Texas Instruments | Prevent Battery Overdischarge with Precise Threshold Enable Pin | Application notes | Texas Instruments Prevent Battery Overdischarge with Precise Threshold Enable Pin Application notes

Texas Instruments Prevent Battery Overdischarge with Precise Threshold Enable Pin Application notes
Prevent Battery Overdischarge
with Precise Threshold Enable Pin
Using a buck-boost converter is a convenient way to
obtain a fixed supply voltage within the wide voltage
range of typical batteries used in low-power devices
such as smart meters, wearables or those in the
Internet of Things. In order to extract as much energy
as possible from the battery, it helps if the converter
can operate at low input voltages.
Table 2. EN pin threshold voltages for TPS63802
Parameter
In non-inverting buck-boost devices such as the
TPS63802, the minimum operating input voltage is as
low as 1.3 V once the device is started. With most
rechargeable batteries, however, such a deep
discharge can cause irreversible reduction in capacity
or even catastrophic failure and damage to the rest of
the system. In such situations, it becomes essential to
cut off the battery from the rest of the system at a
desired voltage value.
Min
Typ
Max
Unit
VIT+(EN)
Rising threshold
voltage for EN pin
1.07
1.1
1.13
V
VIT-(EN)
Falling threshold
voltage for EN pin
0.97
1
1.03
V
Having precise voltage thresholds, it is possible to
easily set a user-defined minimum supply voltage. This
is done using a voltage divider connected to VIN pin,
EN pin and ground, as shown in Figure 1.
Most DC/DC converters have an input pin to enable or
disable the device. However, the threshold voltage at
this pin can have a huge tolerance, making it difficult
to accurately set the desired cutoff voltage. As an
example, according to the datasheet, the threshold
voltage of the enable (EN) pin for the TPS63020 is
within the range listed in Table 1. In this case, the EN
pin voltage threshold can be anywhere between 0.4 V
and 1.2 V.
Figure 1. Setting the input cutoff voltage with a
voltage divider
Table 1. EN pin voltage levels for TPS63020
Parameter
VIL
EN input low voltage
VIH
EN Input high voltage
Min
1.2
Typ
Max
Unit
0.4
V
Equation 1 calculates the falling threshold supply
voltage where the converter is turned off:
V
This threshold range is fine for on/off control using
logic-level signals, but not for setting a precise cutoff
voltage derived from the input voltage. To achieve
higher accuracy, it is possible to add a comparator and
a voltage reference, or a voltage supervisor circuit, but
this increases complexity and cost. Instead, it would
be more useful if the EN pin has more precisely
defined threshold voltages.
The concept of achieving a precise threshold voltage
for buck converters was introduced in the application
report Achieving a Clean Startup by Using a DC/DC
Converter with a Precise Enable-pin Threshold. The
new TPS63802 non-inverting buck-boost converter
also has a very precise threshold voltage for the EN
pin, with approximately 3% tolerance and 100 mV
hysteresis, as listed in Table 2.
SLVAE79 – April 2019
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R1 ·
§
V,7± = V,7± (1 ¨ 1 +
R2 ¸¹
©
(1)
Equation 2 calculates the rising threshold supply
voltage where the converter is turned on:
R1 ·
§
VIT+ = VIT+(EN) ¨ 1 +
¸
R2
©
¹
(2)
The additional voltage divider will increase the current
consumption, therefore, aim for large resistances.
Considering that the EN input leakage current for
TPS63802 is 0.2 μA maximum, aim for at least 20 μA
of current in the voltage divider. The application report
Optimizing Resistor Dividers at a Comparator Input
has more details about how to optimize a resistor
divider at a comparator input.
Prevent Battery Overdischarge with Precise Threshold Enable Pin
Copyright © 2019, Texas Instruments Incorporated
1
References
www.ti.com
As an example, to set the cutoff input voltage to
VIT- = 2.5 V, we can first choose R1 + R2 = 125 kΩ to
have a 20 μA resistor divider current. Solving
Equation 1, we choose R1 = 75 kΩ and R2 = 49.9 kΩ
resistors with 1% tolerance. The turn-on input voltage
is now VIT+ = 2.75 V according to Equation 2.
The previous example shows how simple it is to
protect a battery from overdischarge by adding only
two resistors. The same solution is applicable not only
to buck-boost but also to other buck or boost devices
having the precise threshold voltage EN pin.
Figure 2 shows that the achieved cutoff and turn-on
input voltages are VIT- = 2.56 V and VIT+ = 2.8 V,
respectively. This is within the equivalent tolerance of
approximately 80 mV (3.1%) caused by the EN pin
threshold voltage and resistor tolerances, not taking
into account the oscilloscope accuracy. The
application report Method for Calculating Output
Voltage Tolerances in Adjustable Regulators has more
details on calculating equivalent voltage tolerances.
1
References
TPS63802 Datasheet
TPS63020 Datasheet
Achieving a Clean Startup by Using a DC/DC
Converter with a Precise Enable-pin Threshold
Optimizing Resistor Dividers at a Comparator
Input
Method for Calculating Output Voltage
Tolerances in Adjustable Regulators
Figure 2. Achieved cutoff and turn-on input voltage
thresholds
1.1
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2
Prevent Battery Overdischarge with Precise Threshold Enable Pin
Copyright © 2019, Texas Instruments Incorporated
SLVAE79 – April 2019
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