NCP1410_05 Datasheet, PDF (12/14 Pages) ON Semiconductor – 250 mA Sync-Rect PFM Step-Up DC-DC Converter with Low-Battery Detector

NCP1410
GENERAL DESIGN PROCEDURES
Switching mode converter design is considered as black
magic to most engineers, some complicate empirical
formulae are available for reference usage. Those formulae
are derived form the assumption that the key components,
i.e. power inductor and capacitors are available with no
tolerance. Practically, its not true, the result is not a matter
of how accurate the equations you are using to calculate the
component values, the outcome is still somehow away from
the optimum point. In below a simple method base on the
most basic first order equations to estimate the inductor and
capacitor values for NCP1410 operate in Continuous
Conduction Mode is introduced. The component value set
can be used as a starting point to fine tune the circuit
operation. By all means, detail bench testing is needed to get
the best performance out of the circuit.
Design Parameters:
V_IN = 1.8 V to 3.0 V, Typical 2.4 V
V_OUT = 3.3 V
I_OUT = 200 mA (250 mA max)
V_LB = 2.0 V
V_{OUT−RIPPLE} = 40 mV_P−P at I_OUT = 250 mA
Calculate the feedback network:
Select R_FB2 = 200 K
ǒ Ǔ RFB1 ₊ RFB2
VOUT
VREF
*
1
ǒ Ǔ RFB1 + 200 K
3.3 V
1.19 V
*
1
+ 355 K
Calculate the Low Battery Detect divider:
V_LB = 2.0 V
Select R_LB2 = 330 K
ǒ Ǔ _{RLB1 + RLB2}
VLB
VREF
*
1
ǒ Ǔ RLB1 + 330 K
2.0 V
1.19 V
*
1
+ 225 K
Determine the Steady State Duty Ratio, D for typical V_IN,
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, I_LAVG at
maximum I_OUT:
ILAVG
+
IOUT
1*D
+
250 _mA
1 * 0.273
+
344
mA
Determine the peak inductor ripple current, I_{RIPPLE−P} and
calculate the inductor value:
Assume I_{RIPPLE−P} is 20% of I_LAVG, the inductance of the
power inductor can be calculated as in below:
I_{RIPPLE−P} = 0.20 x 344 mA = 68.8 mA
L
+
2
VIN tON
IRIPPLE *
P
+
2.4 V
2(68.8
1.4 mS
mA)
+
24.4
mH
Standard value of 22 mH is selected for initial trial.
Determine the output voltage ripple, V_{OUT−RIPPLE} and
calculate the output capacitor value:
V_{OUT−RIPPLE} = 40 mV_P−P at I_OUT = 250 mA
COUT
u
IOUT
VOUT−RIPPLE *
tON
IOUT
ESRCOUT
where t_ON = 1.4 mS and ESR_COUT = 0.1 W,
COUT
u
40
250 mA
mV * 250
1.4
mA
mS
0.1
W
+
23.33
mF
From above calculation, you need at least 23.33 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, therefore a capacitor value of 33 mF is selected.
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