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CS51221 Datasheet(PDF) 11 Page - ON Semiconductor

Part # CS51221
Description  Enhanced Voltage Mode PWM Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS51221 Datasheet(HTML) 11 Page - ON Semiconductor

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CS51221
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11
0.0001
0.001
0.01
CT (mF)
0
800
400
700
600
500
300
200
100
1000
10000
1000000
RT (W)
0.50
1.00
0.80
0.95
0.90
0.85
0.75
0.70
0.65
0.60
0.55
100000
Figure 10. Typical Performance Characteristics,
Oscillator Frequency vs. CT
Figure 11. Typical Performance Characteristics,
Oscillator Duty Cycle vs. RT
RT = 5.0 K
RT = 10 K
RT = 50 K
that during steady state, VCOMP doesn’t change for input
voltage variations. This intuitively explains why FF voltage
mode control has superior line regulation and line transient
response. Knowing the nominal value of VIN and TON, one
can also select the value of RC to place VCOMP at the center
of its dynamic range.
Select Feedback Voltage Divider
As shown in Figure 12, the voltage divider output feeds to
the FB pin, which connects to the inverting input of the error
amplifier. The non−inverting input of the error amplifier is
connected to a 1.27 V (typ) reference voltage. The FB pin
has an input current which has to be considered for accurate
DC outputs. The following equation can be used to calculate
the R1 and R2 value
R2
R1
) R2
VOUT + 1.27 *
where
∇ is the correction factor due to the existence of the
FB pin input current Ier.
+ (Ri ) R1 R2)Ier
Ri = DC resistance between the FB pin and the voltage
divider output.
Ier = VFB input current, 1.3
mA typical.
Design Voltage Dividers for OV and UV Detection
In Figure 13, the voltage divider uses three resistors in
series to set OV and UV threshold seen from the input
voltage. The values of the resistors can be calculated from
the following three equations, where the third equation is
derived from OV hysteresis requirement.
VIN(LOW)
R2
) R3
R2
) R3 ) R1 +
1.0 V
(A)
VIN(HIGH)
R3
R2
) R3 ) R1 +
2.0 V
(B)
12.5
mA
(R1
) R2) + VHYST
(C)
where:
VIN(LOW), VIN(HIGH) = input voltage OV and UV
threshold;
VHYST = OV hysteresis seen at VIN
It is self−evident from equation A and B that to use this
design, VIN(HIGH) has to be two times greater than
VIN(LOW). Otherwise, two voltage dividers have to be used
to program OV and UV separately.
Figure 12. The Design of Feedback Voltage Divider
Has to Consider the Error Amplifier Input Current
+
+
VOUT
R1
COMP
FB
R2
Ri
1.27
Ier
Figure 13. OV/UV Monitor Divider
VIN
R1
R2
R3
VUV
VOV


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