Datasheet

R3
D1
C1
R1
V
OUT
R2
V
IN
C4
C3
C2
L1
R4
TPS61
175
VIN
EN
PGND
AGND
Syn
FREQ
COMP
SS
NC
FB
PGND
PGND
SW
SW
OUT D IN
O
UT D
V + V V
D =
V + V
-
OUT D OUT SW
2
I
N
2 (V + V ) I L
D =
V
´ ´ ´ ´ ¦
2
O
UT D IN IN
OUT(crit)
2
OUT D SW
(V + V V ) V
I =
2 (V + V ) L
- ´
´ ´ ¦ ´
TPS61175
www.ti.com SLVS892B –DECEMBER 2008–REVISED FEBRUARY 2012
APPLICATION INFORMATION
The following section provides a step-by-step design approach for configuring the TPS61175 as a voltage
regulating boost converter, as shown in Figure 14. When configured as SEPIC or flyback converter, a different
design approach is required.
Figure 14. Boost Converter Configuration
DETERMINING THE DUTY CYCLE
The TPS61175 has a maximum worst case duty cycle of 89% and a minimum on time of 60 ns. These two
constraints place limitations on the operating frequency that can be used for a given input to output conversion
ratio. 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 (DCM), where the inductor current ramps to zero at
the end of each cycle, the duty cycle varies with changes to the load much more than it does when running in
continuous conduction mode (CCM). In continuous conduction mode, where the inductor maintains a dc current,
the duty cycle is related primarily to the input and output voltages as computed below:
(1)
In discontinuous mode the duty cycle is a function of the load, input and output voltages, inductance and
switching frequency as computed below:
(2)
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.
(3)
For loads higher than the result of the equation above, the duty cycle is given by Equation 1 and for loads less
that the results of Equation 2, the duty cycle is given Equation 3. For Equation 1 through Equation 3, 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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