LM3450AEV230V30/NOPB National Semiconductor, LM3450AEV230V30/NOPB Datasheet - Page 21

• 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

PRIMARY BIAS SUPPLY
The LM3450 requires a supply voltage at V
25V. The device has V
rising and falling thresholds of 12.9V and 7.9V respectively.
A 24V Zener diode should be placed from the V
to protect the device from substantial spikes that could cause
damage.
Figure 18
generate the necessary primary bias supply at start-up. Since
the AC line peak voltage is always higher than the rating of
the controller, all designs require an N-channel MosFET
(passFET). The passFET (Q
tached to the rectified AC. The gate of Q
the BIAS pin which has a stack of 2 Zener diodes internal to
the device. These diodes are then biased from the rectified
AC line through series resistance (R
held at a V
through Q
the capacitance is large enough, the source voltage will re-
main relatively constant over the line cycle and this becomes
the input bias supply at V
PS
shows how the LM3450 provides a quick way to
GS
to charge up whatever capacitance is present. If
below the Zener voltage and current flows
CC
CC
under-voltage lockout (UVLO) with
.
PS
) is connected with its drain at-
BS
). The source of Q
PS
CC
is connected to
, not to exceed
FIGURE 18. Primary Bias Circuitry
CC
pin to GND
PS
is
21
This bias circuit enables instant turn-on. However, once the
circuit is operational it is desirable to bootstrap V
iliary winding of the inductor or transformer (also used for
ZCD). The two bias paths are each connected to V
a diode to ensure the higher of the two is providing V
rent. This bootstrapping greatly improves efficiency when
quick start-up is necessary.
To ensure that the auxiliary winding is powering V
times except start-up, the LM3450 has a dual BIAS mode. The
BIAS voltage at startup is 20V through two Zener diodes.
When the V
device turns on, the BIAS pin voltage is reduced to 14V (bot-
tom 6V Zener is shorted). Once the V
old is reached again, the BIAS pin will return to 20V to attempt
to restart the device.
It should be noted that the large hysteresis of V
the dual BIAS mode allow for a large variation of the auxiliary
bias circuitry easing the design of the magnetics.
CC
UVLO rising threshold is exceeded and the
30127439
CC
UVLO falling thresh-
CC
CC
www.national.com
CC
UVLO and
to an aux-
CC
through
CC
at all
cur-