TEA1532
Philips Semiconductors
GreenChip II SMPS control IC
primary
stroke
secondary
stroke
secondary
ringing
drain
valley
secondary
stroke
(2)
(1)
oscillator
mgu235
(1) Start of new cycle at lowest drain voltage.
(2) Start of new cycle in a classical PWM system at high drain voltage.
Fig 8. Signals for valley switching.
7.14 Brown-out protection
During the so called brown-out test, the input voltage is slowly decreased. Since the
on-time depends on Vi, overlong on-times at low Vi can damage the (external) power
device. This is prevented by stopping the converter when the input voltage drops too low.
When the voltage on pin DEM drops below −50 mV during the on-time (QR mode), the
maximum on-time is set to 25 µs. The maximum on-time will be reached while Vi is low.
Subsequently, the IC stops switching and VCC drops below VUVLO. Capacitor CVCC will
only be recharged and the supply will restart only when voltage Vi is high enough (Vm,
also see Section 7.1). In addition to this, a Vi level at which the converter has to enter a
safe restart can be set with a demag resistor. During the primary stroke, the rectified
mains input voltage is measured by sensing the current drawn from pin DEM. This current
depends on the mains voltage, according to the following formula:
V
N × V
mains
aux
I
≈
≈
-------------- --------------------------
(DEM)
R
R
DEM
DEM
N
aux
Where: N =
-----------
N
p
The latter function requires an on-time of at least 2 µs. This on-time ensures that a reliable
demag current can be measured.
9397 750 13113
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Preliminary data sheet
Rev. 01 — 28 May 2004
10 of 27
NXP [ NXP ]
