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LDO
R5 + R4
V
O
V
FB
–1
+ 180 kW
V
O
500 mV
–1
(3)
Programming the LBI/LBO Threshold Voltage
R1 + R2
V
BAT
V
LBI*threshold
* 1 + 390 kW
V
BAT
500 mV *
1
(4)
Inductor Selection
I
L1*A +
I
L1*B +
I
OUT
V
OUT
V
BAT
0.8
(5)
L1 * A + L1 * B +
V
BAT
V
OUT
DIL
V
OUT)
V
BAT
(6)
SLVS431B – JUNE 2002 – REVISED JANUARY 2008
Programming the output voltage of the LDO follows almost the same rules as at the dc/dc converter section. The
maximum recommended output voltage of the LDO is 5.5 V. Since reference and internal feedback circuitry are
similar, as they are at the dc/dc converter section, R4 also should be in the 200-k
range. The calculation of the
value of R5 can be done using the following
Equation 3:
If as an example, an output voltage of 1.5 V is needed, a 360 k
-resistor should be chosen for R5.
The current through the resistive divider should be about 100 times greater than the current into the LBI pin. The
typical current into the LBI pin is 0.01
A, and the voltage across R2 is equal to the LBI voltage threshold that is
generated on-chip, which has a value of 500 mV. The recommended value for R2 is therefore in the range of 500
k
. From that, the value of resistor R1, depending on the desired minimum battery voltage VBAT, can be
The output of the low battery supervisor is a simple open-drain output that goes active low if the dedicated
battery voltage drops below the programmed threshold voltage on LBI. The output requires a pullup resistor with
a recommended value of 1 M
. The maximum voltage which is used to pull up the LBO outputs should not
exceed the output voltage of the dc/dc converter. If not used, the LBO pin can be left floating or tied to GND.
A SEPIC converter normally requires three main passive components for storing energy during the conversion.
Two inductors, a flying capacitor, and a storage capacitor at the output are required. To select the two inductors,
it is recommended to keep the possible peak inductor current below the current limit threshold of the power
switch in the chosen configuration. For example, the typical current limit threshold of the TPS6113x's switch is
1300 mA at an output voltage of 3.3 V. The highest peak current through the switch is the sum of the two
inductor currents and depends on the output load, the input (VBAT), and the output voltage (VOUT). Estimation of
the maximum average inductor current of each inductor can be done using
Equation 5:For example, for an output current of 300 mA at 3.3 V, at least 680 mA of average current flows through each of
the the inductors at a minimum input voltage of 1.8 V.
The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is
advisable to work with a ripple of around ±20% of the average inductor current. A smaller ripple reduces the
magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in the same way,
regulation time at load changes rises. In addition, a larger inductor increases the total system costs. With those
parameters, it is possible to calculate the value for the inductor by using
Equation 6:Parameter f is the switching frequency and
Δ IL is the ripple current in the inductor, i.e., 40% Δ IL. In this example,
the desired inductance is in the range of 20
H. With this calculated value and the calculated currents, it is
possible to choose a suitable inductor. In typical applications a 22
H inductance is recommended. The device
Copyright 2002–2008, Texas Instruments Incorporated
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