Datasheet

OUT D
IN
V V
1
V 1 D
+
=
-
IN
OUT D
V
D 1
V V
æ ö
æ ö
= -
ç ÷
ç ÷
ç ÷
+
è ø
è ø
( )
( )
f
OUT D OUT SW
2
IN
2 V V I L
D
V
´ + ´ ´ ´
=
( )
( )
( )
f
2
OUT D IN IN
OUT(crit)
2
OUT D SW
V V V V
I
2 V V L
+ - ´
=
´ + ´ ´
TPS40210, TPS40211
SLUS772E –MARCH 2008– REVISED OCTOBER 2011 www.ti.com
APPLICATION INFORMATION
Minimum On-Time and Off Time Considerations
The TPS40210 has a minimum off time of approximately 200 ns and a minimum on time of 300 ns. These two
constraints place limitations on the operating frequency that can be used for a given input to output conversion
ratio. See Figure 2 for the maximum frequency that can be used for a given duty cycle.
The duty cycle at which the converter operates is dependent on the mode in which the converter is running. If the
converter is running in discontinuous conduction mode, the duty cycle varies with changes to the load much
more than it does when running in continuous conduction mode.
In continuous conduction mode, the duty cycle is related primarily to the input and output voltages.
(1)
(2)
In discontinuous mode the duty cycle is a function of the load, input and output voltages, inductance and
switching frequency.
(3)
All converters using a diode as the freewheeling or catch component have a load current level at which they
transition from discontinuous conduction to continuous conduction. This is the point where the inductor current
just falls to zero. At higher load currents, the inductor current does not fall to zero but remains flowing in a
positive direction and assumes a trapezoidal wave shape as opposed to a triangular wave shape. This load
boundary between discontinuous conduction and continuous conduction can be found for a set of converter
parameters as follows.
(4)
For loads higher than the result of Equation 4, the duty cycle is given by Equation 2 and for loads less that the
results of Equation 4, the duty cycle is given Equation 3. For Equations 1 through 4, the variable definitions are
as follows.
• V
OUT
is the output voltage of the converter in V
• V
D
is the forward conduction voltage drop across the rectifier or catch diode in V
• V
IN
is the input voltage to the converter in V
• I
OUT
is the output current of the converter in A
• L is the inductor value in H
• f
SW
is the switching frequency in Hz
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