Datasheet

1.07k
Rfbb
2.26k
Rfbt
107
Rtkb
226
Rtkt
3.3V Master
2.5Vout
50 PA
Int VCC
SS/TRK FB
LMZ23603
www.ti.com SNVS711I –MARCH 2011–REVISED OCTOBER 2013
TRACKING SUPPLY DIVIDER OPTION
The tracking function allows the module to be connected as a slave supply to a primary voltage rail (often the
3.3V system rail) where the slave module output voltage is lower than that of the master. Proper configuration
allows the slave rail to power up coincident with the master rail such that the voltage difference between the rails
during ramp-up is small (i.e. <0.15V typ). The values for the tracking resistive divider should be selected such
that the presence of the internal 50uA current source is minimized. In most cases the ratio of the tracking divider
resistors is the same as the ratio of the output voltage setting divider. Proper operation in tracking mode dictates
the soft-start time of the slave rail be shorter than the master rail; a condition that is easy satisfy since the C
SS
cap is replaced by R
TKB
. The tracking function is only supported for the power up interval of the master supply;
once the SS/TRK rises past 0.8V the input is no longer enabled and the 50 uA internal current source is switched
off.
Figure 52. 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 17 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. R
LRA
is included
on the typical application schematic and evaluation board for this purpose. It is not required in the end design.
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 100 mV on a 3.3V output design.
C
O
≥I
O-Tran
/((*V
O-Tran
– ESR * I
O–Tran
) * ( F*
SW
/ V
O
)) (8)
Solving:
C
O
≥ 2.5A / ((0.1V – .007*2.5A) * ( 800000 Hz / 3.3V) ≥ 125μF (9)
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.
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