Datasheet

1.07k
Rfbb
2.26k
Rfbt
107
Rtkb
226
Rtkt
SS/TRK
3.3V Master
FB
2.5Vout
50 PA
Int VCC
LMZ22005
www.ti.com SNVS686I –MARCH 2011–REVISED OCTOBER 2013
Figure 47. Tracking Option Input Detail
C
O
SELECTION
None of the required C
O
output capacitance is contained within the module. A minimum value of 200 μF is
required based on the values of internal compensation in the error amplifier. Low ESR tantalum, organic
semiconductor or specialty polymer capacitor types are recommended for obtaining lowest ripple. The output
capacitor C
O
may consist of several capacitors in parallel placed in close proximity to the module. The output
capacitor assembly must also meet the worst case minimum ripple current rating of 0.5 * I
LR P-P
, as calculated in
Equation 14 below. Beyond that, additional capacitance will reduce output ripple so long as the ESR is low
enough to permit it. Loop response verification is also valuable to confirm closed loop behavior.
For applications with dynamic load steps; the following equation provides a good first pass approximation of C
O
for load transient requirements. Where V
O-Tran
is 100mV on a 3.3V output design.
C
O
≥I
O-Tran
/((V
O-Tran
– ESR * I
O-Tran
)*(Fsw / V
O
) (6)
Solving:
C
O
≥ 4.5A / ((0.1V – .007*4.5A) * ( 800000HZ / 3.3V) ≥ 271μF (7)
Note that the stability requirement for 200 µF minimum output capacitance will take precedence.
One recommended output capacitor combination is a 220uF, 7 milliohm ESR specialty polymer cap in parallel
with a 100 uF 6.3V X5R ceramic. This combination provides excellent performance that may exceed the
requirements of certain applications. Additionally some small ceramic capacitors can be used for high frequency
EMI suppression.
C
IN
SELECTION
The LMZ22005 module contains only a small amount of input capacitance. Additional input capacitance is
required external to the module to handle the input ripple current of the application. The input capacitor can be
several capacitors in parallel. This input capacitance should be located in very close proximity to the module.
Input capacitor selection is generally directed to satisfy the input ripple current requirements rather than by
capacitance value. Input ripple current rating is dictated by the equation:
I(C
IN(RMS)
) ≊ 1 /2 * I
O
* SQRT (D / 1-D) (8)
where D ≊ V
O
/ V
IN
(As a point of reference, the worst case ripple current will occur when the module is presented with full load
current and when V
IN
= 2 * V
O
).
Recommended minimum input capacitance is 22uF X7R (or X5R) ceramic with a voltage rating at least 25%
higher than the maximum applied input voltage for the application. It is also recommended that attention be paid
to the voltage and temperature derating of the capacitor selected. It should be noted that ripple current rating of
ceramic capacitors may be missing from the capacitor data sheet and you may have to contact the capacitor
manufacturer for this parameter.
If the system design requires a certain minimum value of peak-to-peak input ripple voltage (ΔV
IN
) be maintained
then the following equation may be used.
C
IN
≥ I
O
* D * (1–D) / f
SW-CCM
* ΔV
IN
(9)
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