TEA1504 데이터 시트보기 (PDF) - Philips Electronics
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TEA1504 Datasheet PDF : 20 Pages
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Philips Semiconductors
GreenChip™ SMPS control IC
Preliminary specification
TEA1504
Pulse width modulator
The PWM comprises an inverting error amplifier and a
comparator (see Fig.1) which drives the power MOSFET
with a duty cycle that is inversely proportional to the
voltage at CTRL (pin 9). A signal from the oscillator sets a
latch that turns on the power MOSFET. The latch is then
reset either by the signal from the PWM or by a duty cycle
limiting signal from the oscillator. The latch stops the
power MOSFET from being switched incorrectly if the
PWM output signal becomes unstable. The maximum duty
cycle is internally set to 80%. The IC switching signals
during normal operation are shown in Fig.7.
Oscillator
The oscillator determines the switching duty cycle.
Its ramp signal voltage is compared to the output of the
error amplifier by the PWM. The fully integrated oscillator
circuit works by charging and discharging an internal
capacitor between two voltage levels to create a sawtooth
waveform with a rising edge that is 80% of the oscillator
period (high frequency mode). This ratio sets a maximum
switching duty cycle of 80% for the IC. The accuracy of the
oscillator frequency is internally set to 5%. Its frequency
can be adjusted between 50 and 100 kHz by changing the
value of RREF. This gives the power supply designer
greater flexibility in the choice of system components.
The relationship between frequency and the value of RREF
is shown in Fig.6. The range of RREF values and the
frequencies of foscL and foscH are specified in Chapter
“Characteristics”.
Multi frequency control
When the power supply operates at or below 1⁄9 of its peak
power, the IC changes to low power operation mode.
This lowers the frequency of the oscillator to reduce the
power supply switching losses. The ratio between the high
and the low oscillator frequency is maintained at 1 : 2.5
(see foscL in Chapter “Characteristics”). An innovative
design ensures that the transfer from high-to-low
frequency and vice versa does not effect output voltage
regulation.
Gate driver
The driver circuit to the Gate of the power MOSFET has a
totem-pole output stage that has current sourcing
capability of 120 mA and a current sink capability of
550 mA. This permits fast turn-on and turn-off of the power
MOSFET for efficient operation. This circuit design allows
the power supply designer to control the source and sink
currents of the Gate driver circuit with a minimum number
of external components.
A low driver source current has been chosen in order to
limit the ∆V/∆t at switch-on. This reduces Electro Magnetic
Interference (EMI) and also the current spike across
Rsense.
Demagnetization protection
The demagnetization protection feature ensures
discontinuous conduction of the power supply, simplifying
the design of feedback control and giving a faster transient
response. It protects against saturation of the
transformer/inductor and also protects the power supply
components against excessive stresses at start-up, when
all energy storage components are completely discharged.
During a system output short-circuit fault condition, it
provides cycle-by-cycle protection of the converter
configuration. The demagnetization resistor (RDEM) value
can be calculated using the formula given in Section
“Sample-and-hold”.
handboo1k,1h0alfpage
f oscH
(kHz)
90
MGS573
55
f oscL
(kHz)
45
70
35
(1)
(2)
50
25
30
15
10
20
30
40
RREF (kΩ)
(1) High frequency mode.
(2) Low frequency mode.
Fig.6 Frequency as function of RREF value.
Negative clamp
The negative clamp circuit ensures correct operation of
the IC by preventing the voltage at DEM (pin 13) dropping
below −0.45 V, during the period when the power
MOSFET turns on and the auxiliary winding voltage goes
negative.
1999 Dec 07
7
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