NCP1126, NCP1129

http://onsemi.com

10

latched−state, a permanent current must be injected in the

part. If too low of a current, the part de−latches and the

converter resumes operation. This current is characterized to

32

mA as a minimum but we recommend including a design

margin and select a value around 60

mA. The test is to latch

the part and reduce the input voltage until it de−latches. If

85 V rms, you are fine. If it precociously recovers, you will

have to increase the start−up current, unfortunately to the

detriment of standby power.

The most sensitive configuration is actually that of the

half−wave connection proposed in Figure 3. As the current

disappears 5 ms for a 10 ms period (50 Hz input source), the

latch can potentially open at low line. If you really reduce the

start−up current for a low standby power design, you must

ensure enough current in the SCR in case of a faulty event.

An alternate connection to the above is shown below

(Figure 4):

Figure 4. The Full−Wave Connection Ensures Latch Current Continuity as well as a X2−Discharge Path

In this case, the current is no longer made of 5 ms “holes”

and the part can be maintained at a low input voltage.

Experiments show that these 2 M

W resistor help to maintain

the latch down to less than 50 V rms, giving an excellent

design margin. Standby power with this approach was also

improved compared to Figure 3 solution. Please note that

these resistors also ensure the discharge of the X2−capacitor

up to a 0.47

mF type.

The de−latch of the SCR occurs when a) the injected

data−sheet (32

mA at room temp) or when the part senses a

brown−out recovery.

Frequency Foldback

The reduction of no−load standby power associated with

the need for improving the efficiency, requires a change in

the traditional fixed−frequency type of operation. This

switcher implements a switching frequency foldback when

around 1.5 V. At this point, the oscillator turns into a

Voltage−Controlled Oscillator and reduces its switching

frequency. The peak current setpoint is following the

feedback pin until its level reaches 1 V. Below this value, the

maximum 0.8 V setpoint) and the only way to further reduce

the transmitted power is to diminish the operating frequency

down to 26 kHz. This value is reached at a voltage feedback

level of 450 mV typically. Below this point, if the output

power continues to decrease, the part enters skip cycle for

the best noise−free performance in no−load conditions.

Figure 5 depicts the adopted scheme for the part.