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參數(shù)資料
| 型號： | L6566BTR |
| 廠商： | 意法半導體 |
| 英文描述： | Multi-mode controller for SMPS |
| 中文描述： | 多模式開關電源控制器 |
| 文件頁數(shù)： | 31/51頁 |
| 文件大?。?/td> | 401K |
| 代理商： | L6566BTR |
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L6566B
Application information
31/51
between the detected event and the oscillator, occasionally the device could stop after the
third detection.
This condition is latched as long as the device is supplied. While it is disabled, however, no
energy is coming from the self-supply circuit; hence the voltage on the Vcc capacitor will
decay and cross the UVLO threshold after some time, which clears the latch. The internal
start-up generator is still off, then the Vcc voltage still needs to go below its restart voltage
before the Vcc capacitor is charged again and the device restarted. Ultimately, this will result
in a low-frequency intermittent operation (Hiccup-mode operation), with very low stress on
the power circuit. This special condition is illustrated in the timing diagram of
Figure 18 on
page 30
.
5.8
Frequency modulation
To alleviate converter’s EMI emissions and reduce cost and size of the line filter, it is
advantageous to modulate its switching frequency, so that the resulting spread-spectrum
action distributes the energy of each harmonic of the switching frequency over a number of
side-band harmonics. Their overall energy will be unchanged but the individual amplitudes
will be smaller. This is what naturally occurs with QR operation, due to the twice-mains-
frequency ripple appearing on the input bulk capacitor, which translates into different DCM-
CCM boundary frequencies.
The L6566B is provided with a dedicated pin, FMOD (6), to perform this function if FF mode
is selected.
Figure 19. Frequency modulation circuit
With reference to
Figure 19
, the capacitor C
MOD
is connected from FMOD to ground and is
alternately charged and discharged between 0.5 and 1.5 V by internal current generators
sourcing and sinking the same current (three times the current defined by the resistor R
T
on
pin OSC). Hence, the voltage across C
MOD
will be a symmetric triangle, whose frequency f
m
is determined by C
MOD
. By connecting a resistor R
MOD
from FMOD to OSC, the current
sourced by pin OSC will be modulated according a triangular profile at a frequency f
m
. If
R
MOD
is considerably higher than R
T
, as normally is, both f
m
and the symmetry of the
triangle will be little affected.
With this arrangement it is possible to set, nearly independently, the frequency deviation
f
sw
and the modulating frequency f
m
, which define the modulation index:
L6566B
OSC
6
R
T
FMOD
13
R
MOD
C
MOD
1.5 V
0.5 V
0 V
1 V
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