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ZHCSCS7A September 2014 – September 2014 TPS61291

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- MPDS216E

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- QFND087M

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8 Applications and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

8.1 Application Information

The TPS61291 is a boost converter with pin selectable output voltages and an integrated bypass mode. In bypass operation, the device provides a direct path from the input to the system and allows a low power micro controller (MCU) to operate directly from a single 3V Li-MnO2 battery or dual alkaline battery cells. In bypass mode, the quiescent current consumption is typically only 15nA and supports low power modes of MCUs such as the MSP430. In boost mode operation, the device provides a regulated output voltage (e.g. 3.3V) to supply circuits which require a higher voltage than provided by the battery. See Figure 8.

The device also extends battery life in applications which can run partially directly from the battery, but need a boost conversion to maintain sufficient system voltage when the battery voltage drops due to discharge. In this case, the system runs off the battery in bypass mode operation until the battery voltage trips the minimum system operating voltage. Then the system turns on the boost converter, providing a sufficient output voltage down to the cut off voltage of the battery. See Figure 9 and Figure 26.

8.2 Typical Application

Figure 8. Typical Application Circuit with Regulated 3.3V VOUT / VBAT

Figure 9. Bypass Mode / Boost Mode Operation to Maintain Sufficient System Voltage

8.2.1 Design Requirements

The TPS61291 is a highly integrated boost converter. The output voltage is set internally via a VSEL pin without any additional components. For operation, only an input capacitor, output capacitor, and an inductor are required. Table 1 shows the components used for the application characteristic curves.

Table 1. Components for Application Characteristic Curves⁽¹⁾

Reference	Description	Value	Manufacturer
TPS61291	Low Iq Boost Converter with Bypass Operation		Texas Instruments
C_IN	Input capacitor	10µF	Murata	GRM219R61A106KE44D
C_OUT	Output capacitor	22µF	Murata	GRM21BR60J226ME39L
L	Inductor	3.3µH	Coilcraft	LPS3314 3R3

(1) See the Third-Party Products Disclaimer in the Device Support section.

8.2.2 Detailed Design Procedure

The external components have to fulfill the needs of the application but also the stability criteria of the device's control loop. The TPS61291 is optimized to work within a range of L and C combinations. The LC output filter inductance and capacitance must be considered together. The output capacitor sets the corner frequency of the converter while the inductor creates a Right-Half-Plane-Zero degrading the stability of the converter. Consequently with a larger inductor a bigger capacitor has to be used to guarantee a stable loop. Table 2 shows the output filter component selection.

Table 2. Recommended LC Output Filter Combinations

Output voltage [V]	Inductor value [µH]⁽²⁾	Output capacitor value [µF]⁽¹⁾
Output voltage [V]	Inductor value [µH]⁽²⁾	22	22 + 10	2 x 22
3.3 / 3.0	3.3	√⁽³⁾	√	√
3.3 / 3.0	4.7			√
2.5	2.2	√	√	√
2.5	3.3		√⁽³⁾	√

(1) Capacitance tolerance and bias voltage de-rating is anticipated. The effective capacitance can vary by 20% and -50%.

(2) Inductor tolerance and current de-rating is anticipated. The effective inductance can vary by 20% and -30%.

(3) This LC combination is the standard value and recommended for most applications.

8.2.2.1 Inductor Selection

The device is optimized to operate with a 3.3µH inductor value. Other inductor values can be used, per Table 2. The maximum inductor current can be approximated by the I_LMAX, from Equation 1. For proper operation, the inductor needs to be rated for a saturation current which is higher than the switch current limit of typically 1A. Table 3 lists inductors that have been tested with the TPS61291.

Equation 1.

Table 3. List of Inductors⁽¹⁾

INDUCTANCE	DIMENSIONS [mm³]	TYPE	SUPPLIER
3.3	3.3 x 3.3 x 1.3	LPS3314	Coilcraft
3.3	2.95 x 2.95 x 1.4	LPS3015	Coilcraft
3.3	3 x 2.5 x 1.5	VLF302515	TDK
3.3	2 x 2 x 1.2	MDMK2020T3R3M	Taiyo Yuden
3.3	2.5 x 2.0 x 1.2	DFE252012	Toko
3.3	3.0 x 3.0 x 1.5	74438335033	Würth

(1) See the Third-Party Products Disclaimer in the Device Support section.

8.2.2.2 Input and Output Capacitor Selection

For best output and input voltage filtering, low ESR X5R or X7R ceramic capacitors are recommended. The input capacitor minimizes input voltage ripple, suppresses input voltage spikes and provides a stable system rail for the device. At least a 10μF or larger input capacitor is recommended for operation. In applications in which the power source (e.g. certain battery chemistries) shows an internal resistance characteristic, a larger input capacitor might be used to buffer the supply voltage for the TPS61291. The recommended typical output capacitor value is 22 μF and can vary as outlined in the output filter selection Table 2.