at the lowest cost. In addition, this integrated solution concept offers an achievable real-world power dissipation rating, not the ideal rating usually cited when the amplifier case is somehow kept at 25^oC. The user no longer needs to specify, procure or assemble separate components.

CIRCUIT & CONNECTIONS

EQUIVALENT CIRCUIT

PINOUT & CONNECTIONS

ABSOLUTE MAXIMUM RATINGS

COMMON MODE RANGE

   The PAD157 is a rail-to-rail operational amplifier. This means that it works equally well with the input pins biased to either supply rail or at any voltage in between. The most common application utilizing this function is the single supply voltage amplifier where the +IN pin and the –Vs supply pin are grounded.

OUTPUT SWING

   With no load the output voltage of the PAD157 can swing to either supply voltage rail. As the load current increases the maximum output swing is reduced, but at 10A output the swing from the positive supply rail is less than 1V and less than 1.5V from the negative supply rail. This does not include any voltage drop due to the sensing voltage required for the current limit circuit to operate.

CURRENT LIMIT

   The current limiting function of the PAD157 is a versatile circuit that can be used to implement a four-wire current limit configuration or, in combination with some external components can be configured to implement a fold-over current limit circuit. The four-wire current limit configuration ensures that parasitic resistance in the output line, Rp, does not affect the programmed current limit setting. See Figure 1. The sense voltage for current limit is 0.65V. Thus:

Where I_L is the value of the limited current and R_S is the value of the current limit sense resistor.

   In addition, the sense voltage has a temperature coefficient approximately equal to –2.2mV/^oC. The fold-over function reduces the available current as the voltage across the output transistors increases to help ensure that the SOA of the output transistors is not exceeded. Refer to Application Circuits for details on how to connect the current limit circuitry to implement either a four-wire current limit or current limit with a fold-over function.

   In some applications better current limiting protection and a lower sense voltage may be desired. In this case the PAD157 can be operated with the PAD125 Current Limit Accessory Module. See Figure 3 in the applications section and the PAD125 data sheet for more details.

COOLING FAN

The PAD157 relies on its fan for proper cooling of the amplifier. Make sure that air flow to the fan and away from the heat sink remains unobstructed. To eliminate electrical noise created by the cooling fan we recommend a 47µF capacitor placed directly at the point where the fan wires connect to the PCB. See application note AN-24 for further details.

MOUNTING THE AMPLIFIER

   The amplifier is supplied with four 4-40 M/F hex spacers at the four corners of the amplifier. Since the male threaded ends of the spacers extend beyond the amplifier pins the spacers provide a convenient alignment tool to guide the insertion of the amplifier pins into the circuit board. Once the amplifier is seated secure the module with the provided     4-40 nuts and torque to 4.7 in lb [53 N cm] max. See “Dimensional Information” for a detailed drawing. It is recommended that the heat sink be grounded to the system ground. This can easily be done by providing a grounded circuit board pad around any of the holes for the mounting studs.

TEMPERATURE REPORTING

   An analog output voltage is provided (pin 6, TMP) relative to ground and proportional to the temperature in degrees C. The slope is approximately -10.82mV/^oC. The output voltage follows the equation:

T = (2.127 ─ V) (92.42)

Where V is the TMP output voltage and T is the substrate temperature in degrees C.

   This high impedance output circuit is susceptible to capacitive loading and pickup from the output of the amplifier. When monitoring TMP filter the voltage as shown in Figure 4. See Applications Circuits.

THERMAL SHUTDOWN

   The temperature monitoring circuit automatically turns off the output transistors when the substrate temperature reaches 110^oC. When the substrate cools down 10^oC the output is enabled once again. The thermal shutdown feature is activated either by amplifier overloads or a failure of the fan circuit. 

EXTERNAL SHUTDOWN

   When pin 8 () is taken low (ground) the output stage is turned “off” and remains “off” as long as pin 8 is low. When pin 8 is monitored with a high impedance circuit it also functions as a flag, reporting when the amplifier is shut down. A “high” (+5V) on pin 8 indicates the temperature is in the normal range. A “low” (ground) indicates a shutdown condition. See Application Circuits for details on how to implement an external shutdown circuit and how to monitor the shutdown status.

PHASE COMPENSATION

   The PAD157 must be phase compensated to operate correctly. The compensation capacitor, C_C, is connected between pins 4 and 5. On page 7, Typical Performance Graphs, you will find plots for small signal response and phase response using compensation values of 100pF and 470pF. The compensation capacitor must be an NPO type capacitor rated for the full supply voltage (100V). On page 2, under Amplifier Pinout and Connections, a table gives recommended compensation capacitance values for various gains and the resulting slew rate for each capacitor value.    

NO DEGENERATION RESISTORS

   As mentioned on page 1, the PAD157, unlike the PAD127 that it replaces, needs no degeneration (balancing) resistors even though the amplifier relies on its parallel output devices for its high-power rating. Special circuitry has been added to the design of the PAD157 to ensure current in the output devices is equally shared. In most cases the PAD157 is backwards compatible with PAD127 circuits previously developed. Minor performance differences may be noted. The PAD157 can be used in the evaluation kit for the PAD127 (EVAL127) if need be, but a new evaluation kit is available (EVAL157) that has deleted the requirement for the degeneration resistors. If the EVAL127 is used the degeneration resistors can be replaced with 18ga. jumper wire. 

SPECIAL PIN FUNCTIONS

 There are several special function pins of the PAD157 that do not exist on the PAD127 amplifier that the PAD157 replaces. On the PAD127 these pins are a NC, but for the PAD157 these pins service a power booster amplifier model PAD159. These pins are 13 (SH2), 18 (SL2), 27 (SL1) and 32 (SH1). These pins provide reference voltages that allow the PAD159 to follow the output stage of the PAD157, thus extending the output current and internal power dissipation of the combined PAD157-PAD159 amplifier. See the data sheet for the PAD159 for connection details. Without any connection to the PAD159 these special function pins must be left open.

Performance Graphs

1kHz sine clipped by standard current limit into 4Ω load	20kHz into 4Ω load, G=-10, Cc=100pF

	SHUTDOWN RESPONSE, NEGATIVE OUTPUT TO ZERO TRANSITION The oscilloscope display at the left shows an expanded view of a 1kHz 1.2A p-p amplifier output signal being interrupted near the negative peak by a shutdown signal on Ch2. The Ch1 display shows the output current going to zero about 2µS after the shutdown signal goes low. The ringing in the output signal is due to inductance in the output line.
SHUTDOWN RESPONSE, POSITIVE OUTPUT TO ZERO TRANSITION The oscilloscope display at the right shows an expanded view of a 1kHz 1.2A p-p amplifier output signal being interrupted near the positive peak by a shutdown signal on Ch2. The Ch1 display shows the output current going to zero about 0.5µS after the shutdown signal goes low. The ringing in the output signal is due to inductance in the output line.
Pulse Response, Positive to Negative, 4Ω LoadG=-10, Cc=100pF	Pulse Response, Negative to Positive , 4Ω LoadG=-10, Cc=100pF

Safe Operating Area

SAFE OPERATING AREA

The safe operating area (SOA) of a power amplifier is its single most important specification. The SOA graph presented above serves as a first approximation to help you decide if the PAD157 will meet the demands of your application. But a more accurate determination can be reached by making use of the   PAD Power™ spreadsheet which can be found in the Power Amp Design website. While the graph above adequately shows DC SOA and some pulse information it does not take into account ambient temperatures higher than 30^OC, AC sine, phase or non-symmetric conditions that often appear in real-world applications. The PAD Power™ spreadsheet takes all of these effects into account.

Dimensions

Application Circuits