2003 May 12
10
Philips Semiconductors
Product specification
GreenChip
II SMPS control IC
TEA1654
LOCK input
Pin LOCK is a general purpose (high-impedance) input
pin, which can be used to switch off the IC. As soon as the
voltageonthispinisraisedabove2.5 V,switchingwillstop
immediately.ThevoltageontheV
CC
pinwillcyclebetween
V
CC(start)
and V
CC(UVLO)
, but the IC will not start switching
again until the latch function is reset. The latch is reset as
soon as the V
CC
drops below 4.5 V (typical value). The
internal OVP and OTP will also trigger this latch (see
Fig.2).
The detection level of this input is related to the V
CC(5V)
pin
voltage in the following way: 0.5
×
V
CC(5V)
±
4%.
An internal Zener diode clamp of 5.6 V will protect this pin
from excessive voltages. No internal filtering is done on
this input.
OverTemperature Protection (OTP)
An accurate temperature protection is provided in the
circuit. When the junction temperature exceeds the
thermal shutdown temperature, the IC will stop switching.
When V
CC
drops to UVLO, capacitor C
VCC
will be
recharged to the V
start
level, however the IC will not start
switching again. Subsequently, V
CC
will drop again to the
UVLO level, etc.
Operation only recommences when the V
CC
voltage drops
below a level of approximately 4.5 V (practically when the
V
mains
has been disconnected for a short period).
Soft start-up
To prevent transformer rattle during hiccup, the
transformer peak current is slowly increased by the soft
start function. This can be achieved by inserting a resistor
and a capacitor between pin I
sense
and the sense resistor
(see Fig.10). An internal current source charges the
capacitor to V = I
SS
×
R
SS
, with a maximum of
approximately 0.5 V.
The start level and the time constant of the increasing
primary current level can be adjusted externally by
changing the values of R
SS
and C
SS
.
The charging current I
SS
will flow as long as the voltage on
pin I
sense
is below approximately 0.5 V. If the voltage on
pin I
sense
exceeds 0.5 V, the soft start current source will
start limiting the current I
SS
. At the V
CC(start)
level, the I
SS
current source is completely switched off.
Since the soft start current I
SS
is subtracted from pin V
CC
chargingcurrent,theR
SS
valuewillaffecttheV
CC
charging
current level by a maximum of 60
μ
A (typical value).
5 V output
Pin V
CC(5V)
can be used for supplying external circuitry.
The maximum output current must be limited to 1 mA.
If higher peak currents are required, an external RC
combination should limit the current drawn from this pin to
1 mA maximum.
The 5 V output voltage will be available as soon as the
start-up voltage is reached. As the high voltage supply can
not supply the 5 V pin during start-up and/or shutdown,
during latched shutdown (via pin LOCK or other latched
protection such as OVP or OTP), the voltage is switched
to zero.
Driver
The driver circuit to the gate of the power MOSFET has a
current sourcing capability of typically 170 mA and a
current sink capability of typically 700 mA. This permits
fast turn-on and turn-off of the power MOSFET for efficient
operation. A low driver source current has been chosen to
limit the
V/
t at switch-on. This reduces Electro Magnetic
Interference (EMI) and also limits the current spikes
across R
sense
.
I
primary(max)
V
---------------R
I
R
×
(
)
–
sense
=
τ
R
SS
C
SS
×
=
handbook, halfpage
CSS
RSS
Rsense
ISS
Vocp
start-up
MCE405
2
0.5 V
Isense
Fig.10 Soft start-up.