參數(shù)資料
型號(hào)： TEA1541
廠商： NXP Semiconductors N.V.
英文描述： SMPS control IC with synchronization function
中文描述： 開(kāi)關(guān)電源與同步功能控制IC
文件頁(yè)數(shù)： 9/19頁(yè)
文件大小： 112K
代理商： TEA1541
2003 Aug 11
9
Philips Semiconductors
Product specification
SMPS control IC with
synchronization function
TEA1541
6.6
Minimum and maximum ‘on-time’
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
smps
is the converter switching frequency in either
synchronized or unsynchronized mode. If the system
requires a longer on-time, a fault condition is assumed, for
example, if C
VIN
is removed, the IC will stop switching and
enter the safe restart mode.
6.7
Overvoltage protection
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
flowingintopin DEMduringthedemagnetizingcycleofthe
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
OVP(DEM)
, the OVP circuit turns off
the power MOSFET. The controller then waits until the
V
CC(UVLO)
condition is reached. This is followed by a safe
restart cycle, before switching recommences.
This process is repeated until the OVP condition ends.
TheoutputvoltageatwhichOVPactivates,V
o(ovp)
issetby
the value of resistor, R
DEM
, (see Fig.8) using the equation:
N
aux
where N is the number of turns on the transformer
windings; V
clamp(DEM)(pos)
is the positive clamp voltage on
pin DEM; reference current I
OVP(DEM)
is set internally.
6.8
Overcurrent protection and overpower
protection
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 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
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
peak
)
2
×
frequency
stays constant. This arrangement also implements OPP,
ensuringthatthemaximumoutputpowerisindependentof
theswitchingfrequency,otherwisetheoutputpowerwould
increase in direct proportion to the switching frequency.
6.9
Soft start
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
SS
and
capacitor C
SS
to be connected between pin I
sense
and the
sense resistor R
sense
(see Fig.7). C
SS
is charged by an
internal current source I
SS
to V = I
SS
×
R
SS
, to a maximum
of approximately 0.5 V.
The rate at which the primary current increases can be
adjusted by changing the values of R
SS
and C
SS
to change
the circuit time constant:
τ
=
The maximum primary current is calculated by the
equation:
smps
f
V
o(ovp)
N
I
OVP DEM
)
R
DEM
V
clamp DEM
)
pos
(
)
+
×
(
)
×
=
handbook, halfpage
CSS
RSS
Isense
Rsense
ISS
Vocp
start-up
MBL503
5
0.5 V
Fig.7 Soft start.
R
SS
C
SS
×
I
primary max
)
V
-------------------------R
I
R
×
(
)
–
sense
=
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