
www.ti.com
I
WLED +
K
ISET
1.229
R
ISET
)
1.229 * VDC
R
1 )
10K
for PWM signal input
(10)
Filter
ISET
PWM Signal
0.1 mF
10 kW
R1
RISET
ISET
DC Voltage
R1
RISET
INDUCTOR SELECTION
I
L_DC +
V
iout
Iout
V
in
h
(11)
SLVS624B – JANUARY 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
OR
where KISET = current multiplier between the ISET pin current and the IFB pin current.
VDC= voltage of the DC voltage source or the DC value of the PWM signal source.
Figure 19. Analog Dimming Uses an External Voltage Source to Control the Output Current
Because the selection of the inductor affects the power supply's steady state operation (e.g., efficiency and
output ripple), transient behavior and loop stability, the inductor is the most important component in power
regulator design. There are three specifications most important to the performance of the inductor, inductor
value, DC resistance and saturation current. Considering inductor value alone is not enough.
The inductor’s inductance value determines the inductor ripple current. It is generally recommended setting the
peak to peak ripple current given by
Equation 3 to 30–40% of the dc current. It is a good compromise of power
losses and inductor size. For this reason, 10
H inductors are recommended for TPS61140/1. Inductor DC
current can be calculated as
Use the maximum load current and minimum Vin for calculation.
The internal loop compensation for PWM control is optimized for the external component values, including
typical tolerances, shown in the typical application circuit. Inductor values can have
±20% tolerance with no
current bias. When the inductor current approaches saturation level, its inductance can decrease 20 to 35%
from the 0A value depending on how the inductor vendor defines saturation. Using an inductor with a smaller
inductance value forces discontinuous PWM operation in which the inductor current ramps down to zero before
the end of each switching cycle. It reduces the boost converter’s maximum output current, and causes large
input voltage ripple. An inductor with larger inductance will reduce the gain and phase margin of the feedback
loop, possibly resulting in instability.
Inductor selection is also important for PFM operation. As seen in I(out_max) calculation, the maximum output
current in PFM mode goes up with the inductor’s inductance value. A smaller value inductor, such as 4.7
H,
reduces the available output current, while a larger inductor raises the risk of instability by entering continuous
operation.
Regulator efficiency is dependent on the resistance of its high current path and switching losses associated with
the PWM switch and power diode. Although the TPS61140/1 has optimized the internal switches, the overall
efficiency still relies on inductor’s DC resistance (DCR); Lower DCR improves efficiency. However, there is a
trade off between DCR and inductor size, furthermore, shielded inductors typically have higher DCR than
unshielded ones. DCR in range of 150 m
to 350 m is suitable for applications requiring both on mode. DCR
is the range of 250 m
to 450 m is a good choice for single output application.
Table 2 and
Table 3 list
recommended inductor models.
16