Sensing Output Voltage
Although Q1 turns on when the MAX810 output goes low, the MAX810's reset threshold cannot accurately sense the main output voltage. That's because its ground terminal is referred to the negative-charge pump output. Accordingly, the MAX810 ground terminal connects to ground. Its output drives a level shifter comprised of Q2 and Q3, such that Q1's gate is pulled to the negative rail for turn-on.
At light loads, the MAX668 features idle-mode pulse-frequency modulation (PFM). So it can skip charging pulses when load current from the main supply is low. When that occurs, Q2's emitter current (set by R1) discharges C3. This action may cause insufficient supply voltage to the MAX4544, even with the main output voltage in regulation. In turn, this effect can force the on-resistance to skyrocket in the internal analog switch. The feedback voltage at the MAX668 would then drop towards ground.
The MAX668 would try to compensate by raising its output voltage, possibly causing an overvoltage condition. As an antidote, watch the feedback resistors, which are the minimum dc load on the main output voltage. Make sure that they're small enough to discharge VOUT slightly faster than the discharge of C3 by Q2's emitter current. Whether Q1 conducts or not, the following inequality allows the sizing of C3:
(VOUT − VBE)/(R1 × C3) < VOUT/[(RA + RB) × (C1 + C2)]
If no charging pulses are skipped during normal PWM operation, C4 can be small compared to C3. But the more pulses that are skipped, the larger C4 has to be. Boosting action resumes after pulse skipping, while Q2 is held off. C4 should then be large enough to charge C3 before C1 becomes fully charged.
Many components and interconnects affect the MAX668 feedback path in Figures 3 and 4. A fault in these components can produce an overvoltage VOUT that destroys the load. For added safety, a zener diode (not shown) is connected from C1 to the MAX668 FB pin. Its anode is joined to FB. This zener diode can provide an overriding local feedback loop that clamps the output at (VZ + VFB). To prevent a large overvoltage, set VZ equal to the maximum regulated VOUT minus the maximum VFB.
If the system must control multiple loads individually while the boost converter remains on, replace the MAX810 with a MAX812 (in a 4-pin SOT143 package type). The MAX812's fourth pin is designed for manual-reset applications. But it can force a disconnect between the local load and main boosted output by acting as a logic-level signal that overrides each smart solid-state fuse. With this approach, the user is able to control each load on the main boost supply independently.
Note that this smart solid-state fusing technique auto-resets without power cycling and requires no replacement or field troubleshooting. Most importantly, it doesn't have to be limited to boost-converter outputs. It can replace a fuse on the dc-power bus of virtually any system, regardless of voltage.
Above 60 V
Of course, bus voltages higher than 60 V may require non-logic-level FETs and level shifters for the MAX810 output. Using just two precision resistors to set an appropriate external bias for the higher voltages, the solid-state fuse can be set to be triggered by a programmed sag in the bus supply voltage (Fig. 6).
Suppose that −48 V is to be protected against overcurrent. Interrupt the rail side instead of the ground side, because the voltage source is negative. Use an n-channel FET plus a MAX809T reset circuit, which has a reset-output polarity that's opposite to that of the MAX810. The supply voltage can range down to −36 V under normal operation (Fig. 7). Design equations are as follows:
The MAX809 quiescent current is about 100-µA maximum over temperature. The current through RH and RL should be about 100× higher to minimize the quiescent-current effect on trip voltage: 36/(RH + RL) = 10 µA. Therefore:
(RH + RL) = 3600 Ω
The MAX809 threshold is much lower than the supply trip voltage. So RL is smaller than RH, approximately by the ratio VTHRESHOLD/(VTHRESHOLD + VSUPPLY TRIP) = 3/(36 + 3) = 0.077. MAX809 IQ flows through approximately 93.3% of (RH + RL), causing a voltage-trip contribution of about 0.336 V. Taking this fact into account, set the initial trip voltage for calculating RH and RL at 36 V − 0.336 V = 35.664 V. Using 1% resistors for RH and RL, VSUPPLY TRIP = 35.664 V. This threshold occurs when the MAX809T threshold is at its minimum (3.15 V over the temperature range −40° to + 85°C):
Calculated values for RL and RH are 323.81 Ω and 3276.19 Ω, respectively. The closest 1% values are 320 and 3280. Consider these resistor values and the 100-µA IQ. The maximum supply-trip voltage becomes 36.09 V, slightly surpassing 36 V. This result also occurs only for simultaneous worst-case values for all errors. In practice, that's a rare scenario. For most applications, this design would be quite acceptable. The MAX809's nominal threshold voltage gives a nominal trip voltage of −34.65 V.
RH should have a power rating of 0.5 W. But the voltage across RL exceeds the MAX809's maximum input-voltage rating when VSUPPLY goes above its minimum limit. To deal with this issue, place a 5-V ±5% zener diode across RL, as shown in Figure 7.
Eugene Carey is a corporate field applications engineer for Maxim Integrated Products Inc., Sunnyvale, Calif.; (408) 737-7600. He holds a BSEE from the University of Notre Dame, South Bend, Ind.
Michael Hess is an applications engineer for the customer applications department at Maxim Integrated Products. He holds a BSEE from San Jose State University, Calif.