nx2138 Microsemi Corporation, nx2138 Datasheet - Page 12

• Manufacturer Part Number: nx2138
• Description: Single Channel Mobile Pwm Controller
• Manufacturer: Microsemi Corporation

is chosen as output capacitor, the ESR and inductor
current typically determines the output voltage ripple.
When VIN reach maximum voltage, the output volt-
age ripple is in the worst case.
tiple capacitors in parallel are needed. The number of
output capacitor can be calculate as the following:
integer. Choose N =1.
Based On Transient Requirement
sient is specified as
transient is composed of two sections. One section 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 over-
shoot when load from high load to light load with a
DI
tem is high enough, the overshoot can be estimated
as the following equation.
Rev. 1.6
12/09/09
STEP
ESR
and the second term can be neglected.
For this example, one POSCAP 2R5TPE330MC
If low ESR is required, for most applications, mul-
N
N =1
The number of capacitor has to be round up to a
Typically, the output voltage droop during tran-
During the transient, the voltage droop during the
where
where
N
V
transient load, if assuming the bandwidth of sys-
V droop
overshoot
0
L
desire
12m
V
E S R
OUT
60m V
=
if L L
I
step
ESR
is the a function of capacitor,etc.
V
I
E
RIPPLE
V
RIPPLE
V tran @step load DI
5A
R I P P L E
ESR C
crit
I
I
R I P P L E
step
60mV
OUT
5A
2 L C
V
OUT
if L L
12m
OUT
STEP
2
crit
.
..(7)
...(6)
...(8)
...(9
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 60mV for 3A load step.
ESR) is used, the crticial inductance is given as
than critical inductance. In that case, the output volt-
age transient mainly dependent on the ESR.
L
Choose N=1.
crit
where ESR
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 one POSCAP 2R5TPE330MC(330uF, 12mohm
The selected inductor is 1.5uH which is smaller
number of capacitor is
N
N
12m
0.6
0
L
ESR C
L
12m
60mV
ESR
ESR
V
crit
OUT
if L L
I
step
ESR
E
E
V
V
3A
3300 F 1.8V
E
tran
OUT
I
tran
3A
step
and C
E
ESR
I
I
step
step
C
I
V
step
crit
E
OUT
E
E
represents ESR and capaci-
2 L C
V
C
OUT
E
ESR
23.76 H
V
For low frequency ca-
OUT
if L L
E
E
L L
C
I
step
V
E
tran
NX2138
crit
V
crit
OUT
is true. In
2
...(12)
...(10)
...(11)
12