**LOW TEMPERATURE EVALUATION OF THE TC170 CMOS CURRENT-MODE PWM
CONTROLLER**

Test Report

Scott Gerber

ZIN Technologies

Ahmad Hammoud

QSS Group, Inc.

Malik Elbuluk

University of Akron

&

Richard Patterson

NASA Glenn Research Center

NASA Glenn Research Center

Cleveland, Ohio

February 25, 2002

**Low Temperature Evaluation of the TC170 CMOS**

**Current-Mode PWM Controller**

**Background**

The Microchip Technology TC170 is a current-mode pulse-width-modulation
controller based on CMOS technology. It features fast rise/fall times
with dual push-pull outputs \[1\]. The device requires low supply
current with an input voltage range from 8 to 16 V. The dual totem-pole
CMOS outputs can drive power MOSFETS or bipolar transistors. The device
is capable of high frequency switching (200 kHz) and is specified for
operation in the temperature range of 0 °C to +70 °C. Other features
include double-pulse suppression, soft-start operation, programmable
current limit, undervoltage lockout, and excellent line and load
regulation. Two or more of these controllers can be slaved together for
parallel operation. The circuits can operate from a master TC170
internal oscillator or an external system oscillator \[1\]. CMOS-based
devices typically perform well at low temperatures. This device was,
therefore, selected for evaluation for potential use in the development
of a low temperature dc/dc converter module.

**Test Setup**

A circuit board, populated with the TC170 chip, a transistor, and few
passive components, was designed and built for evaluation in the
temperature range of +20 °C to --185 °C. The circuit layout used in this
investigation is shown in Figure 1. The device was characterized at test
temperatures of 20, 0, -25, -50, -75, -100, -125, -150, -175 and ‑185 °C
in a liquid nitrogen cooled environmental chamber. At each test
temperature, the device was allowed to soak for 15 minutes before
measurements were made. The device performance in terms of its switching
frequency and duty cycle control was evaluated as a function of
temperature. The switching frequency was set to about 30 kHz through
selected external resistive and capacitive components. The duty cycle of
the two outputs was controlled by varying the external voltage control
level applied to the differential input of the error amplifier.

![](media/image1.emf){width="4.811805555555556in"
height="3.401388888888889in"}

Figure 1. Test setup for the TC170.

**Results and Discussion**

Testing of the device was initially performed at 20 °C after which
measurements were taken at lower temperatures with an increment of 25
°C. At each test temperature, the device was allowed to soak for 15
minutes before measurements were made. Performance of the device as a
function of temperature is depicted in Table I. Listed are the source
current (Is), minimum and maximum duty cycle (D), internal reference
voltage (Vref), and the switching frequency at the different test
temperatures. The data, which is listed in the last row of Table II,
represents measured parameters obtained at room temperature after
completion of the thermal cycling. Throughout these tests, an input of
12 V was supplied to the device and a control voltage between 2 and 4 V
was used to vary the duty cycle.

It can be clearly seen that while the internal reference voltage
remains, in general, steady throughout the entire test temperature
range, the frequency undergoes a slight increase as temperature is
decreased. The duty cycle exhibits similar behavior to that of the
frequency with change in temperature. This increase in the duty cycle is
more apparent at its maximum. Unlike the frequency and the duty cycle,
the supply current changes considerably with temperature. It can be seen
that the current increases as the temperature is decreased. At the
extreme temperature of --185 °C, for example, the supply current amounts
to about 5 mA; a value that is more than double its room temperature
counterpart. These temperature-induced changes in the device
characteristics tend to be transitory as all investigated parameters
regain their original values after the temperature stress has been
removed, as indicated in Table I.

Table I. TC170 device characteristics at various temperatures.

+----------------+---------+---------+---------+---------+---------+
| **Temperature  | **Is**  | *       | *       | *       | *       |
| (°C)**         |         | *Dmin** | *Dmax** | *Vref** | *Freq** |
|                | *       |         |         |         |         |
|                | *(mA)** | **(%)** | **(%)** | **(V)** | **      |
|                |         |         |         |         | (kHz)** |
+----------------+---------+---------+---------+---------+---------+
| 20             | 2.4     | 21.7    | 43.3    | 5.00    | 29.2    |
+----------------+---------+---------+---------+---------+---------+
| 0              | 2.6     | 21.7    | 44.2    | 5.00    | 30.0    |
+----------------+---------+---------+---------+---------+---------+
| -25            | 2.8     | 21.6    | 45.0    | 5.00    | 30.7    |
+----------------+---------+---------+---------+---------+---------+
| -50            | 3.1     | 21.5    | 45.8    | 5.02    | 31.4    |
+----------------+---------+---------+---------+---------+---------+
| -75            | 3.5     | 21.9    | 46.5    | 5.03    | 32.1    |
+----------------+---------+---------+---------+---------+---------+
| -100           | 3.9     | 21.6    | 47.0    | 5.04    | 32.7    |
+----------------+---------+---------+---------+---------+---------+
| -125           | 4.4     | 21.1    | 47.5    | 5.07    | 33.2    |
+----------------+---------+---------+---------+---------+---------+
| -150           | 4.9     | 22.0    | 47.8    | 5.00    | 34.0    |
+----------------+---------+---------+---------+---------+---------+
| -175           | 5.4     | 22.8    | 48      | 5.08    | 33.6    |
+----------------+---------+---------+---------+---------+---------+
| -185           | 5.1     | 22.6    | 47.9    | 5.02    | 33.6    |
+----------------+---------+---------+---------+---------+---------+
| 20             | 2.4     | 22.0    | 43.6    | 5.00    | 29.6    |
+----------------+---------+---------+---------+---------+---------+

Waveforms of the device reference voltage, oscillator, and the two
modulated output voltages, which were recorded with the duty cycle at
its maximum, are shown in Figure 2 at test temperature of 25 °C. These
waveforms were also recorded at --185 °C under the same conditions and
are depicted in Figure 3. It is quite evident that the device undergoes
very little changes in its operational behavior as a result of the low
temperature exposure.

**Conclusion**

The Microchip Technology TC170 CMOS current-mode PWM controller, which
is a commercial-grade device rated for 0 to 70 °C operation, has been
evaluated for potential use in low temperature applications. The device
was characterized in terms of its switching frequency, internal
reference voltage, and duty cycle control in the temperature range of 20
°C to ‑185 °C. Although very slight changes occur in some of these
parameters, little effect on the device overall performance is observed
as a result of exposure to low temperature. The results of this
preliminary work indicate that the device has the potential of
utilization in circuits and systems designed for operation in low
temperature applications. Further testing is needed, however, to
establish operational performance and reliability of these devices under
long-term temperature exposure and thermal cycling.

**References**

1\. TC170 CMOS Current-Mode PWM Data Sheet DS21395A, Microchip
Technology, Inc.

**Acknowledgments**

This work was performed under the NASA Glenn Research Center GESS
Contract \# NAS3-00145. Support was also provided from the NASA
Electronic Parts and Packaging (NEPP) Program, EPAR Task "Electronics
for Extreme Environments".

![](media/image2.emf)

Figure 2. Waveforms of the TC170 controller at 20 °C.

![](media/image3.emf)

Figure 3. Waveforms of the TC170 controller at -185 °C.