Hot-swap controllers must contend with both steady-state and startup conditions. In steady-state operation, the MOSFET used as the controlled switch element must be designed to operate above the maximum current load of the internal FET and maintain junction temperature below the rated maximum junction temperature.
The steady-state power dissipation is basically the product of the square of the load current and the RDS(ON).
|PD = ILOAD2 × RDS(ON)||(Eq. 1)|
Dynamic requirements such as startup must be considered when designing a hot-swap IC with integrated FETs. This is an important consideration when driving capacitive loads that serve as the energy reservoirs for downstream point-of-loads (POLs). The MAX15090/MAX15090B use a technique that monitors the VIN - VOUT difference and uses a current foldback technique to limit the current during startup, which will be discussed in greater detail.
The devices integrate a hot-swap controller, 6mΩ power MOSFET and electronic circuit-breaker protection in a single package. These devices implement a foldback current limit during startup to control inrush current lowering di/dt and keep the MOSFET operating under safe operating area (SOA) conditions. This feature is very important at 12V when the load is highly capacitive. Figure 1 outlines the foldback feature during device startup.
Figure 1. Variable speed/bi-level response. This should read startup inrush current foldback characterisics.
As shown in Figure 1, the device limits the amount of current to the load based on the VIN - VOUT difference. When the VIN - VOUT difference is 2V or less, the current is limited to RCB/3333.3 × 0.5. If VOUT rises above 0.9 × VIN before the internal 50mS timer times out, the current limit reverts back to RCB/3333.3. To use this part for lower voltage applications with loads that are resistive in nature, the foldback function may prevent the device from actually starting up. For example, for a 3.3V application with a 1.5Ω load, the load current should be 3.3V/1.5 or 2.2A. In this example, with RCB = 10kΩ, the normal current limit is 3A while the foldback current is 1.5A. To exit the startup phase before the internal 50mS timer expires, the output voltage must be greater than 90% of the input to return to the normal current limit of RCB/3333.3. Since 1.5A × 1.5Ω = 2.25V the part will not start up and will never increase the current limit to RCB/3333.3 and will latch off (MAX15090) or retry (MAX15090B).
For low-voltage operation, the current foldback feature is not needed and can be disabled by forcing a voltage on the CB pin. This can be done using a resistive divider on the CB pin. In looking at Figure 2 and examining the current-limit equation of RCB/3333.3, the CB voltage setting should be 12µA × RCB. For this example, to set the current limit to 3A, a 10kΩ resistor will be used. In normal current-limit mode, the voltage on the CB pin is 10kΩ × 12µA or 0.120V.
Figure 2. Setting a fixed current limit by disabling the current foldback feature.
To minimize the voltage-setting error at the CB pin, the 12µA can be taken into account. For a 3A current limit, ideally 0.120V must be at the CB pin. As such, the current through R2 must be 0.120V/R2. If R2 is equal to 1000Ω then IR2 is equal to 120µA. Therefore, the current from R1 must be 120µA - 12µa or 108µA. So R1 must be 3.3V - 0.120/108µA or 29444. From the standard 1% resistor chart, the closest value is 29400, which provides a nominal error of only 160µV. The bigger error will be derived from the tolerance of the input supply voltage. The tolerance of the resistor-dividers must be taken into account. Since the divider ratio is 29:1, any voltage ripple on the 3.3V power supply will be divided down and should not affect the current-limit threshold.
Figure 3. Safe operating area when disabling the foldback feature.
Referring back to Figure 1, since the maximum foldback current is RCB/3333.3 × 0.5, RCB1 can be set to 2 × RCB/3333.3 to increase the current limit to enable to startup in resistive loads. For example, if the normal current limit is 3A then setting RCB1 and RCB2 to 20kΩ allows for a maximum foldback current limit of 20k/3333.3 = 6A during startup and 20k||20k/3333.3 = 3A for the desired current limit after a successful startup. This technique has also been used for other Maxim Integrated hot-swap ICs with internal FETs. Application note 48721 outlines this technique when using the MAX5976.
Figure 4. Dual-level current-limit control.
Figure 5. Current-limit zones with dual -level control.