TDA8385 Datasheet, PDF (7/21 Pages) NXP Semiconductors – Control circuit for a Self-Oscillating Power Supply SOPS

Philips Semiconductors
Control circuit for a Self-Oscillating
Power Supply (SOPS)
VO
(5)
(4)
(3)
(2)
(1)
IO
MCD406
(1), (2), (3) = V_TCG.
(4), (5) = V_diff.
Fig.7 Current foldback characteristic; stabilized output voltage (V_O)
as function of load current (I_O).
Preliminary specification
TDA8385
The voltage V_Ton(min) determines the
minimum on-time of the switching
transistor. This voltage can be
determined externally with a resistor
on pin 4. With this resistor the current
foldback characteristic can be
influenced (see dotted line in Figs 6
and 7).
The minimum on-time is of
importance for the following.
• Stand-by operation
• Starting-up of power supply
• Overload and short-circuit
conditions.
The output of the differential amplifier
(V_diff), the output of the TCG (V_TCG)
and the voltage V_ss + V_Ton(min) are
compared in a minimum voltage
clamping circuit (see Fig.1
sub-section 6). The output voltage is
equal to the lowest input voltage.
Some relevant characteristics of the
control part are depicted in Fig.8.
handbook, full pagewidth_{Ic (max)}
Ic
Vmv
Ic _(min)
Vss + VTon(min)
xy
V _ref
V _fb
VTCG
Vdiff
Vmv
external peak-current
setting (pin 7)
MCD407
The voltage V_mv determines the collector peak current I_c of transistor T1. The right-hand curve is passed through at start-up. When the feedback voltage
slowly increases from zero, the peak current starts at I_c(min) and rises along the straight line until I_c(max) is reached. At a slightly higher feedback voltage
the regulation slope is reached, which is approximately V_ref.
The plateau of the top between the points x and y has to be kept as small as possible.
The voltage V_diff decreases with the decreasing load. For good no-load operation the peak current has to be made zero with V_diff.
Due to the characteristic of the TCG open- and short-circuit feedback loop will result in low peak current.
An additional signal on pin 13 can be supplied which is subtracted from the signal V_mv. This input can be used for feed forward information.
If no feed forward information is used, pin 13 should be connected to ground.
Fig.8 Characteristics of the control part.
March 1994
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