NCP1417(2002) Ver la hoja de datos (PDF) - ON Semiconductor
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NCP1417 Datasheet PDF : 16 Pages
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NCP1417
With the feedback resistor divider, additional small
capacitor, CFB1 in parallel with RFB1 is required to ensure
stability. The value can be in between 68 nF to 220 nF, the
rule is to select the lowest capacitance to ensure stability.
Also a small capacitor, CFB2 in parallel with RFB2 may also
be needed to lower the feedback ripple hence improve
output ripple and regulation. In this example, only CFB1 is
used and the value is 150 nF.
Calculate the Low Battery Detect divider:
VLB1 = 2.3 V
Select RLB2 = 330 K
ǒ Ǔ RLB1 + RLB2
VLB1
VREF
*
1
ǒ Ǔ RLB1 + 330 K
2.3 V
1.19 V
*
1
+ 308 K
Once the VLB1 is set, the next low battery detection point,
VLB2 will be fixed automatically.
Determine the Steady State Duty Ratio, D for typical VIN,
operation will be optimized around this point:
VOUT
VIN
+
1
1
*
D
D
+
1
*
VIN
VOUT
+
1
*
2.4
3.3
V
V
+
0.273
Determine the average inductor current, ILAVG at maximum
IOUT:
ILAVG
+
IOUT
1*D
+
200 mA
1 * 0.273
+
275
mA
Determine the peak inductor ripple current, IRIPPLE–P and
calculate the inductor value:
Assume IRIPPLE–P is 25% of ILAVG, the inductance of the
power inductor can be calculated as follows:
IRIPPLE–P + 0.25 275 mA + 68.8 mA
L
+
VIN tON
2IRIPPLE–P
+
2.4 V 1.4 mS
2(68.8 mA)
+
24.4
mH
Standard value of 22 mH is selected for initial trial.
Determine the output voltage ripple, VOUT–RIPPLE and
calculate the output capacitor value:
VOUT * RIPPLE + 40 mVP–P at IOUT + 200 mA
COUT
u
IOUT
VOUT–RIPPLE *
tON
IOUT
ESRCOUT
where tON + 1.4 mS and ESRCOUT + 0.15 W,
COUT
u
40
200 mA
mV * 200
1.4
mA
mS
0.15
W
+
28
mF
From above calculation, you need at least 28 mF in order
to achieve the specified ripple level at conditions stated.
Practically, a one level larger capacitor will be used to
accommodate factors not take into account in the
calculation. So a capacitor value of 33 mF is selected as
initial trial.
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