3 A Wide input Adjustable Switching Regulator

The PTN78060 is a series of high-efficiency, buck-boost, integrated switching regulators (ISR) from good old Texas Instruments (TI). The caseless, double-sided package has excellent thermal characteristics, and is RoHs compliant. The PTN78060 devices operate from a remarkably wide input voltage range:

Note that the –A version supplies a negative output voltage. The devices provide high-efficiency stepdown voltage conversion for loads of up to 3 A. The PTN78060 devices are suited to a wide variety of general-purpose applications that operate off 12-V, 24-V, or tightly regulated 28-V dc power, hence are ideal for running low-voltage electronics from a very high power 24-V battery unit salvaged from an electric wheel chair and migrated into a robot.  The output voltage VO can be set to any value over a wide adjustment range using a single external resistor RSET, using the equation RSET = 54.9kΩ×(1.25V/VO–VMIN) – Rp  If pin 4 is left open, the output voltage defaults to the lowest value.

Limiting ourselves to the two positive-output regulators, for the -W version, VMIN and Rp are 2.5 V and 6.49 kΩ respectively; for the -H device, the values 11.824 V and 6.65 kΩ should be used. For the output to remain in regulation, the input voltage must exceed the output by a minimum differential voltage. Another consideration is the pulsewidth modulation (PWM) range of the regulator’s internal control schema. For stable operation, its operating duty cycle should not be lower than a certain minimum percentage. This defines the maximum advisable ratio between the regulator input and output voltage magnitudes. For satisfactory performance, the operating input voltage range of the PTN78060x must satisfy the following requirements.

1. For PTN78060W devices supplying output voltages lower than 10 V, the minimum input voltage is (VO+2 V) or 7 V, whichever is higher.

2. For PTN78060Ws supplying output voltages of 10 V and higher, the minimum input voltage is (VO+2.5 V).

3. The maximum input voltage for PTN78060W is 10VO or 36 V, whichever is less.

4. For PTN78060H output voltages lower than 19 V, the minimum input voltage is (VO+3 V) or 15 V, whichever is higher.

5. For PTN78060H output voltages equal to 19 V and higher, the minimum input voltage is (VO+4 V).

 

As an example, the Table gives the operating input voltage range for some commonly used output bus voltages. The modules are protected against load faults with a continuous current limit characteristic. Under a load-fault condition, the output current increases to the current limit threshold. Attempting to draw current that exceeds the current limit threshold causes the module to progressively reduce its output voltage. Current is continuously supplied to the load until the fault is removed. Once it is removed, the output voltage promptly recovers. When limiting output current, the regulator experiences higher power dissipation, which increases its temperature. If the temperature increase is excessive, the module overtemperature protection begins to periodically turn the output voltage off.

The inhibit feature can be used wherever there is a requirement for the output voltage to be turned off. The power module switches off the output voltage when the Inhibit control (pin 3) is pulled to ground,for example, by a switching FET. Finally, good attention should be paid to the quality of the capacitors on VI and VO as they determine the regulator stability and overall performance to a substantial degree. Summarizing the extensive information on capacitor selection found in the datasheets, the minimum requirement for C1 is 2.2 μF (!) worth of ceramic capacitors for the –W device and 14.1 μF (!!) for the-H device. Tantalum caps are not recommended.  Similarly, at the regulator output, C2 should be at least 100 μF worth of low- ESR electrolytics.