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Here is a set of notes on the MC smps controller chip.
This chip has been around for quite a while and is very inexpensive. It can provide switched power conversion datasheer currents of a few hundred milliamps to over an amp depending on the configuration. It comes in an 8 pin DIP and requires only a handful of additional components.
It is useful in applications such as providing analogue positive and negative supplies from a single 5V logic supply in small circuit boards. The control functions provided consist simply of a comparator that gives hysteretic or bang-bang control. This is thought to be very effective, giving excellent transient performance, but its stability is not well understood.
The aim of these notes is to provide some insight into the operation of the chip and associated circuits. Practical circuits and problems experienced.
The theory of operation is well covered datashedt reference 2 of the resources. The charge current is modified by a voltage at the Ipk-sense pin to shorten the charge time and so reduce the maximum duty cycle.
EX Datasheet, PDF – Datasheet Search Engine
This provides a form of current limiting. A set-reset flip flop is held in the reset state during the discharge cycle, causing the output switching transistor to turn off. Datazheet the charging period the flip flop is set when the voltage at the feedback input pin 5 falls below 1. If the feedback voltage remains above 1.
This program is opensource and has been rewritten many times to improve performance and to adapt it to a number of specialist application areas. While it is no replacement for measurements on hardware, it can provide quite good datashedt modelling of complex circuits, allowing many errors datashewt problems to be corrected before the hardware is built.
It is also valuable for gaining an intuition into how circuits work. Two opensource simulators are provided with the gEDA design suite: The conversion to PSpice mentioned in reference 3 uses different syntax again.
Ex 34063 Datasheet Pdf Storage
The model is different to the AEi model but makes use of some information about MC behaviour. It is provided as opensource under the GPLv2 licence. The following diagram of the model is presented in terms of comparators datashset standard digital gates for clarity.
The gschem circuit shown above is given here for illustrative purposes. It includes a number of simple logic gates built from nonlinear dependent voltage sources. This file is not suitable in itself for generating a subcircuit as the gates need to have their supplies referenced to the common pin of the MC rather than to ground this could be done by defining custom symbols.
A well documented subcircuit file is given here. This has been tested using a number of test circuits including the buck, boost and buck-boost example configurations shown in the datasheets.
The above circuit can be run as a simulation. The circuit in the top left of the diagram above is the oscillator. This consists of a comparator working with hysteresis to provide the 1. To improve performance this was replaced with a controlled hysteresis switch similar to that used in the AEi model. The oscillator-source block generates the timing capacitor current in response to the output of the comparator. This current is modified by the transistor attached to the Ipk input to cause the capacitor to charge up faster if Ipk increases above about mV.
This provides the current limiting capabilities of the device. The signal “oscillator” provides a pulse that goes low when the capacitor is being discharged. In the top right of the circuit is the flip flop that drives the switching transistor.
The differentiator formed by Cdelay and Rdelay along with the squaring comparators provide two short pulses, one following the leading edge of the oscillator pulse delayminus and one following the trailing edge delayplus. These are used with the gates Xminus and Xplus to ensure that the reset pulse is shortened at its leading edge, and the set pulse is shortened at its leading edge. This is needed to prevent the set and reset inputs of the flip flop being active at the same time, and so causing the simulator to choke.
The RC integrators in the flip flop are also needed for the same reason. The time constant of these must be much shorter than dattasheet width of the delay pulses. In the lower centre of the circuit is the comparator that compares the feedback input with the reference voltage to gate the set pulses to the flip 30463. The low-pass filter at this point was necessary to ensure convergence.
Xoutgate ensures that the flip flop output signal is turned off when the feedback control signal is off. The switch circuit includes a drive transistor fed by a current source that ensures it is firmly turned on and off in response to the gated control signal.
The transistors need to have some capacitances and resistances to be specified to avoid convergence problems. The latter can be obtained using the method described in the gEDA setup page. To get a basic simulation going try the following commands. Ignore any error messages about unrecognized parameters in the diode model. Right-click on the graphs and drag a small box to zoom in on the detail.
MC Analysis and SPICE Model
This is about all that can be done with these graphs. Note that the uic parameter in the transient analysis command, which says “use initial conditions”, is eseential to allow convergence to occur. The reason for this is that at least one capacitor inside the MC model has initial conditions.
The focus of this page is to study the behaviour of some switching power circuits using the MC For designing dc-dc converters, the datasheets have a table of formulas that may be used, however this should not be a replacement for understanding of the circuit operation as the designs may need to be tweaked for a variety of reasons. As well as the spreadsheet provided here, more comprehensive resources can be found on the web, see in particular references 4 and 5.
On a practical note, take extra care with board layout. These chips can be sensitive to noise on the power input rails, resulting in the flip flop triggering prematurely and resulting in poor regulation and overvoltage.
Ceramic capacitors in parallel with the power input electrolytic capacitor should be used to attenuate high frequency switching noise.
This is a straightforward dc-dc converter that produces an output voltage less than the input voltage. We consider the conversion from a 12V input to a 5V output for driving logic circuitry. This is a bit lower than that for the AEi model and closer to the mV trigger specified for the MC The 3R load allows a clear demonstration of the current limiting in action.
The following plot shows a detailed portion of the resulting simulation in ngspice.
The blue plot is the voltage across the current limiting resistor R0, which is proportional mainly to the current through the output switch BJT. The yellow plot is the feedback comparator gating signal that turns the switching process on and off. Datasheet a large filter capacitor, this signal typically spans a number of oscillator cycles. The upward slope of the switch current is due to the ramping up of the inductor current.
This represents a circuit near to the current limiting point, having a current of 1. Examination of the red oscillator trace shows that the charge time is shorter during a gated on period. The changes in oscillator slope datsaheet in the cycle period are clearly visible as the feedback gating signal turns on and off.
Some of the difference may be in the datahseet capacitor ESR and inductor series resistance which we have assumed as zero. Data Sheet from ON Semiconductor. Lazar’s Power Electronics Corner. Switching Mode Power Supply Design. Practical Circuits The focus of this page is to study the behaviour of some switching power circuits using the MC Buck Converter This is a straightforward dc-dc converter that produces an output fx less than the input voltage.