TDA 16888
Current Amplifier OP2
Being part of the inner loop the error amplifier OP2 controls the shape of the line current
by comparison of the controlled current
I
QM
with the measured average line current. This
is achieved by setting the pulse width of the PFC gate drive in conjunction with the
comparator C1. In order to limit the voltage range supplied at pin 4 (PFC CS) and at pin 5
(GND S), clamping diodes D1, D2 and D3 are connected with these pins and ground. To
allow for individual feedback the output of OP2 is connected to pin 3 (PFC CC).
Ramp Amplifier OP3
For crosstalk suppression between PFC and PWM section a signal-to-noise ratio
comparable to voltage mode controlled PWMs is set by operational amplifier OP3
performing a fivefold amplification of the PWM load current, which is sensed by an
external shunt resistor. In order to suppress leading spikes, which are due to parasitic
capacitances being discharged whenever the power transistor is switched on, the
resulting signal is subsequently increased by the constant voltage of
V
1
and finally fed
into a 1st order low pass filter. By combination of voltage source
V
1
and the following low
pass filter a step response with a leading notch is created, which will fully compensate a
leading spike (see
provided, the external capacitor at pin 15 (PWM RMP)
and the external current sensing shunt resistor are scaled properly.
Operational Transconductance Amplifier OTA1
The TDA 16888’s auxiliary power supply mode is controlled by the fast operational
transconductance amplifier OTA1. When under low load or no load conditions a voltage
below 5 V is sensed at pin 20 (AUX VS), it will start to superimpose its output on the
output Q
M
of the multiplier and in this way will replace the error amplifier OP1 and the
multiplier. At normal operation, however, when the voltage at pin 20 (AUX VS) is well
above 5 V, this operational transconductance amplifier is disabled.
Operational Transconductance Amplifier OTA2
By means of the operational transconductance amplifier OTA2 sensing at pin 19
(PFC VS) a fast but soft overvoltage protection of the PFC output voltage is achieved,
which when being activated (
V
PFC VS
> 5.5 V) will cause a well controlled throttling of the
multiplier output Q
M
(see
Operational Transconductance Amplifier OTA3
In order to achieve offset compensation of error amplifier OP2 under low load conditions,
that will not suffice to start OTA1, the operational transconductance amplifier OTA3 is
introduced. It will start operation as soon as these conditions are reached, i.e. the voltage
at pin 18 (PFC VC) falls below 1.2 V.
Data Sheet
12
2000-02-28
INFINEON [ INFINEON TECHNOLOGIES AG ]
