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Programming the LBI/LBO Threshold Voltage
R1 + R2
V
BAT
V
LBI-threshold
–1
+ 390 kW
V
BAT
450 mV
–1
(3)
V
BAT +
V
LBI-threshold
R1
R2 )
1 + 500 mV
680 kW
390 kW )
1
(4)
Inductor Selection
I
L +
I
OUT
V
OUT
V
BAT
0.8
(5)
TPS61100, TPS61103
TPS61106, TPS61107
SLVS411B – JUNE 2002 – REVISED APRIL 2004
APPLICATION INFORMATION (continued)
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 400 mV, 450 mV or 500 mV. The recommended value for R2is therefore
in the range of 500 k
. From that, the value of resistor R
1, depending on the desired minimum battery voltage
VBAT, can be calculated using Equation 3.
For example, if the low-battery detection circuit should flag an error condition for the 450 mV threshold on the
LBO outputs at a battery voltage of 1.23 V, a 680-k
resistor should be chosen for R1. The resulting battery
voltages of the other thresholds can be calculated using Equation 4:
The result for the 500-mV threshold in our example is 1.37 V and for the 400-mV threshold 1.1 V. This results in
the following truth table for the battery supervisor outputs:
VBAT [V]
LBO1
LBO2
0-1.1
0
1.1-1.23
1
0
1.23-1.37
0
1
1.37-VBAT max
1
If the application requires only a simple LBI/LBO function both LBO outputs can be connected together. The LBI
threshold then is 500 mV.
The outputs of the low battery supervisor are simple open-drain outputs that go 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 boost converter. If not used, the LBO pin can be left floating or tied to GND.
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 TPS6110x's switch is 1200 mA at an
output voltage of 3.3 V. The highest peak current through the inductor and the switch depends on the output
load, the input (VBAT), and the output voltage (VOUT). Estimation of the maximum average inductor current can be
done using Equation 5:
For example, for an output current of 100 mA at 3.3 V, at least 515 mA of current flows through the inductor at a
minimum input voltage of 0.8 V.
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