Monolithic Switching Regulator [µa78s40]

Monolithic Switching Regulator [µa78s40]:

The µA78S40 consists of a temperature compensated voltage reference, duty cycle controllable oscillator with an active current limit circuit, a high gain comparator, a high- current, high voltage output switch, a power switching diode & an uncommitted op-amp. Important features of the µA78S40 switching regulators are:

·           Step up, down & Inverting operation

·           Operation from 2.5 to 40V input

·           80dB line & load regulations

·           Output adjustable from 1.3 to 40V

·           Peak current to 1.5A without external resistors

·           Variable frequency, variable duty cycle device

The internal switching frequency is set by the timing capacitor CT, connected between pin12 & ground pin 11. The initial duty cycle is 6:1. The switching frequency & duty cycle can be modified by the current limit circuitry, IPK sense, pin14, 7 the comparator, pin9 & 10.

Comparator:

The comparator modifies the OFF time of the output switch transistor Q1 & Q2. In the step – up & step down modes, the non-inverting input(pin9) of the comparator is connected to the voltage reference of 1.3V (pin8) & the inverting input (pin10) is connected to the output terminal via the voltage divider network.

·           In the Inverting mode the non – inverting input is connected to both the voltage reference & the output terminal through 2 resistors & the inverting terminal is connected to ground.

·           When the output voltage is correct, the comparator output is in high state & has no effect on the circuit operation.

·           However, if the output is too high & the voltage at the inverting terminal is higher than that at the non-inverting terminal, then the comparator output goes low.

·           In the LOW state the comparator inhibits the turn on of the output switching transistors. This means that, as long as the comparator output is low, the system is in off time.

·           As the output current rises or the output voltage falls, the off time of the system decreases.

·           Consequently, as the output current nears its maximum IoMAX, the off time approaches its minimum value.

In all 3 modes (Step down, step up, Inverting), the current limit circuit is completed by connecting a sense resistor Rsc, between IPK sense & Vcc.

·           The current limit circuit is activated when a 330mV potential appears across Rsc.

·           Rsc is selected such that 330mV appears across it when the desired peak current IPK, flows through it. 

·           When the peak current is reached, the current limit circuit is turned on.

The forward voltage drop, VD, across the internal power diode is used to determine the value of inductor L off time & efficiency of the switching regulator.

Another important quantity used in the design of a switching regulator is the saturation voltage Vs:

In the step down mode an “output saturation volt” is 1.1V typical, 1.3VMAX.

In the step up mode an “Output saturation volt” is 0.45V typical, 0.7 maximum.

The desired peak current value is reached; the current limiting circuit turns ON & immediately terminates the ON time & starts OFF time.

·           As we increase IL (load current), Vout will decrease, to compensate for this, the ON time of the output is increased automatically.

If the IL decreased then Vout increased, to compensate for this, the OFF time of the output is increased automatically.

(i)       Step – Down Switching Regulator:

·           CT is the timing capacitor which decides the switching frequency.  Rsc is the current sensing resistance. Its value is given by

·           The Non-inverting terminal of the internal op-amp (pin9) is connected to the 1.3V reference (pin8).

·           Resistances R1 & R2 from a potential divider, across the output voltage Vo. Their value should be such that the potential at the inverting input of the op-amp should be equal to 1.3V ref when Vo is at its desired level.

The output capacitance Co is used for reducing the ripple contents in the output voltage. It acts as a filter along with the inductor L.

·           The inductor L is a part of filter connected on the output side, to reduce the ripple percentage.

·           The 0.1μF capacitor connected between pin8 & ground bypasses any noise voltage coupled to the reference (pin8).

(ii)    Step – Up Switching Regulator:

·           Inductor is connected between the collectors of Q1 & Q2.

·           When Q1 is ON, the output is shorted & the collector current of Q1 flows through L.

·           The diode D1 is reverse biased & Co supplies the load current.

·           The inductor stores the energy. When the Q1 is turned OFF, there is a self induced emf that appears across the inductor with polarities.

·           The output voltage is given by,

V_o = V_in + V_L

·           Hence it will be always higher than Vin & step up operation is achieved.

(iii)  Inverting Switching Regulator:

Inverting switching regulator converts a positive input voltage into a negative output voltage which is higher in magnitude.