[Design Application]
Smart Solid-State Fuse Helps Designers Cure Boost-Converter Ailments
The challenge is to get desired load disconnect while retaining use of the humble catch diode and unadorned boost topology.
The majority of non-synchronous, inductor-based boost converters exhibit a dc-current path between power source and load (Fig. 1). They're the step-up switching types, and their path can have two undesirable consequences.
The first problem occurs if a grounded output or other overload draws heavy output current for more than a few hundred milliseconds. Then the catch diode, which is usually a Schottky type, may exude that blended aroma of molten silicon and potting compound familiar to all true hackers. Or, say switching action is disabled for any reason, such as intentional shutdown. The load voltage remains just one diode drop below the supply voltage. If this residual voltage is outside the load circuit's expected steady-state operating range, the result can be indeterminate circuit behavior.
Both problems are neatly solved for relatively low-output-current applications—those less than 5 A. These employ monolithic current-mode controllers and high-side current sensing. The circuits replace the catch diode with a synchronous switching transistor that can be disabled by shutdown or the removal of input power.
Essentially, disabling this internal transistor or turning it off during shutdown removes the path for dc current flow. The load then sees a requisite high-impedance disconnect. When not in shutdown, the circuit's cycle-by-cycle current-sensing mechanism protects against catastrophic meltdown from internal current overloads. It does this through the use of an internal, high-side current-sense resistor. Finally, thermal-overload protection provides the circuit a safe area of operation.
For applications with higher output current, in which pricing makes synchronous switching impractical for monolithic devices, the load-disconnect function demands a high-side switch external to the controller die. A discrete current-mode topology using a high-side current-sense resistor and synchronous switching transistor is possible. But that approach suffers from pc-board parasitics and layout dependencies, especially at high switching frequencies. The result is a relatively complex design, particularly when system constraints mandate a low input voltage (less than 3.6 V).
A synchronous, high-side external switch becomes feasible at higher levels of peak inductor current—those greater than 5 A. But at the more moderate levels—approximately 1.5 to 5 A—cost and complexity override heat and efficiency considerations. A simple catch diode is again the most desirable solution. The challenge is to achieve the desired load disconnect while retaining use of the humble catch diode and the unadorned boost topology.
A Simple Solution A simple and smart solution does exist. In it, a MAX668 controller illustrates the demanding task of boosting from low input voltages (Fig. 2). This current-mode boost controller drives a logic-level, n-channel, enhancement-mode MOSFET that's configured on the low side. That MOSFET is driven in series with a low-side current-sense resistor. The high-side switch is a Schottky catch diode chosen for its low forward-voltage drop, which is now standard. The simple boost topology remains intact. This application boosts 3.3 to 5 V and delivers load currents to 3 A. The MAX668 boosts only from 3 V or higher, but the MAX669 can accept inputs as low as 1.8 V.
The key element in implementing a smart load disconnect is the p-channel enhancement-mode MOSFET, Q1. The system can enable this boost circuit (SHDN) or shut it down. D1 conducts during shutdown. At the MAX810L supply terminal, it produces 3.3 V minus one diode drop.
The MAX810 is a tiny power-on-reset device with an SOT23-3 package. It draws about 24 µA of quiescent current, and guarantees operation at 1 V. In this case, the MAX810L output is high because its nominal reset threshold is 4.65 V. This forces Q1 off and disconnects the load from the main supply.
The controller's feedback resistors are set to produce a 5-V output when that device exits shutdown. When the rising output exceeds the MAX810L input threshold, an internal one-shot turns on for approximately 240 ms. After this timeout period, the output goes low and turns on Q1.
While Q1 is on, the MAX810 constantly monitors the supply line for overload currents. An overload causes the output to sag below the MAX810's internal threshold voltage. That output then goes high with a nominal 20-µs delay, turning off Q1 and disconnecting the load. Soon after, the MAX668's boosting action raises the MAX810 input voltage above its threshold. After timeout, the MAX810 automatically reconnects the load. This cycle repeats until the excessive load is removed or the boost circuit is disabled. Q1 and the MAX810 together act as a smart solid-state switch.
As a micropower device, the MAX810 has a rather wimpy push-pull output stage. It approximately resembles a 6-kΩ resistor when sourcing current and a 125-Ω resistor when sinking it. When the device turns off or on, these resistances slow things down by acting against Q1's Miller capacitance and the associated CGS. The associated RC time constant for a large pass transistor is about 0.6 µs. That's assuming a total effective capacitance of 5000 pF acting against the MAX810's 125-Ω sinking stage. So a full voltage transition can be approximated as 10RC = 6 µs.
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