The TPS61098x is an ultra low power solution optimized for products powered by either a one-cell or two-cell alkaline, NiCd or NiMH, one-cell coin cell battery or one-cell Li-Ion or Li-polymer battery. To simplify system design and save PCB space, the TPS61098x integrates an LDO or load switch with a boost converter (different configurations for different versions) to provide two output rails in a compact package. The boost output V(MAIN) is designed as an always-on supply to power a main system, and the LDO or load switch output V(SUB) is designed to power peripheral devices and can be turned off.
The TPS61098x features two modes controlled by MODE pin: Active mode and Low Power mode. In Active mode, both outputs are enabled, and the transient response performance of the boost converter and LDO/load switch are enhanced, so it is able to respond load transient quickly. In Low Power mode, the LDO/load switch is disabled, so the peripherals can be disconnected to minimize the battery drain. Besides that, the boost consumes only 300 nA quiescent current in Low Power mode, so up to 88% efficiency at 10 µA load can be achieved to extend the battery run time. The TPS610982 is an exception. Its LDO is always on in both Active mode and Low Power mode. The main differences between the two modes of the TPS610982 are the quiescent current and performance. Refer to Operation Modes by MODE Pin for details.
The TPS61098x supports automatic pass-through function in both Active mode and Low Power mode. When VIN is detected higher than a pass-through threshold, which is around the target V(MAIN) voltage, the boost converter stops switching and passes the input voltage through inductor and internal rectifier switch to V(MAIN), so V(MAIN) follows VIN; when VIN is lower than the threshold, the boost works in boost mode and regulates V(MAIN) at the target value. The TPS61098x can support different V(MAIN) target values in Active mode and Low Power mode to meet various requirements. For example, for TPS61098, the set value of V(MAIN) is 4.3 V in Active mode but 2.2 V in Low Power mode.
The TPS61098x boost converter is controlled by a hysteretic current mode controller. This controller regulates the output voltage by keeping the inductor ripple current constant in the range of 100 mA and adjusting the offset of this inductor current depending on the output load. Since the input voltage, output voltage and inductor value all affect the rising and falling slopes of inductor ripple current, the switching frequency is not fixed and is decided by the operation condition. If the required average input current is lower than the average inductor current defined by this constant ripple, the inductor current goes discontinuous to keep the efficiency high under light load conditions. Figure 53 illustrates the hysteretic current operation. If the load is reduced further, the boost converter enters into Burst mode. In Burst mode, the boost converter ramps up the output voltage with several pulses and it stops operating once the output voltage exceeds a set threshold, and then it goes into a sleep status and consumes less quiescent current. It resumes switching when the output voltage is below the set threshold. It exits the Burst mode when the output current can no longer be supported in this mode. Refer to Figure 54 for Burst mode operation details.
To achieve high efficiency, the power stage is realized as a synchronous boost topology. The output voltage V(MAIN) is monitored via an internal feedback network which is connected to the voltage error amplifier. To regulate the output voltage, the voltage error amplifier compares this feedback voltage to the internal voltage reference and adjusts the required offset of the inductor current accordingly.
The TPS61098x supports automatic pass-through function for the boost converter. When the input voltage is detected higher than the pass-through threshold V(PSTH), which is around V(MAIN) set value, the boost converter enters into pass-through operation mode. In this mode, the boost converter stops switching, the rectifier is constantly turned on and the low side switch is turned off. The input voltage passes through external inductor and the internal rectifier to the output. The output voltage in this mode depends on the resistance between the input and the output, calculated as Equation 1:
where RL is the DCR of external inductor, and RDS(on)_HS is the resistance of internal rectifier.
When the input voltage is lower than V(PSTH), the boost converter resumes switching to regulate the output at target value.
The TPS61098x can support automatic pass-through function in both Active mode and Low Power mode.
The TPS61098x uses a PMOS as a pass element of its integrated LDO / load switch. The input of the PMOS is connected to the output of the boost converter. When the MODE pin is pulled logic high, the PMOS is enabled to output a voltage on VSUB pin.
For load switch version, the PMOS pass element is fully turned on when enabled, no matter the boost converter works in boost operation mode or pass-through operation mode. So the output voltage at VSUB pin is decided by the output voltage at VMAIN pin and the current passing through the PMOS as Equation 2:
where I(SUB) is the load of VSUB rail and the RLS is the resistance of the PMOS when it is fully turned on.
For LDO version, the output voltage V(SUB) is regulated at the set value when the voltage difference between its input and output is higher than the dropout voltage V(Dropout), no matter the boost converter works in boost operation mode or pass-through operation mode. The V(SUB) is monitored via an internal feedback network which is connected to the voltage error amplifier. To regulate V(SUB), the voltage error amplifier compares the feedback voltage to the internal voltage reference and adjusts the gate voltage of the PMOS accordingly. When the voltage drop across the PMOS is lower than the dropout voltage, the PMOS will be fully turned on and the output voltage at V(SUB) is decided by Equation 2.
When the MODE pin is pulled low, the LDO or load switch is turned off to disconnect the load at VSUB pin. For some versions, active discharge function at VSUB pin is offered, which can discharge the V(SUB) to ground after MODE pin is pulled low, to avoid any bias condition to downstream devices. For versions without the active discharge function, the VSUB pin is floating after MODE pin is pulled low, and its voltage normally drops down slowly due to leakage. Refer to the Device Comparison Table for version differences.
When MODE pin is toggled from low to high, soft-start is implemented for the LDO versions to avoid inrush current during LDO startup. The start up time of LDO is typically 1 ms. For load switch versions, the load switch is turned on faster, so the output capacitor at VSUB pin is suggested 10X smaller than the output capacitor at VMAIN pin to avoid obvious voltage drop of V(MAIN) during load switch turning on process.
The boost converter of the TPS61098x is designed always-on, so there is no enable or disable control of it. The boost converter starts operation once input voltage is applied. If the input voltage is not high enough, a low voltage startup oscillator operates the switches first. During this phase, the switching frequency is controlled by the oscillator, and the maximum switch current is limited. Once the converter has built up the output voltage V(MAIN) to approximately 1.8 V, the device switches to the normal hysteretic current mode operation and the VMAIN rail starts to supply the internal control circuit. If the input voltage is too low or the load during startup is too heavy, which makes the converter unable to build up 1.8 V at V(MAIN) rail, the boost converter can't start up successfully. It will keep in this status until the input voltage is increased or removed.
The TPS61098x is able to startup with 0.7 V input voltage with ≥ 3 kΩ load. The startup time depends on input voltage and load conditions. After the V(MAIN) reaches 1.8 V to start the normal hysteretic current mode operation, an internal ramp-up reference controls soft-start time of the boost converter until V(MAIN) reaches its set value.
The TPS61098x does not support undervoltage lockout function. When the input voltage drops to a low voltage and can't provide the required energy to the boost converter, the V(MAIN) drops. When and to what extent V(MAIN) drops are dependent on the input and load conditions. When the boost converter is unable to maintain 1.8 V at VMAIN rail to supply the internal circuit, the TPS61098x powers down and enters into startup process again.
The boost converter of the TPS61098x supports a cycle-by-cycle current limit function in boost mode operation. If the peak inductor current reaches the internal switch current limit threshold, the main switch is turned off to stop a further increase of the input current. In this case the output voltage will decrease since the device cannot provide sufficient power to maintain the set output voltage. If the output voltage drops below the input voltage, the backgate diode of the rectifying switch gets forward biased and current starts to flow through it. Because this diode cannot be turned off, the load current is only limited by the remaining DC resistance. After the overload condition is removed, the converter automatically resumes normal operation.
The overload protection is not active in pass-through mode operation, in which the load current is only limited by the DC resistance.
The integrated LDO / load switch also supports over load protection. When the load current of VSUB rail reaches the ILIM_SUB, the V(SUB) output current will be regulated at this limit value and will not increase further. In this case the V(SUB) voltage will decrease since the device cannot provide sufficient power to the load.
The TPS61098x has a built-in temperature sensor which monitors the internal junction temperature in boost mode operation. If the junction temperature exceeds the threshold (150°C typical), the device stops operating. As soon as the junction temperature has decreased below the programmed threshold with a hysteresis, it starts operating again. There is a built-in hysteresis (25°C typical) to avoid unstable operation at the overtemperature threshold. The over temperature protection is not active in pass-through mode operation.
The TPS61098x features two operation modes controlled by MODE pin: the Active mode and Low Power mode. It can provide quick transient response in Active mode and ultra-low quiescent current in Low Power mode. So a low power system can easily use the TPS61098x to get high performance in its active mode and meantime minimize its power consumption to extend the battery run time in its sleep mode.
The MODE pin is usually controlled by an I/O pin of a controller, and should not be left floating.
The TPS61098x works in Active mode when MODE pin is logic high. In Active mode, both of the boost converter and the integrated LDO/load switch are enabled, and the TPS61098x can provide dual outputs simultaneously. The transient response performance of the boost converter is enhanced in Active mode, and the device consumes around 15 µA quiescent current. It is able to respond load transient quickly.
When MODE pin is toggled from low to high, soft-start is implemented for the LDO versions to avoid inrush current during startup. For load switch versions, the load switch is turned on faster, so the output capacitor at VSUB pin is suggested 10X smaller than the output capacitor at VMAIN pin to avoid obvious voltage drop of V(MAIN) during turning on process.
The TPS61098x works in Low Power mode when MODE pin is logic low. In Low Power mode, the LDO/load switch is turned off, so the peripherals can be disconnected to minimize the battery drain. The VSUB pin either outputs high impedance or is pulled to ground by internal active discharge circuit, depending on different versions. The boost converter consumes only 300 nA quiescent current typically, and can achieve up to 88% efficiency at 10 µA load.
The Low Power mode is designed to keep the load device powered with minimum power consumption. For example, it can be used to keep powering the main system, like an MCU, in a system's sleep mode even under < 0.7 V input voltage condition.
The TPS610982 is an exception. Its LDO is always on in both Active mode and Low Power mode with higher quiescent current consumption than other versions. The TPS610982 can be used to replace discrete boost and LDO solutions where the LDO output is always on, and its two modes provide users two options of different quiescent current consumption and performance. Refer to the Device Comparison Table, Specifications and Typical Characteristics for details.
The boost converter of TPS61098x enters into Burst Mode operation under light load condition. Refer to Boost Controller Operation for details.
The boost converter of TPS61098x automatically enters into pass-through mode operation when input voltage is higher than the target output voltage. Refer to Pass-Through Operation for details.