NCP3127
http://onsemi.com
20
tss
Output
Current
Output
Voltage
3.3 V
Figure 28. Resistive Load Current
Alternatively, if the output has an under voltage lockout,
turns on at a defined voltage level, and draws a consistent
current, then the RMS connected load current is:
I
CLI+
V
OUT
* V
OUT_TO
V
OUT
Ǹ
I
OUT
(eq. 51)
492 mA +
ǒ
ǒ
3.3 V * 2.5 V
Ǔ
3.3 V
Ǔ
Ǹ
1A
I
OUT
= Output current
V
OUT
= Output voltage
V
OUT_TO
= Output voltage load turn on
tss
t
1.0 V
3.3 V
Output
Current
Output
Voltage
Figure 29. Voltage Enable Load Current
If the inrush current is higher than the steady state input
current during max load, then an input fuse should be rated
accordingly using I
2
t methodology.
Layout Considerations
As in any high frequency switching regulator, layout is
very important. Switching current from one power device to
another can generate voltage transients across the
impedances of the interconnecting bond wires and circuit
traces. The interconnecting impedances should be
minimized by using wide short printed circuit traces. The
critical components should be located as close together as
possible using ground plane construction or single point
grounding. For optimal performance, the NCP3127 should
have a layout similar to the one shown in Figure 30. An
important note is that the input voltage to the NCP3127
should have local decoupling to PGND. The recommended
decoupling for input voltage is a 1 mF general purpose
ceramic capacitor and a 0.01 mF COG ceramic capacitor
placed in parallel.
Top
Bottom
AGND
COG
0.01 uF
1.0 uF
RF
R2
RC
PGND
ISET
FB
COMP
AGND
BST
VIN
PGND
VSW
RISET
R1
CF
CP
CC
Single Point
Grounding
AGND
COG
0.01 uF
1.0 uF
RF
R1
RC
PGND
ISET
FB
COMP
AGND
BST
VIN
PGND
VSW
RISET
R2
CF
CP
CC
Figure 30. Recommended Layout
The typical applications are shown in Figures 31
and NO TAG for output electrolytic and ceramic bulk
capacitors, respectively.
