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
PULSE STANDBY on
RIPPLE(mV)
OUT
I I t
V =
C
- g
( )
ESR PULSE STANDBY
V = I I ESRD - g
TPS61251
SLVSAF7 –SEPTEMBER 2010 www.ti.com
(7)
Regarding the example from above the current ripple (ΔI
L
) will be 290 mA and therefore an inductor with a rated
current of about 800 mA should be used.
The TPS61251 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 7). 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. TPS61251 allows the design engineer 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
The second energy storing device is the output capacitor. When selecting output capacitors for large pulsed
loads, the magnitude and duration of the pulsing current, together with the ripple voltage specification, determine
the choice of the output capacitor. Both the ESR of the capacitor and the charge stored in the capacitor each
cycle contribute to the output voltage ripple. The ripple due to the charge is approximately what results from
Equation 8
(8)
where I
PULSE
and t
ON
are the peak current and on time during transmission burst and I
STANDBY
is the current in
standby mode. The above is a worst-case approximation assuming all the pulsing energy comes from the output
capacitor.
The ripple due to the capacitor ESR is defined by Equation 9
(9)
High capacitance values and low ESR can lead to instability in some internally compensated boost converters.
The internal loop compensation of the TPS61251 is optimized to be stable with output capacitor values greater
than 150mF with very low ESR.
Since big bulk capacitors can not be placed very close to the IC it is required to put a small ceramic capacitor of
about 4.7 µF as close as possible to the output terminals. This will reduce parasitic effects that can influence the
functionality of the converter.
Table 2. List of Bulk Capacitors
VENDOR (alphabetical order) CAPACITANCE PART NUMBER
Kemet 470 µF, 6.3 V, 55 mΩ T520W477M006ATE055
Sanyo 470 µF, 6.3 V, 35 mΩ 6TPE470MAZU
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