The ADALM-SR1 (Analog Devices Active Learning Module, Switching Regulator #1) board is a companion module for several switching regulator exercises covering buck and boost regulators:
The circuits required for these exercises exceed the complexity that can be constructed on a breadboard, so the ADALM-SR1 is required to run them, although the simulations can be run beforehand to gain insight into the circuits' operation.
The Figures 1a and 1b show an overview of the board, along with connections to an ADALM2000 (m2k) and meters.
Figure 1a. ADSRALM overview
Figure 1b. ADSRALM overview
The ADALM-SR1 uses 0.635mm (0.025-mil) headers for configuration jumpers, signal inputs, and signal outputs. Signal inputs and outputs are 2-conductor headers with 5.08mm (200-mil) pitch so that they cannot be confused with configuration jumpers. The lower conductor is always a ground connection (that is not always used) and an arrow indicates whether the upper conductor is an input or output.
Figure 2. Topology, Mode selection, and Loads
A Wurth 749196141 6-winding coupled inductor is used in both boost and buck configurations. The datasheet inductance for a single winding is 8.5μH, with a DC resistance of 344 milliohms. The windings are connected in series on the ADALM-SR1, allowing the inductance to be changed as noted in the table below. (Values in the table are measured from a typical board.)
|Inductance Selection (P3)|
|Position||EXT IND||1 TAP||2 TAPS||3 TAPS||4 TAPS||5 TAPS||6 TAPS|
|Function||Ext. inductor at TP3, TP4||7.7μH inductance||31.5μH inductance||72.6μH inductance||131.5μH inductance||216.2μH inductance||313.4μH inductance|
A 4.7μF capacitor is always connected to the output of the circuit. An additional 47μF and 470μF can be added by installing jumpers according to the table below.
|Output Capacitors Selection|
|Function||P8 connect aditional 47μF capacitance||No additional capacitance|
|P11 connect aditional 470μF capacitance||No additional capacitance|
A resistive load may be connected to the output of the circuit. Resistances range from 12.5Ω to 200Ω, and may be added in parallel according to the table below. The jumpers are 3-position, with the righ-hand position connecting the resistor to ground, and the left-hand position allowing the effective resistance to be adjusted by Pulse-width-modulating the ground connection.
|Load Resistors Selection|
|Jumper||P18, P14, P13, P17, P16, P15|
|Function||P18 connect 200 Ω load resistance||No aditional load resistor connected|
|P14 connect 200 Ω load resistance||No aditional load resistor connected|
|P13 connect 100 Ω load resistance||No aditional load resistor connected|
|P17 connect 50 Ω load resistance||No aditional load resistor connected|
|P16 connect 25 Ω load resistance||No aditional load resistor connected|
|P15 connect 12.5 Ω load resistance||No aditional load resistor connected|
|Notes||7V max across 25Ω, 12.5Ω resistors will turn on the Over Power LED illuminates as warning.|
The R87 (LOAD CONTROL) potentiometer controls the duty cycle of all load resistors whose jumper is placed in the adjustable position, by swithcing the ground connection. (YES this is super weird, but it's convenient and it works much better than you'd think!) Duty cycle is guaranteed to be zero when fully counter-clockwise and 100% when fully clockwise. Thus the load can be a combination of fixed and variable resistances, and the exact duty cycle of the onboard PWM circuit can be measured at P40. The signal at P40 has a 1k impedance and maybe overdriven by a 3.3V logic signal, allowing the load to be stepped.
The load PWM frequency is fixed at 200kHz, approximately 10x the typical operating frequency of most experiments, thus appearing as a steady (DC) load.
A 0.1 ohm current sense resistor is in the ground return of the load resistors, allowing the total load current to be easily measured with meter set to the 200mV range at either the LOAD turret post (TP25) or the LOAD CURRENT jumper (P39).
The selection between boost or buck topologies is made by jumpers P25, P35, and P37.
|FET Selection||Current Sense Selection|
|Function||Select buck topology||Select boost topology||Select the proper FET for buck topology||Select the proper FET for boost topology||Used for buck topology||Used for boost topology|
|Notes||Proper selection allows complete inductor waveform to be viewed.|
In the boost configuration, P37 does not bypass Schottky diode D4, P35 routes the FET driver control to the low-side switch, and P25 selects the high-side current sense amplifier.
In the buck configuration, P37 bypasses D4, P35 routes the FET driver control to the high-side switch, and P24 selects the low-side current sense amplifier.
Close inspection of the operation of the circuit will show that in theory, either current sense amplifier will work for both topologies, but the amplifier that is NOT at the switch node is chose in order to minimize errors due to common-mode excursion.
The ADALM-SR1 has several operational modes, set by the jumpers noted below.
The MODE jumper selects between peak current mode and duty cycle control:
|Control Mode Selection|
|Position||Peak Current||Duty Cycle|
|Function||Start on clock rising edge, stop on peak current||Direct duty cycle control|
|Notes||Closed/open-loop determined by Duty Cycle or Current Threshold jumper.|
The peak current circuit is always active, even in duty-cycle mode; increasing robustness in the event of an output short circuit.
An LTC6992 Pulse-Width Modulator allows the switching MOSFET's duty cycle to be controlled directly, either manually by adjusting the DUTY CYCLE knob, or under the control of the error amplifier.
|Duty Cycle Source Selection|
|Function||Duty controlled by error amplifier||Duty controlled by potentiometer|
|Notes||Current Threshold still operational for safety, determined by Current Threshold Potentiometer.|
In peak-current control modes, the peak current can be controlled either manually by adjusting the CURRENT THRESHOLD knob, or under the control of the error amplifier. The current threshold is also always active in voltage control modes, maintaining per-cycle current limit as an added safety and robustness feature.
|Current Threshold Source Selection|
|Function||Threshold controlled by potentiometer||Threshold controlled by error amplifier|
|Notes||Set Duty Cycle pot to approx. 50%. (not critical, rising edge controls start of the ramp)|
Three options for feedback network are provided, assuming a 1.25V reference voltage. The 5V option is usually used for buck experiments (input voltage greater than 5V), and the 12V option is usually used for boost experiments (input less than 12V). Footprints for optional 0603-sized resistors are included for user-defined feedback networks, selected by the third jumper option.
|Function||Usually used for buck topology||Usually used for boost topology||Select user-installed feedback resistors|
The ADALM-SR1 provides numerous test points for stimulating (modulating) and measuring the operation of the circuit, such as input and output voltage, inductor current, and output current. Several aspects of the circuit's dynamic response can also be measured:
* In open-loop voltage mode, modulate the duty cycle control voltage to characterize the response of the power stage * In open-loop current mode, modulate the current threshold (ITH) to characterize the response of the power stage * In any closed-loop mode, modulate the feedback divider to characterize the closed-loop response of the whole circuit.
Connections are summarized in Figure X below, and the following tables.
|TP1||P_PROT||Overvoltage/overcurrent protected power.|
|P1||SW_NODE_HI||Switch node in buck topology.|
|P4||SW_NODE_LOW||Switch node in boost topology.|
|P2||IHIGH||Current sense amplifier output, boost configurations (0.1Ω sense R, G=7, 1.429A/V net output).|
|P12||ILOW||Current sense amplifier output, buck configurations (0.1Ω sense R, G=7, 1.429A/V net output).|
|P10, P9, TP5, TP7||V_OUT||Experiment output voltage. P10 can be AC coupled by removing P9.|
|P5||FB_MEAS||Feedback perturbation measurement (AC coupled).|
|P33||CLOCK||Master clock, 3.3V logic level.|
|P34||GATE CONTROL||FET gate control signal, 3.3V logic level.|
|P28||ITH MOD||Peak current control modulation (1.429A/V). Install P29 to DC couple.|
|P27||DUTY CYCLE MOD||Duty Cycle modulation (100%/V). Install P26 to DC couple.|
|P7||FB_MOD||Feedback modulation for loop gain measurements (XXV/V). Install P6 to DC couple.|
Refer to individual lab exercises for detailed setup information.
The setup and operation of circuitry associated with the lab exercises is described in detail in the exercises themselves. The ADALM-SR1 includes various auxiliary housekeeping and protection circuits described here.
The ADALM-SR1 has two power inputs. The experiment power input is supplied by the user, and the voltage will vary depending on the experiment being run. An additional micro USB connector is the input for a 5V “housekeeping” supply that powers all of the control circuitry, allowing the experiment power to vary over a wide range. An LT3472 boosts / inverts the 5V supply to +15 / -2V, respectively. This provides a high voltage and slightly negative voltage for the LT1995 current sense amplifiers, and a negative supply for the error amplifier.
An LT1970-1.25 provides an accurate reference for the error amplifier and duty cycle, current threshold adjustment potentiometers.
The Figure below shows an LTspice schematic of the ADALM-SR1's protection circuitry. The simulation file is available at OL_engineer_proofing.asc and running the simulation in LTspice will exercise some of the various fault conditions.
An LTC4368 and associated circuitry protects the experiment power input by only turning on when the supply is between 3V and 15V. The circuit is protected from voltages between -40V and +60V. The LTC4368 also functions as a fuse, shutting off the supply if the current exceeds 2A.
In boost mode, the ADSRALM can produce high voltages under certain conditions: if the duty cycle is high and the load is light, or if feedback is disconnected. An LTC2912 overvoltage / undervoltage supervisor will disable the switching circuitry if the output exceeds 22V. An SMAJ24A, 24-volt TVS diode provides additional protection.
The inductor and onboard load resistors can get warm during certain experiments or if the board is misconfigured. Three temperature sensors measure the inductor temperature and the temperature of the high-dissipation areas of the load resistor bank. If any temperature exceeds 60ºC, switching is disabled for a 1.9 second cool-down period.
The low-resistance loads consist of parallel, single 100Ω, 1/2W resistors - An orange LED near the associated jumpers illuminates when the output voltage exceeds 7V as a warning that these must be disconnected.
The high resistance loads consist of multiples of two 100-ohm, 1/4W resistors in series, which will handle voltages up to 14V. A temperature sensor is still included in case the output voltage exceeds 14V or if the experiment is left running for an extended period of time.