Obviously, long battery life is a significant design goal for portable devices. Much effort is put into maximizing the efficiencies of the DC-DC converters in these products since that clearly extends battery life. However, another feature of the DC-DC converters that should be examined is the startup circuitry.
Power applied to a DC-DC converter causes a large peak inrush current due to the application of the high dv/dt to the input filter capacitance. These filter capacitors initially act like a short circuit, allowing an immediate inrush current with a fast rise time. The peak inrush current can be significantly greater than the steady-state current. The peak current and current ramp must be controlled to prevent potential damage to the circuit.
Battery-powered devices pose unique challenges in managing inrush current. While batteries perform reliably and predictably when new or freshly charged, a battery that is at a fraction of full capacity will be unable to support high start-up or inrush currents. As battery capacity is used up, its internal resistance increases, causing increased voltage drop under load. Under start-up conditions the increased voltage drop that results can trigger the power supply under-voltage lockout mechanism.
During shutdown, the battery voltage recovers, allowing the power supply to restart, creating the high inrush currents that then re-trigger another under-voltage event. The resulting oscillation between ON and OFF states is unsatisfactory and can force the user to prematurely replace or recharge the battery.
Battery life is a key buying decision for any portable product and any mechanism that can potentially extend battery life can add value to the end product and be touted in terms of end product performance.
Implementing a soft-start routine eliminates the problems caused by inrush current, as it allows the current to build up over a controlled period of time to the required value. There are two major elements to the operation of a soft-start routine.
- It prevents the output voltage from overshooting, or severely reduces the scale of the overshoot, by ensuring that the output voltage does not rise too fast.
- It reduces the large voltage drop experienced when a partially discharged battery releases a large inrush current.
A soft-start routine reduces the size of the inrush current, thus also reducing the voltage drop at start-up and enabling the system to maintain voltage above the threshold for triggering the system’s under-voltage lockout (IVLO) mechanism.
Some linear regulators require significant inrush current at start-up to charge their output capacitors and to provide current to their loads. With no inrush-control or soft start circuitry, the inrush current is clamped to the regulator’s current limit. If the input power supply is current-limited, the voltage at the regulator’s input could droop, potentially falling below the regulatorís UVLO limit. For most low dropout regulators, soft-start and inrush current protection can be implemented simply by gate control.
For DC-DC converters, the approach depends on whether the device is a buck or boost regulator. Most buck converters use a fairly simple start-up scheme. If the input voltage is high enough to be able to supply all blocks in the chip even in the worst case (that is, when the input voltage is at its minimum), then the situation is simple, because the current can be limited by use of a maximum current detector. The limit can be released after a fixed time or when a set output voltage is reached. This is an attractive option because the chip remains in its normal operating mode at all times.
Buck converters may have an internal soft-start function that ramps the output voltage in a controlled manner upon startup thus limiting the inrush current. This prevents input voltage from the battery from dropping when it is connected to the input of the converter. After the device is enabled, the internal circuit begins the power-up cycle.
Certain boost converters, however, do contain a start-up block. When operating in boost mode, the input voltage is below the required voltage, and the converter’s output operates as the power supply. In this case, implementing soft-start is more complicated, because the routine must limit the current throughout the whole start-up process until the required output voltage is reached. This means that the soft-start mechanism must operate across both the battery power supply and the converter output.
Of course, standard linear chips are designed to be used in a wide variety of applications alongside a variety of associated components, further complicating the implementation of soft-start. This means that the time taken to reach the required output is different from application to application. So applying a universal soft-start solution across multiple products and applications would appear to be extremely challenging.
Austriamicrosystems’ AS1344 boost converter reduces the complexity of implementing soft-start across a platform of products has been considerably reduced. The AS1344 not only addresses start-up in almost all applications, but it also allows selection of the maximum current during start-up. This makes it possible to adjust the time that is needed to reach the desired output voltage.
Fig. 1 is a block diagram of the AS1344. Both PMOS switches are OFF during shut-down (the output is disconnected from the input to stop current flowing during shut-down), and both are ON during normal operating mode.
During start-up, however, only one PMOS switch is ON (the one connected to VIN). So the maximum current that will flow during start-up can be adjusted by changing the value of the external resistor (Rv) that is connected between the battery and VIN (Table 1). This resistor will permanently limit the current that flows into VIN.
After the DC-DC converter has reached the required output level, and after a small delay, the switch from VCC to SWOUT will be ON — in other words, the device will be in normal operating mode. This in turn allows a larger current to flow, which supports a larger load current, and produces a more efficient system.
These effects are presented in Fig. 2 and Fig. 3. In Fig. 2, with an input voltage of 1.8 V, the inrush current can be limited to around 200 mA with an external resistor of 3 Ω. Fig. 3 demonstrates that for an input voltage of 2.4 V, the maximum inrush current is around 400 mA, with the same value of resistor.
Table 1 shows the different values for maximum inrush current, and also start-up times, for an input voltage of 2.4 V, and for different values of the external resistor (0 Ω to 3 Ω). This table shows clearly that, with no external resistor (Rv=0 Ω), the system starts fast and experiences a large inrush current (Ipeak). As the value of the resistor increase, the inrush current is less and the start-up time is longer.
| Table 1. Timing for Softstart @ Vcc = 2.4 V | ||||||
| Rv (Ω) | 0 | 1.0 | 1.5 | 2.0 | 2.4 | 3.0 |
| Ipeak (A) | 1.75 | 0.81 | 0.66 | 0.56 | 0.50 | 0.42 |
| Time (ms) | 25 | 76 | 100 | 130 | 156 | 204 |
Through selecting a single external resistor value, the AS1344 gives the designer a simple means to control inrush current. By using one boost converter in multiple circuit designs, the OEM can potentially reduce procurement costs and simplify inventory issues.
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