SCY99079ADR2G View Datasheet(PDF) - ON Semiconductor
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Description
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SCY99079ADR2G Datasheet PDF : 28 Pages
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RoppU
This point will
be adjusted to
reduce the ref
at hi line to the
desired level.
Iopp
RoppL
OPP
DAP018A/B/C/D/F
VCC aux
+
Swings to:
N1Vout during toff
−N2Vin during ton
VDD
from FB
K1 SUM2
ref
K2
0.8 V ±5%
ref = 0.8 V + VOPP
(VOPP is negative)
Reset
-
+
CS
Figure 52. The OPP Circuitry Affects the Maximum Peak Current Setpoint by
Summing a Negative Voltage to the Internal Voltage Reference
Let us assume that we have the following converter
characteristics:
Vout = 19 V
Vin = 85 to 265 Vrms
N1 = Np:Ns = 1:0.2
N2 = Np:Naux = 1:0.16
Given the turn ratio between the primary and the auxiliary
windings, the on−time voltage at high line (265 Vac) on the
auxiliary winding swings down to:
Vaux + −N2Vin,max + −0.16 375 + −60 V (eq. 5)
To obtain a level as imposed by Equation 3, we need to
install a divider featuring the following ratio:
Div
+
0.16
60
+
0.00266
(eq. 6)
If we arbitrarily fix the pulldown resistor ROPPL to 1 kW,
then the upper resistor can be obtained by:
ROPPU
+
60 * 0.16
0.16ń1 k
+
374
kW
(eq. 7)
If we now plot the peak current setpoint obtained by
implementing the recommended resistor values, we obtain
the following curve:
The OPP pin is made of zener diodes arranged to protect
the pin against ESD pulses. These diodes accept some peak
current in the avalanche mode and are designed to sustain a
certain amount of energy. On the other side, negative
injection into these diodes (or forward bias) can cause
substrate injection wich can lead to an erratic circuit
behaviour. To avoid this problem, the pin is internall
clamped slightly below –300 mV which means that if more
current is injected before reaching the ESR forward drop,
then the maximum peak reduction is kept to 40%. If the
voltage finally forward biases the internal zener diode, then
care must be taken to avoid injecting a current beyond
–2 mA. Given the value of ROPPU, there is no risk in the
present example.
On Figure 53, we can see that the OPP starts to fold the
setpoint immediately from a low mains operation. This is no
different than a standard OPP circuitry built from a resistive
string placed between the bulk rail and the current sense pin.
However, in some applications, it is good to remove the OPP
at low line and place a small threshold so that our OPP only
changes the circuit power capability at high line only.
Figure 54 offers a possible solution built on a zener diode
connection.
Peak currentsetpoint
100%
20%
375
Vbulk
Figure 53. The Peak Current Regularly Reduces
Down to 20% at 375 Vdc
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