TEA2018A 데이터 시트보기 (PDF) - STMicroelectronics
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TEA2018A Datasheet PDF : 7 Pages
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TEA2018A
GENERAL DESCRIPTION
(see application note AN-086)
Operating Principles (Figure 1)
On every period, the beginning of the conduction
time of the transistor is triggered by the fall of the
oscillator sawtooth which acts as clock signal. The
period Tosc is given by : Tosc ≅ 0.66 Ct (Rt + 200)
(Tosc in seconds, Ct in Farad, Rt in Ω)
The end of the conduction time is determined by a
signal issued from comparing the following sig-
nals :
a) the sawtooth waveform representing the
collector current of the switching transistor,
sampled across the emitter shunt resistor,
b) the output of the error amplifier.
Base Drive
- Fast turn-on : On each period, a current pulse
ensures fast transistor switch-on.
This pulse performs also the ton(min) function at
the beginning of the conduction.
- Proportional base drive : In order to save power,
the positive base current after the starting pulse
becomes an image of the collector current.
The
ratio
IC
IB
is
programmed
as
follows
Figure
2)
:
TEST CIRCUIT
IC
IB
=
RB
Re
- Efficient and fast switch-off : When the positive
base drive is removed, 1ms (typically) will elapse
before the application of negative current there-
fore allowing a safe and rapid collector current
fall.
Safety Functions
- Overload & short-circuit protection : When the
voltage applied to pin 3 exceeds the current
limitation threshold voltage [Vth(Ic)], the output
flip-flop is reset and the transistor is turned off.
The shunt resistor Re must be calculated so as to
obtain the current limitation threshold on pin 3 at
the maximum allowable collector current.
- Demagnetization sensing : This function disables
any new conductioncycle of the transistor as long
as the core is not completely demagnetized.
When not used, pin 7 must be grounded.
- ton(max) : Outside the regulation area and in the
absence of current limitation, the maximum con-
duction time is set at about 70 % of the period.
- ton(min) : A minimum conducting time is ensured
during each period (see Figure 2)
- Supply voltage monitoring : The TEA2018A will
stop operating if VCC+ on pin 6 falls below the
threshold level VCC(stop)
Vi
VREF
OS CILLATOR
E RROR
AMPLIFIER
FLIP-FLOP
S
Q
E RROR
SI GNAL
R
COMPARATO R
Re
I C SENSE
OUTPUT
FILTER
IC
LO AD
OSCILLATOR
SAW-TOOTH
I C (sample)
FLIP-FLOP
OUTPUT
t
Error signal
t
t
4/7
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