Datasheet
L
L Lmax outmax
Vout
1
ΔI
Vin
ΔI = Vout I = I +
L 2f
-
´
´
c
1
= with L = 1.5 H, Cout = 10 F
2π L Cout
f m m
´
TPS65708
SLVSAE1A –OCTOBER 2010– REVISED FEBRUARY 2011
www.ti.com
LED DRIVER
TPS65708 contains a LED driver for a current of up to 30mA. ISINK is an open drain current sink that regulates a
current in an LED. The anode of the LED needs to be tied to a positive supply voltage; for example, V
CC
or the
output voltage of one of the dcdc converters in TPS65708, depending on the forward voltage of the LED. The
cathode of the LED is connected to ISINK which sets a constant current to GND. ISINK is regulated internally
based on the default current set internally. In addition, the LED current can be PWM dimmed by a signal applied
to pin PWM. If pin PWM is pulled LOW, the LED driver is disabled and its output ISINK is high resistive. If PWM
is HIGH, the current sink regulates to the current defined by EEPROM (TI factory set in two ranges. 7.5mA to
15mA with 0.5mA resolution and 15mA to 30mA in 1mA resolution). The maximum PWM frequency is 50kHz
with a duty cycle range of 5% to 100%. TPS65708 is set to 7.5mA as a default. Contact TI about different default
settings.
APPLICATION INFORMATION
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Inductor Selection
The converter operates typically with a 1.5µH or 2.2µH output inductor. The selected inductor has to be rated for
its dc resistance and saturation current. The DC resistance of the inductance will influence directly the efficiency
of the converter. Therefore an inductor with lowest dc resistance should be selected for highest efficiency.
Equation 1 calculates the maximum inductor current under static load conditions. The saturation current of the
inductor should be rated higher than the maximum inductor current as calculated with Equation 1. This is
recommended because during heavy load transient the inductor current will rise above the calculated value.
(1)
With:
f = Switching Frequency (2.25MHz typical)
L = Inductor Value
ΔI
L
= Peak-to-Peak inductor ripple current
I
Lmax
= Maximum Inductor current
The highest inductor current will occur at maximum Vin.
Open core inductors have a soft saturation characteristic and they can usually handle higher inductor currents
versus a comparable shielded inductor.
A more conservative approach is to select the inductor current rating just for the maximum switch current of the
corresponding converter. It must be considered, that the core material from inductor to inductor differs and will
have an impact on the efficiency especially at high switching frequencies.
Note that the step down converter has internal loop compensation. The internal loop compensation is designed
to work with an output filter corner frequency calculated as follows:
(2)
This leads to the fact the selection of external L-C filter has to be coped with the above equation. As a general
rule the product of L x C
OUT
should be constant while selecting smaller inductor or increasing output capacitor
value.
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