TEA1541P PHILIPS [NXP Semiconductors], TEA1541P Datasheet - Page 9

• Manufacturer Part Number: TEA1541P
• Description: SMPS control IC with synchronization function
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
6.6
The minimum on-time of the converter is not limited by the
leading edge blanking time, and therefore can be zero.
The IC limits the maximum on-time to
where f
synchronized or unsynchronized mode. If the system
requires a longer on-time, a fault condition is assumed, for
example, if C
enter the safe restart mode.
6.7
The TEA1541 allows OVP to be set accurately.
The flyback converter output voltage is accurately
represented by the voltage across the auxiliary winding.
The auxiliary winding voltage is monitored by the current
flowing into pin DEM during the demagnetizing cycle of the
transformer. The inevitable voltage spikes at pin DEM are
reduced using an internal filter.
If the output voltage causes the current into pin DEM to
exceed the OVP level l
the power MOSFET. The controller then waits until the
V
restart cycle, before switching recommences.
This process is repeated until the OVP condition ends.
The output voltage at which OVP activates, V
the value of resistor, R
where N is the number of turns on the transformer
windings; V
pin DEM; reference current I
6.8
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
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
Leading Edge Blanking circuit (LEB shown in Fig.2).
Leading edge blanking is required to inhibit OCP for a
short period when the power MOSFET turns on.
This ensures that the MOSFET is not turned off
2003 Aug 11
V
CC(UVLO)
o(ovp)
SMPS control IC with
synchronization function
Minimum and maximum ‘on-time’
Overvoltage protection
Overcurrent protection and overpower
protection
smps
=
condition is reached. This is followed by a safe
-----------
N
N
clamp(DEM)(pos)
is the converter switching frequency in either
aux
VIN
s
is removed, the IC will stop switching and
I
OVP DEM
DEM
OVP(DEM)
is the positive clamp voltage on
, (see Fig.8) using the equation:
OVP(DEM)
R
leb
, the OVP circuit turns off
DEM
generated by the
+
is set internally.
------------ -
f
V
smps
1
clamp DEM pos
o(ovp)
sense
is set by
.
9
prematurely due to the false sensing of an overcurrent
condition caused by current spikes produced by the
discharge of primary-side snubber and parasitic
capacitances.
The OCP level is adjusted proportionally to the switching
frequency such that the product of (I
stays constant. This arrangement also implements OPP,
ensuring that the maximum output power is independent of
the switching frequency, otherwise the output power would
increase in direct proportion to the switching frequency.
6.9
The soft start function allows the transformer peak current
to slowly increase at every start-up and restart, to prevent
transformer rattle.
The soft start function requires a resistor R
capacitor C
sense resistor R
internal current source I
of approximately 0.5 V.
The rate at which the primary current increases can be
adjusted by changing the values of R
the circuit time constant:
The maximum primary current is calculated by the
equation:
handbook, halfpage
Soft start
0.5 V
I
primary max
SS
to be connected between pin I
sense
Fig.7 Soft start.
(see Fig.7). C
start-up
V ocp
=
I SS
SS
V
----------------------------------------------------------------- -
to V = I
5
sense max
=
I sense
R
SS
SS
R
SS
R SS
C SS
SS
peak
C
sense
Product specification
–
SS
is charged by an
R
and C
SS
)
I
2
SS
TEA1541
, to a maximum
SS
frequency
sense
R sense
SS
MBL503
R
and
SS
to change
and the