nx2113 Microsemi Corporation, nx2113 Datasheet - Page 10

• Manufacturer Part Number: nx2113
• Description: 300khz & 600khz Synchronous Pwm Controller With Programmable Bus Uvlo
• 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 ESR is used. The amount of output ripple is
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
transient load, if assuming the bandwidth of system is
high enough, the overshoot can be estimated as the fol-
lowing equation.
tance of each capacitor if multiple capacitors are used
in parallel.
put inductor is smaller than the critical inductance, the
voltage droop or overshoot is only dependent on the ESR
Rev. 2.0
11/18/05
L
crit
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
OUT
if L L
I
< V
step
is the a function of capacitor, etc.
2m
6mV 6.2mV 12.2mV
E
ESR
I
OUT
TRAN
step
and C
ESR C
crit
V
@ step load DI
3A
OUT
E
I
represents ESR and capaci-
step
OUT
8 600kHz 100uF
ESR
2 L C
if L L
V
E
OUT
3A
C
I
step
STEP
OUT
E
V
crit
OUT
2
...(8)
...(6)
...(7)
DI
STEP
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 10A load step.
used, the critical inductance is given as:
than critical inductance. In that case, the output voltage
transient not only dependent on the ESR, but also ca-
pacitance.
and transient requirement. Overall, we can choose N=3.
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 0.78uH which is bigger
number of capacitors is
The number of capacitors has to satisfy both ripple
N
2 1.5 H 220 F 80mV
0
L
1.7
12m
0.78 H 10A
L
12m
ESR
V
L
crit
ESR
80mV
OUT
V
1.6V
if L L
I
OUT
E
V
step
I
ESR
tran
step
E
V
10A
220 F 1.6V
1.6V
10A
tran
I
ESR
step
E
ESR
I
step
crit
C
I
L L
12m
E
step
2 L C
E
E
2 L C
C
V
C
NX2113/2113A
E
crit
OUT
E
V
220 F 2.24us
is true. In that case, the
OUT
0.42 H
E
V
if L L
(2.24us)
OUT
E
V
tran
V
tran
2
crit
2
2
...(10)
...(9)
10