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
TPS61251
www.ti.com SLVSAF7 –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)
2.9 V
Output Voltage V
OUT
5.5 V
Input Current Limit set by R
ILIM
I
LIM
500 mA
Feedback Voltage V
FB
1.2 V
Switching Frequency f 3.5 MHz
Estimated Efficiency h 90 %
Inductor Value of Choice L1 1.0 µH
OUTPUT VOLTAGE SETTING
The output voltage can be calculated by Equation 4:
(4)
To minimize the current through the feedback divider network and therefore increase efficiency during snooze
mode operation, R2 should be >240k. To keep the network robust against noise the resistor divider can also be
in the lower 100k values. Regarding the example, R1 is 1000 kΩ and R2 is 280 kΩ.
An external feed forward capacitor C1 is required for optimum load transient response. The value of C1 should
be 1000pF. 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.
AVERAGE INPUT CURRENT LIMIT
The average input current is set by selecting the correct external resistor value correlating to the required current
limit. Equation 5 is a guideline for selecting the correct resistor value:
(5)
For a current limit of 500 mA the resistor value will be 20 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 6:
(6)
Following the example I
OUT(max)
will be 295 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 TPS61251 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. Estimation of the
maximum peak inductor current can be done using Equation 7.
Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): TPS61251