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Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Preliminary data sheet
Rev. 01 — 28 May 2004
7 of 27
Philips Semiconductors
TEA1532
GreenChip
II SMPS control IC
7.4 Oscillator
The fixed frequency of the oscillator is set by an internal current source and capacitor.
7.5 Cycle skipping
(V
sense(max)
) lower than 25 mV will inhibit switch-on of the external power MOSFET until
this voltage increases to a higher value; see
Figure 6
.
7.6 Demagnetization (QR operation)
The system will be in Discontinuous Conduction Mode (DCM) (QR operation) when
resistor R
DEM
is applied. The oscillator will not start a new primary stroke until the
secondary stroke has ended.
Demagnetization features a cycle-by-cycle output short-circuit protection which
immediately reduces the frequency (longer off-time), thereby reducing the power level.
Demagnetization recognition is suppressed during the first t
supp
time. This suppression
may be necessary in applications where the transformer has a large leakage inductance
and at low output voltages or start-up.
7.7 Continuous Conduction Mode (CCM)
It is also possible to operate the IC in the so-called Fixed Frequency Continuous
Conduction Mode (FF CCM). This mode is activated by connecting pin DEM to ground
and connecting pin DRAIN to the rectified constant V
i
voltage; see
Figure 13
.
7.8 Overcurrent Protection (OCP)
The current in the transformer primary is measured accurately by the internal
cycle-by-cycle source current limit circuit using the external sense resistor R
sense
.
The accuracy of the current limit circuit allows the transformer core to have a minimum
specification for the output power required. The OCP circuit limits the ‘sense’ voltage to an
internal level, and is activated after the leading edge blanking period, t
leb
generated by the
Leading Edge Blanking (LEB) circuit.
7.9 Control pin protection
If pin CTRL becomes open-circuit or is disconnected, a fault condition is assumed and the
converter will stop operating immediately. Operation recommences when the fault
condition is removed.
7.10 Adjustable slope compensation
A slope compensation function has been added at pin CTRL; see
Figure 7
. The slope
compensation function prevents sub-harmonic oscillation in CCM at duty cycles over
50 %. The CTRL voltage is modulated by sourcing a (non-constant) current out of
pin CTRL and adding a series resistor R
slopecomp
. This increases the CTRL voltage
proportionally with the on-time, which therefore limits the OCP level. Thus, a longer
on-time results in a higher CTRL voltage, however, this increase in CTRL voltage will
actually decrease the on-time. Slope compensation can be adjusted by changing the