NX2119 Datasheet, PDF (7/16 Pages) Microsemi Corporation – SYNCHRONOUS PWM CONTROLLER WITH CURRENT LIMIT PROTECTION

NX2119/2119A
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 .
For example, one 100uF, X5R ceramic capacitor
with 2mΩ ESR is used. The amount of output ripple is
∆V_RIPPLE
=
2mΩ× 2.56A +
2.56A
8 × 300kHz ×100uF
= 15mV
Although this meets DC ripple spec, however it
needs to be studied for transient requirement.
Based On Transient Requirement
Typically, the output voltage droop during transient
is specified as:
∆V_DROOP <∆V_TRAN @ step load DI_STEP
During the transient, the voltage droop during the
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_STEP
transient load, if assuming the bandwidth of system is
high enough, the overshoot can be estimated as the fol-
lowing equation.
∆V_overshoot
=
ESR × ∆I_step
+
V_OUT
2 × L × C_OUT
× τ²
...(6)
where τ is the a function of capacitor, etc.
τ
=
0


L

if
× ∆I_step
V_OUT
L
−
≤ L_crit
ESR ×
C_OUT
where
if L ≥ L_crit
...(7)
L_crit
=
ESR × C_OUT × V_OUT
∆I_step
=
ESR _E × C_E × V_OUT
∆I_step
...(8)
where ESR_E and C_E represents ESR and capaci-
tance of each capacitor if multiple capacitors are used
in parallel.
The above equation shows that if the selected out-
put inductor is smaller than the critical inductance, the
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 L ≤ L_crit is true. In that case, the
transient spec is dependent on the ESR of capacitor.
In most cases, the output capacitors are multiple
capacitors in parallel. The number of capacitors can be
calculated by the following
N = ^{ESR E × ∆Istep} +
V_OUT
× τ²
∆V_tran
2 × L × C_E × ∆V_tran
where
...(9)
τ
=
0
 _L

if
× ∆I_step
V_OUT
L
−
≤ L_crit
ESR _E
×
C_E
if L ≥ L_crit
...(10)
For example, assume voltage droop during tran-
sient is 100mV for 9A load step.
If the POSCAP 2R5TPE220MC(220uF, 12mΩ ) is
used, the critical inductance is given as
L_crit
=
ESR _E × C_E × V_OUT
∆I_step
=
12mΩ × 220µF×1.9V _{= 0.56µH}
9A
The selected inductor is 1.5uH which is bigger than
critical inductance. In that case, the output voltage tran-
sient not only dependent on the ESR, but also capaci-
tance.
number of capacitors is
τ
=
L × ∆I_step
V_OUT
− ESR _E
× C_E
= ^{1.5µH × 9A} −12mΩ × 220µF = 4.86us
1.8V
N = ^{ESR E × ∆Istep} +
V_OUT
× τ²
∆V_tran
2 × L × C_E × ∆V_tran
= ^{12mΩ × 9A} +
100mV
1.8V
× (4.86us)²
2×1.5µH × 220µF×100mV
= 1.7
The number of capacitors has to satisfied both ripple
and transient requirement. Overall, we can choose N=2.
Rev.3.2
7
04/10/08