nx2142 Microsemi Corporation, nx2142 Datasheet - Page 10

• Manufacturer Part Number: nx2142
• Description: Single Channel Pwm Controller With Feedforward And 5v Bias Regulator
• Manufacturer: Microsemi Corporation

pacitors, both terms in equation (3) need to be evalu-
ated to determine the overall ripple. Usually when this
type of capacitors are selected, the amount of capaci-
tance per single unit is not sufficient to meet the tran-
sient specification, which results in parallel configura-
tion of multiple capacitors.
tor
is
this can meet DC ripple spec, however it needs to be
studied for transient requirement.
Based On Transient Requirement
sient is specified as
the transient is composed of two sections. One sec-
tion is dependent on the ESR of capacitor, the other
section is
a function of the inductor, output capacitance as well
as input, output voltage. For example, for the
overshoot when load from high load to light load
with a DI
width of system is high enough, the overshoot can
be estimated as the following equation.
Rev. 1.1
10/28/07
with 2m
L
If ceramic capacitors are chosen as output ca-
For example, one 100uF, X5R ceramic capaci-
One ceramic capacitors are needed. Although
Typically, the output voltage droop during tran-
During the transient, the voltage droop during
where
where
where ESR
V
crit
V
V droop
overshoot
RIPPLE
STEP
0
L
ESR C
V
OUT
transient load, if assuming the band-
ESR is used
if L L
I
step
1.838mV 1.9mV 3.738mV
2m
ESR
is the a function of capacitor,etc.
E
and C
I
V tran @step load DI
OUT
step
ESR C
0.919A
crit
I
V
step
E
OUT
represents ESR and capaci-
.
The amount of output ripple
OUT
2 L C
8 600kHz 100uF
ESR
V
OUT
if L L
E
0.919A
OUT
C
I
step
STEP
E
2
V
crit
OUT
...(6)
...(7)
...(8)
tance of each capacitor if multiple capacitors are used
in parallel.
output inductor is smaller than the critical inductance,
the voltage droop or overshoot is only dependent on
the ESR of output capacitor.
pacitor such as electrolytic capacitor, the product of
ESR and capacitance is high and
that case, the transient spec is mostly like to depen-
dent on the ESR of capacitor.
pacitor in parallel. The number of capacitor can be cal-
culated by the following
sient is 150mV for 1.5A load step.
ESR) is used, the crticial inductance is given as
smaller than critical inductance. In that case, the out-
put voltage transient not only dependent on the ESR,
but also capacitance.
and transient requirement. Overall, we choose N=1.
The above equation shows that if the selected
Most case, the output capacitor is multiple ca-
where
For example, assume voltage droop during tran-
If the Electrolytic 6ME1000WG(1000uF, 30mohm
The selected inductor is 6.8uH which is much
number of capacitor is
The number of capacitors has to satisfy both ripple
N
0
L
L
30m
ESR
V
crit
N
OUT
0.225
if L L
30m
I
step
E
V
ESR
ESR
200mV
1.5A
tran
1000 F 5V
ESR
I
E
E
step
V
1.5A
tran
crit
C
I
E
step
E
I
2 L C
step
NX2142/2142A
C
V
E
OUT
V
100 H
For low frequency ca-
OUT
if L L
E
L L
V
tran
crit
crit
is true. In
2
...(10)
...(9)
10