LM3450AEV230V30/NOPB National Semiconductor, LM3450AEV230V30/NOPB Datasheet - Page 19

• Manufacturer Part Number: LM3450AEV230V30/NOPB
• Description: LM3450A EVAL BOARD 230V30
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3450AEV230V30/NOPB

• Current - Output / Channel: 700mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 50V
• Features: -
• Voltage - Input: 180 ~ 265VAC
• Utilized Ic / Part: LM3450A
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

DYNAMIC HOLD
A forward phase “triac” dimmer requires a minimum amount
of current to be flowing through it during the entire conduction
angle. This is referred to as hold current. If the minimum hold
current requirement is not met, the triac will shut off (misfire).
During normal operation, the converter will demand some
amount of input current. However, at any point during the cy-
cle, the input current can be low enough to cause a misfire.
During an LM3450 sampling period, the triac should not mis-
fire or the decoded angle will be inaccurate as shown in
15. Since the triac is asymmetrical phase-to-phase, misfires
can occur at different points in the waveform. After the triac
misfires, the voltage returns to zero exponentially. This can
create a large difference between decoded angles which can
be observed as a “fluttering” of the light.
To ensure the triac does not misfire during a sampling period
and the angle is correctly decoded, a dynamic hold function
is enabled. The input current is sensed with a resistor (R
from GND to I
voltage across this resistor is less than 200mV, the device
adds holding current via the HOLD circuitry to maintain
200mV across R
The hold current is added by linearly adjusting the gate volt-
age of Q
Q
HLD
is increased, the HOLD pin voltage decreases, forcing
HLD
FIGURE 15. Forward Phase Waveform
as shown in
SEN
SEN
(the return of the full bridge rectifier). If the
.
Figure
14. As the gate voltage of
FIGURE 14. Dynamic Hold Circuit
30127436
Figure
SEN
)
19
a voltage across the resistance (R
Q
through the triac, but is not processed by the converter.
16
is a non-sampled conduction angle, 2 is the firing angle, and
3 is a sampled conduction angle.
The dynamic hold function is also necessary for reverse
phase dimmers, but for a different reason. Reverse phase
dimmers do not use triacs, therefore they do not require a
minimum “holding” current. Instead, they need what is com-
monly called bleeder current. When a reverse phase dimmer
turns off, the AC voltage is at a high value. There is an RC
time constant associated with discharging the total effective
input capacitance (EMI capacitors, PFC capacitor, damper
capacitance). The decoder does not record the angle until the
voltage reaches the 356mV threshold. This can cause the
decoded angle to be much larger than it actually is and de-
pendent on the RC time constant as shown in
PS
shows a typical dynamic hold waveform where interval 1
to HOLD. This extra current is drawn from the input
FIGURE 16. Dynamic Hold Waveform
30127435
HLD
) from the source of
Figure
www.national.com
30127437
17.
Figure