TLE 6387 G
Target Datasheet Rev. 0.5
13
2001-08-27
4.2
Functional Description
Internal Power Supply
The TLE 6387 G internal power supply consists of an internal voltage supply, a
subsequent auxiliary voltage supply and the internal voltage reference. As long as the
supply voltage provided at pin 2 (
V
S) is high enough, the entire controller is supplied from
this voltage by means of the internal voltage supply. If the regulated voltage of the
internal voltage supply, however, drops below the converter’s output voltage sensed at
pin 9 (FB), the auxiliary voltage supply and the internal voltage reference are
immediately cut off the internal voltage supply and are supplied from the converter’s
output voltage, instead. In this way the output voltage supervisor, which drives the reset
output, keeps on running even though the supply voltage at pin 2 (
V
S) has dropped down
to zero. The same switching operation is performed whenever the controller is set into
stand-by, either by external request via pin 8 (STBY), by overtemperature protection or
by wirebreak detection (cf. “Stand-by Management”).
Stand-by Management
The same switching operation as described above (cf. “Internal Power Supply”) is also
performed, whenever the controller is set into stand-by mode, either by external request,
by overtemperature protection or by wirebreak detection. While the latter are failure
modes, the external stand-by request is controlled by pin 8 (STBY). As long as the
voltage at pin 8 exceeds 2 V the controller is disabled. However, if the voltage at pin 8
drops below 0.8 V the controller is enabled. To avoid unintentional start-ups pin 8 is
internally pulled-up. In this way the controller is disabled even if this pin is not connected.
Observer-Based Adaptive Current Mode Control
IIn order to achieve a fast and stable control with excellent line and load regulation and
with a precisely controlled output voltage, TLE 6387 G uses observer-based adaptive
current mode control. Like conventional current mode control it consists of two control
loops. The first loop serves to control the output voltage sensed at pin 9 (FB) by means
of error amplifier OTA
EA. The second loop is used to control the load current. Unlike
conventional approaches the load current is calculated by OCA
CM integrating the voltage
across the choking coil, i.e. between pin 14 (SRC) and pin 9 (FB). In this way the current
loop can do without an additional resistor, which is usually needed to sense the load
current and which reduces the overall efficiency of the DC/DC converter. To achieve a
stable duty cycle at values above 50%, the current loop is furthermore equipped with an
appropriate slope-compensation. In order to achieve a precise output voltage regardless
of the respective load current the voltage loop and the current loop are coupled by
OTA
CC. OTACC serves to compensate the DC voltage offset of the choking coil, which