TPS613222A See References no. 1 is a very compact boost converter with only 3 active pins. Only two external components (one inductor and one output capacitor) are needed when designing a 5-V fixed output power rail. The quiescent is 6.5uA typically and the total power loss is very small even though the boost converter is always in active state. This part reduces the system cost significantly and makes the system design quite simple. But it has no short circuit protection.

TPS613222A is a hysteretic current control boost converter. There is a current comparator inside turns on and off the power MOSFETs. During the off-phase, the high-side MOSFET is on and the low side MOSFET is off. As the output voltage is higher than the input voltage, the inductor current ramps down. When the inductor current triggers the target value set by the output of the error amplifier, the high-side MOSFET is turned off and the low-side MOSFET is turned on (dead time control is integrated). Then the inductor current starts ramping up. When the inductor current ramps up to the target value set by the hysteretic current comparator, the low-side MOSFET is turned off and high-side MOSFET is turned on again. The TPS613222A operates in this behavior back and forth. If the output load goes beyond the TPS613222A’s capability, the peak switch current will be limited to 1.8A typically and the output voltage starts to drop. When the output voltage drops to the input voltage level, the inductor current is no longer controllable because even in off-phase, the inductor current ramps up not down and the boost converter may be damaged.

Figure 2-1 Typical Application

The technique used in this application note to achieve the short circuit protection adds two general purpose active switches and some passive components as shown in Figure 2-1. The resistor R_sense together with the R_ds,on of the Q2 monitors the output current of the sub-system. The V_o- voltage is fed to the base of the NPN signal switch Q1. The Q1 is general purpose bipolar switch, for example, MMBT3904. When the V_o- ramps to the V_be threshold of the Q1, which is 0.6 V typically, the collector current of Q1 increases significantly and pull the V_gs voltage of Q2 lower than the gate threshold voltage V_th. In this way, the current path is cut off immediately and both the power system and the sub-system is protected.

The short circuit current threshold I_LIMIT is set by the Equation 1:

Equation 1.

where

• R_sense is the resistance of the sense resistor
• R_ds,on is the on-resistance of Q2
• V_be is the open threshold of the bipolar switch Q1

When the short circuit condition is removed, the V_o- voltage is pulled to GND slowly by the base-to-emitter current I_be of the Q1. The recovery time is related to the output capacitor C3, and the leakage current set by Equation 2:

Equation 2.

where

• T_recovery is the recovery time when an short-circuit accident is removed and the output voltage recovers to 5 V. (V_o- ramps down to 0 V).
• I_leak is the leakage current into the base of the Q1 switch, in other words I_be.
• C₃ is the output capacitance.
• V_o- is the voltage at the negative input side of the sub-system.

For example, the R₂ is 1000 Ω, and the output capacitor is 4.7 µF. The I_leak is 4.4 mA, maximum. A simple 2.2-mA average leakage current can be used in the calculation. Then the recovery time is longer than 10 ms.

To accelerate the response time when SCP happens (or recovery time from SCP to normal mode), the resistor of R2 with smaller resistance can be used. The leakage current is inversely proportional to the R2 resistance according to the equation below. The recommended value for this resistor is between 100 Ω to 100 KΩ. It is a tradeoff between the recovery time/response time and the leakage current.

The following figures show the response time when a fast short-circuit accident happens. The R_sense is set to 0.5 Ω to get a 1.1-A target short-circuit current threshold. The actual trigger point is 1.1 A when the R₂ is 100 Ω while the trigger point shifts to higher than 2 A when the R₂ is higher than 10 kΩ.

Figure 2-2 R₂ = 100 Ω

Figure 2-4 R₂ = 10000 Ω

Figure 2-3 R₂ = 1000 Ω

Figure 2-5 R₂ = 100000 Ω

The following explains how to select external components in such applications is another important topic. Especially the two external active switches.

Pay attentions to the following requirements:

• The maximum V_ds voltage of the switch must be higher than the TPS613222A output voltage.
• If no R_sense, the R_ds,on should also be balanced with the total solution efficiency, 3% is good tradeoff under the maximum load for the whole system power consumption.
• The maximum V_gs specification should exceed the maximum output voltage of TPS613222A under the worst condition.
• The parasitic capacitance is also problem for Q2. When a short circuit accident happens, the high slew rate di/dt introduces high spike at the drain-source of Q2. The maximum spike should not exceed the maximum V limitation.