nx2114 Microsemi Corporation, nx2114 Datasheet - Page 9

• Manufacturer Part Number: nx2114
• Description: 300khz & 600khz Synchronous Pwm Controller
• Manufacturer: Microsemi Corporation

pacitors, both terms in equation (3) need to be evaluated
to determine the overall ripple. Usually when this type of
capacitors are selected, the amount of capacitance per
single unit is not sufficient to meet the transient specifi-
cation, which results in parallel configuration of multiple
capacitors.
with 2m
needs to be studied for transient requirement.
Based On Transient Requirement
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 overshoot,
when load from high load to light load with a DI
sient load, if assuming the bandwidth of system is high
enough, the overshoot can be estimated as the following
equation.
tance of each capacitor if multiple capacitors are used
in parallel.
put inductor is smaller than the critical inductance, the
Rev. 4.0
06/20/06
L
crit
If ceramic capacitors are chosen as output ca-
For example, one 100uF, X5R ceramic capacitor
Although this meets DC ripple spec, however it
Typically, the output voltage droop during transient
During the transient, the voltage droop during the
where
where
where ESR
The above equation shows that if the selected out-
V
V
V
0
L
RIPPLE
DROOP
overshoot
ESR C
V
ESR is used
OUT
if L L
I
< V
step
is the a function of capacitor, etc.
2m
4.6mV 9.6mV 13.2mV
E
ESR
I
OUT
TRAN
step
and C
ESR C
crit
V
2.3A
@ step load DI
.
OUT
E
The amount of output ripple is
I
represents ESR and capaci-
step
OUT
8 300kHz 100uF
ESR
2 L C
if L L
V
E
OUT
2.3A
C
I
step
STEP
OUT
E
V
crit
OUT
2
STEP
...(8)
...(6)
...(7)
tran-
voltage droop or overshoot is only dependent on the ESR
of output capacitor. For low frequency capacitor such
as electrolytic capacitor, the product of ESR and ca-
pacitance is high and
transient spec is dependent on the ESR of capacitor.
capacitors in parallel. The number of capacitors can be
calculated by the following
sient is 100mV for 6A load step.
used, the critical inductance is given as
critical inductance. In that case, the output voltage tran-
sient not only dependent on the ESR, but also capaci-
tance.
and transient requirement. Overall, we can choose N=2.
N
In most cases, the output capacitors are multiple
where
For example, assume voltage droop during tran-
If the POSCAP 2R5TPE220MC (220uF, 12m ) is
The selected inductor is 1.5uH which is bigger than
number of capacitors is
The number of capacitors has to satisfied both ripple
N
2 1.5 H 220 F 60mV
0
L
1.7
15m
1.5 H 6A
ESR
15m
L
V
L
ESR
60mV
crit
OUT
1.6V
V
if L L
I
E
V
step
OUT
tran
I
E
V
ESR
6A
step
1.6V
tran
220 F 1.6V
I
6A
ESR
step
I
E
step
ESR
crit
15m
L L
E
C
I
2 L C
step
E
2 L C
C
E
NX2114/2114A
E
crit
V
C
V
OUT
220 F
is true. In that case, the
E
OUT
E
V
if L L
0.88 H
OUT
2.3us
E
V
tran
V
2
2.3us
tran
crit
2
2
...(10)
...(9)
9