Datasheet
NCP1421
http://onsemi.com
11
TYPICAL APPLICATION CIRCUIT
LBI/EN
FB
LBO
REF BAT
GND
LX
OUT
1
2
3
4
8
7
6
5
NCP1421
R4
330 k
R2 200 k
Shutdown
Open Drain
Input
Low Battery
Open Drain
Output
C3
200 nF
R1
350 k
C2
22 F
+
V
OUT
=3.3 V
500 mA
C1
22 F
V
IN
L
6.5 H
Figure 26. Typical Application Schematic for 2 Alkaline Cells Supply
R3
220 k
C4
10 p*
*Optional
GENERAL DESIGN PROCEDURES
Switching mode converter design is considered a
complicated process. Selecting the right inductor and
capacitor values can allow the converter to provide
optimum performance. The following is a simple method
based on the basic first−order equations to estimate the
inductor and capacitor values for NCP1421 to operate in
Continuous Conduction Mode (CCM). The set component
values can be used as a starting point to fine tune the
application circuit performance. Detailed bench testing is
still necessary 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
= 500 mA (600 mA max)
V
LB
= 2.0 V
V
OUT−RIPPLE
= 45 mV
p−p
at I
OUT
= 500 mA
Calculate the feedback network:
Select R2 = 200 k
R1 R2
V
OUT
V
REF
1
R1 200 k
3.3 V
1.20 V
1
350 k
Calculate the Low Battery Detect divider:
V
LB
= 2.0 V
Select R4 = 330 k
R3 R4
V
LB
V
REF
1
R3 300 k
2.0 V
1.20 V
1
220 k
Determine the Steady State Duty Ratio, D, for typical
V
IN
. The operation is optimized around this point:
V
OUT
V
IN
1
1 D
D 1
V
IN
V
OUT
1
2.4 V
3.3 V
0.273
Determine the average inductor current, I
LAVG,
at
maximum I
OUT
:
I
LAVG
I
OUT
1 D
500 mA
1 0.273
688 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 follows:
L
V
IN
t
ON
2I
RIPPLEP
2.4 V 0.75 S
2 (137.6 mA)
6.5 H
A standard value of 6.5 H 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
= 500 mA
C
OUT
I
OUT
t
ON
V
OUTRIPPLE
I
OUT
ESR
COUT
where t
ON
= 0.75 uS and ESR
COUT
= 0.05 ,
C
OUT
500 mA 0.75 S
45 mV 500 mA 0.05
18.75 F