Datasheet

Inductor Selection
IN OUT
OUT
V - V
L
I
=
D
(3)
Maximum Output Capacitance
( )
SS
CLx RIPPLE LOAD
OUT max
OUT
t
1
C I I I
V 2
æ ö
æ ö
= - ´ -
ç ÷
ç ÷
è ø
è ø
(4)
Minimum Output Capacitance
Compensation For The Feedback Loop
( )
600000
Fm
TPS55386
6
1.5 10 t
V - V
ON
-6
IN OUT
19.7 e 50 10
L
=
é ù
´ ´
æ ö
ê ú
´ + ´ ´
ç ÷
ê ú
ç ÷
è ø
ê ú
ë û
(5)
TPS55383 , , TPS55386
www.ti.com
......................................................................................................................................................................................... SLUS818 SEPTEMBER 2008
Calculate the inductance value so that an output ripple current between 300 mA and 900 mA results. Lower
ripple current results in discontinuous mode (DCM) operation at a lower DC load current, while higher ripple
current generally allows for higher closed loop bandwidth.
NOTE:
For wide input range converters, highest input voltage results in the highest ripple
current.
NOTE:
The load current at which the overcurrent protection (OCP) engages is dependent on
the amount of ripple current, because it is the peak current in the switch that is
monitored. See Output Overload Protection .
With internal pulse-by-pulse current limiting and a fixed soft-start time, there is a maximum output capacitance
which may be used before startup problems begin to occur. If the output capacitance is large enough so that the
device enters a current-limit protection mode during startup, then there is a possibility that the output never
reaches regulation. Instead, the TPS5538x simply shuts down and attempts a restart as if the output were
short-circuited to ground. The maximum output capacitance (including bypass capacitance distributed at the
load) is given by Equation 4 :
Ensure the value of capacitance selected for closed-loop stability is compatible with the requirements of Soft
Start .
To determine the components necessary for compensating the feedback loop, the controller frequency response
characteristics must be understood and the desired crossover frequency selected. The best results are obtained
if 10% of the switching frequency is used as this closed loop crossover frequency. In some cases, up to 20% of
the switching frequency is also possible.
With the output filter components selected, the next step is to calculate the DC gain of the modulator. For the
TPS55386:
The gain of the TPS55383 modulator is approximated by:
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