Datasheet

1
OUT FB
2
R
V V 1
R
æ ö
= +
ç ÷
è ø
g
ILIM
LIM
1.0V
R 10,000
I
= g
IN
OUT(max) LIM
OUT
V
I I
V
»
hg
g
TPS61252
www.ti.com SLVSAG3 –SEPTEMBER 2010
APPLICATION INFORMATION
EXAMPLE
During the following Application Information section one specific example will be used to define and work with the
different equations.
Parameter Symbol Value Unit
Input Voltage V
IN
3.6 V
Minimum Input Voltage V
IN(min)
2.6 V
Output Voltage V
OUT
5.0 V
Input Current Limit set by R
ILIM
I
LIM
1000 mA
Feedback Voltage V
FB
1.2 V
Switching Frequency f 3.25 MHz
Estimated Efficiency h 90 %
Inductor Value of Choice L1 1.0 µH
OUTPUT VOLTAGE SETTING
The output voltage can be calculated by Equation 5:
(5)
To minimize the current through the feedback divider network, R2 should be between 180k and 360k. The sum
of R1 and R2 should not exceed ~1MΩ, to keep the network robust against noise. Regarding the example, R1 is
768 kΩ and R2 is 240 kΩ.
An external feed forward capacitor C1 is required for optimum load transient response. The value of C1 should
be 100pF. The connection from FB pin to the resistor divider should be kept short and away from noise sources,
such as the inductor or the SW line.
INPUT CURRENT LIMIT
The input current limit is set by selecting the correct external resistor value. Equation 6 is a guideline for selecting
the correct resistor value:
(6)
For a current limit of 1A the resistor value will be 10 kΩ
MAXIMUM OUTPUT CURRENT
The maximum output current is set by R
ILIM
and the input to output voltage ratio and can be calculated by
Equation 7:
(7)
Following the example I
OUT(max)
will be 648 mA at 3.6 V input voltage and will decrease with lower input voltage
values due to the energy conservation.
INDUCTOR SELECTION
As for all switching power supplies two main passive components are required for storing the energy during
operation. This is done by an inductor and an output capacitor. The inductor must be connected between VIN pin
and SW pin to make sure that the TPS61252 device operates. To select the right inductor current rating the
programmed input current limit as well as the current ripple through the inductor is necessary. An estimation of
the maximum peak inductor current can be done using Equation 8.
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