Simple Circuit Disconnects Load
Figure 1. The addition of a couple of transistors enables a switching regulator to start with full load and low input voltages.
To get around this problem and ensure reliable start-ups, most regulator ICs incorporate an undervoltage lockout (UVLO). IC1, for example, is a synchronous boost converter whose bootstrapped operation cannot start until its output voltage exceeds the internal UVLO threshold of 2.3V. You can overcome this start-up limitation with an external power MOSFET, Q1, operating as a load-disconnect switch, and by using the power-OK (POK) comparator built into many low-voltage switching regulators. R3 and R4 set the POK threshold at 2.5V, allowing VIN to rise above the UVLO threshold. Q2 inverts the POK output before driving Q1. Q1 disconnects the load, allowing VOUT to rise to a level (above UVLO) that ensures full enhancement of Q1 when it turns on. As a result, the circuit can start under full load with input voltages as low as 0.8V (Figure 2a). Because the circuit takes the regulator feedback before this switch, the MOSFET you choose for a given application depends on the load current and minimum acceptable level of load regulation. The MOSFET shown is a low-threshold device. Connecting the FB terminal (Pin 2) to ground and removing R1 and R2 produces a 5V regulated output, whose performance is similar to that of the 3.3V version (Figure 2b). (DI #2487)
Figure 2. The load-disconnect switch in Figure 1 allows the regulator to start up with heavy loads and low input voltages (a). A slight modification of the circuit in Figure 1 provides 5V-output operation (b).
Reprinted from EDN magazine February 17, 2000
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