Datasheet

IN(min) IN(min)
L(max) LIM L LIM
OUT
V D V
I I I I with D 1
L f V
h
= + D = + = -
g g
g
( )
OUT OUT IN
MIN
OUT
I V V
C
f V V
-
=
D
g
g g
ESR OUT ESR
V I R= g
TPS61252
SLVSAG3 –SEPTEMBER 2010 www.ti.com
(8)
Regarding the above example the current ripple (ΔI
L
) will be 426 mA and therefore an inductor with a rated
current of about 1.5 A should be used.
The TPS61252 is designed to work with inductor values between 1.0 µH and 2.2 µH. For typical applications a
1.5 µH inductor is recommended. Regarding the conversion factor and the need of a sufficient output current the
rated current for the inductor drives into lower inductance values. Therefore the inductor value can be reduced
down to 1.0 µH without degrading the stability. Reduced inductance values increase the current ripple that needs
to be included in the peak current calculation for the inductor (Equation 8). Using standard boost converters the
current through the inductor is defined by the switch current limit of the converters switches and therefore bigger
inductors have to be chosen. TPS61252 allows you to reduce the current limit to the needs of the application
regardless the maximum switch current limit of the converter. Programming a lower current value allows the use
of smaller inductors without the danger to get into saturation.
OUTPUT CAPACITOR
For the output capacitor, it is recommended to use small ceramic capacitors placed as close as possible to the
VOUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which
cannot be placed close to the IC, a smaller ceramic capacitor in parallel to the large one is highly recommended.
This small capacitor should be placed as close as possible to the converters VOUT and GND pins. To get an
estimate of the recommended minimum output capacitance, Equation 9 can be used.
(9)
Where ΔV is the maximum allowed output ripple.
With a chosen ripple voltage of 10 mV, a minimum effective capacitance of 9.6 mF is needed regarding the
example. The total ripple will be larger due to the ESR of the output capacitor. This additional component of the
ripple can be calculated using Equation 10
(10)
To maintain control loop stability a capacitor with twice the value (I. e. 22 µF) of the calculated minimum
capacitance is required be used due to DC Bias effects. There are no additional requirements regarding
minimum ESR. Larger capacitors cause lower output voltage ripple as well as lower output voltage drop during
load transients but the total output capacitance value should not exceed values above 50 µF.
INPUT CAPACITOR
Multilayer ceramic capacitors are an excellent choice for input decoupling of the step-up converter as they have
extremely low ESR and are available in small form factors. The input capacitors should be located as close as
possible to the device. While a 10mF input capacitor is sufficient for most applications, larger values may be used
to reduce input current ripple without limitations. Also low ESR tantalum capacitors may be used.
NOTE
DC Bias effect: High capacitance ceramic capacitors have a DC Bias effect, which will
have a strong influence on the final effective capacitance. Therefore the right capacitor
value has to be chosen very carefully. Package size and voltage rating in combination with
dielectric material are responsible for differences between the rated capacitor value and
the effective capacitance. A 10 V rated 0805 capacitor with 10 µF can have a effective
capacitance of less than 5 µF at an output voltage of 5 V.
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