LM78S40



LM78S40 Datasheet


Many of the functional blocks in this circuit are disconnected. This gives the designer a great deal of design flexibility. If required, the transistor switch can be used directly in low power applications, or it can be used to drive a high power series pass element.

 

Each SMPS IC has its own design peculiarities. In some cases, the restriction placed on the designer may preclude the use of any SMPS IC and a completely discrete circuit design must be considered. This situation however, is beyond the scope of this presentation.

 

Transistor Driver and Switch

The 78S40 uses a Darlington pair in the switching arrangement. The collectors of both transistors are brought out to external pins. This allows them to be connected together as is the standard configuration, or an external resistor can be placed in the driver collector to control the switch saturation current.

 

Oscillator

The 78S40 chip is designed to operate within a switching frequency range of 100 Hz to 100 kHz. Increasing the switching frequency increases the electromagnetic radiation and PCB layout problems but decreases the size of the inductor. Most designs based on this IC have an operating frequency of 20 - 30 kHz.

The oscillator, the charge/discharge ratio is preset to approximately 6:1. The overall switching duty cycle can be varied from approximately 17% to 50% by means of two feedback loops.

The switching frequency and duty cycle are controlled by current and voltage feedback. This will at times make triggering an oscilloscope to the switching waveforms somewhat problematic. As the load increases, the switching frequency tends to increases.

 

Reference Voltage   

The 78S40 has an internal 1.245 volt temperature compensated, band-gap voltage reference which is available at an external pin. This reference voltage should be bypassed by a 0.1mfd capacitor to ground to help insure stability.

 

Current Feedback   

The current feedback circuit modifies the switch ON time. A current sensing resistor Rsc generates a voltage proportional to the switching current. When this potential exceeds approximately 0.3 volts, the oscillator (and hence switch) is turned OFF. This control mechanism takes priority over voltage feedback.

 

Voltage Feedback  

The voltage feedback loop, consisting of the voltage divider and comparator controls the switch ON time. If the output voltage is too low, the ON time is increased.

The switching waveforms are also synchronized with the output voltage ripple and correspond to the increasing output magnitude. The switching waveforms can disappear if the sampled output magnitude exceeds the reference voltage.

 

Current and Voltage Limitations

The internal Darlington transistor switch can handle a maximum peak current of 1.5 amps during the ton period, and a maximum of 40 volts during the  toff period.

An external transistor switch is needed if the design requires either more current or a higher input voltage.

 

Voltage Sensing Resistors   

The voltage divider at the output represents a minimum load. The voltage at the junction of the two resistors must equal the 1.245 reference voltage when one resistor is attached to ground and the other to the output.

 

Efficiency

The efficiency of a well-designed power converter can be in excess of 90%.

The output voltage divide constitutes a minimum load on the switching converter and therefore reduces the efficiency. The current drawn by the divider can be as low as 100 ma, but is more typically in the region of 1 ma. This may not be significant with high load currents, but it becomes more dominant as load current decreases.

The saturation voltage of the Darlington transistor can be as high as 1.3 volts. This decreases efficiency as load current and    increase.

The internal Darlington transistor has a switching speed of 300 – 500 nSec. During this time, the transistor is neither ON nor OFF, and therefore dissipates power.

Any current sensing resistor in series will also dissipate power.

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