Datasheet

SS
RIPPLE
OUT(max) LOAD
OUT
t
I
C ILIM I
V 2
æ ö
æ ö
= ´ - -
ç ÷
ç ÷
è ø
è ø
FB
11.5 kW
+
Error
Amplifier
0.8 V
REF
BP
GND
+
Offset
f(I
DRAIN
)
PWM to
Switch
I
SLOPE
I
COMP
UDG-09128
I
COMP
– I
SLOPE
x 2
C
COMP
R
COMP
COMP
TPS54290, TPS54291, TPS54292
www.ti.com SLUS973 –OCTOBER 2009
MAXIMUM OUTPUT CAPACITANCE
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 TPS5429x 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
(4)
where
• t
SS
is the soft start time
• ILIM is the current limit level
FEEDBACK LOOP COMPENSATION
In the feedback signal path, the output voltage setting divider is followed by an internal g
M
-type error amplifier
with a typical transconductance of 325 µS. An external series connected R-C circuit from the g
M
amplifier output
(COMPx pin) to ground serves as the compensation network for the converter. The signal from the error amplifier
output is then buffered and combined with a slope compensation signal before it is mirrored to be referenced to
the SW node. Here, it is compared with the current feedback signal to create a pulse-width-modulated (PWM)
signal-fed to drive the upper MOSFET switch. A simplified equivalent circuit of the signal control path is depicted
in Figure 15.
NOTE
Noise coupling from the SWx node to internal circuitry of BOOTx may impact narrow
pulse width operation, especially at load currents less than 1 A.
Figure 15. Feedback Loop Equivalent Circuit
A more conventional small-signal equivalent block diagram is shown in Figure 16. Here, the full closed-loop
signal path is shown. Because the TPS5429x contains internal slope compensation, the external L-C filter must
be selected appropriately so that the resulting control loop meets criteria for stability.
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Product Folder Link(s) :TPS54290 TPS54291 TPS54292