I
L
+
I
OUT
V
OUT
V
BAT
0.8
R1
+
R2
ǒ
V
BAT
V
LBI
*
threshold
*
1
Ǔ
+
390 k
W
ǒ
V
BAT
500 mV
*
1
Ǔ
C
parR3
10 pF
200 k
R4
–1
=
(
)
Ω
×
R3
R4
V
O
V
FB
1
200 k
V
O
500 mV
1
=
(
)
×
(
)
=
×
–
–
Ω
TPS61090, TPS61091, TPS61092
SLVS484C – JUNE 2003 – REVISED DECEMBER 2014
www.ti.com
10.2.1.2 Detailed Design Procedure
The TPS6109x DC-DC converters are intended for systems powered by a dual or triple cell NiCd or NiMH battery
with a typical terminal voltage between 1.8 V and 5.5 V. They can also be used in systems powered by one-cell
Li-Ion with a typical stack voltage between 2.5 V and 4.2 V. Additionally, two or three primary and secondary
alkaline battery cells can be the power source in systems where the TPS6109x is used.
10.2.1.2.1
Programming the Output Voltage
The output voltage of the TPS61090 DC-DC converter section can be adjusted with an external resistor divider.
The typical value of the voltage on the FB pin is 500 mV. The maximum allowed value for the output voltage is
5.5 V. The current through the resistive divider should be about 100 times greater than the current into the FB
pin. The typical current into the FB pin is 0.01 µA, and the voltage across R4 is typically 500 mV. Based on those
two values, the recommended value for R4 should be lower than 500 k
Ω
, in order to set the divider current at 1
µA or higher. Because of internal compensation circuitry the value for this resistor should be in the range of 200
k
Ω
. From that, the value of resistor R3, depending on the needed output voltage (V
O
), can be calculated using
Equation 1
:
(1)
If as an example, an output voltage of 5.0 V is needed, a 1.8-M
Ω
resistor should be chosen for R3. If for any
reason the value for R4 is chosen significantly lower than 200 k
Ω
additional capacitance in parallel to R3 is
recommended. The required capacitance value can be easily calculated using
Equation 2
(2)
10.2.1.2.2
Programming the LBI/LBO Threshold Voltage
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 µA, 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 R2is therefore in the range of 500
k
Ω
. From that, the value of resistor R1, depending on the desired minimum battery voltage V
BAT,
can be
calculated using
Equation 3
.
(3)
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
Ω
. 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.
10.2.1.2.3
Inductor Selection
A boost converter normally requires two main passive components for storing energy during the conversion. A
boost inductor and a storage capacitor at the output are required. To select the boost inductor, 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 current limit threshold of the TPS6109x's switch is 2500 mA at an
output voltage of 5 V. The highest peak current through the inductor and the switch depends on the output load,
the input (V
BAT
), and the output voltage (V
OUT
). Estimation of the maximum average inductor current can be done
using
Equation 4
:
(4)
For example, for an output current of 500 mA at 5 V, at least 1750 mA of average current flows through the
inductor at a minimum input voltage of 1.8 V.
14
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