Simple circuit allows long PWM soft starts

Available from multiple sources, the UC384X family of current-mode, PWM (pulse-width-modulated) power-supply controllers offers good performance and has spawned a variety of similar ICs. All members of the UC384X family and its variants share a common characteristic—an internal voltage-error amplifier that provides a current-limited output. Designated as the COMP pin, the amplifier’s output provides a convenient connection for applying compensation to ensure overall feedback-loop stability. In addition, the COMP pin allows attachment of shutdown and soft-start circuitry and serves as a convenient point for setting an external power switch’s output-current-limit threshold.

Two of the COMP pin’s characteristics enhance its versatility: First, the pin delivers limited output current, and, second, the pin’s voltage is directly proportional to the current flowing through an external power switch. Both features also allow the pin to serve as a control port. For example, perhaps the most common application for the pin involves addition of a soft-start feature to a UC384X-based power-supply design.

In soft-start mode, an external power switch’s output current and the power supply’s output voltage ramp up at a rate controlled by, and proportional to, the voltage at the COMP pin. Figure 1 shows a typical soft-start circuit’s implementation comprising a small-signal PNP transistor, Q₁, connected to the COMP pin. An RC network, R₁ and C_SS, drives Q₁’s base from IC₁’s internally generated, 5V precision-reference source.

When the external power-supply voltage, V_DD, exceeds IC₁’s internally preset UVLO (undervoltage-lockout) threshold, the 5V reference source switches on. The voltage on C_SS ramps upward toward 5V at a rate that the time constant, τ, of R₁×C_SS determines in seconds. Given Q₁’s emitter-follower configuration, Q₁ applies the COMP pin’s voltage, which “follows” Q₁’s base voltage, and the power supply’s output current ramps up proportionally.

As an alternative, the circuit of Figure 2 better satisfies designs such as battery chargers that require a longer soft start or a more accurately timed soft-start ramp. Adding a second transistor to form a PNP-NPN compound transistor maintains the slow-start function. The circuit’s composite current gain (beta) consists of the product of Q₁’s and Q₂’s current gains, or 70×60=4200, which greatly exceeds the single transistor’s current gain of 60. The higher current gain reduces the charging current’s base-current component to only 338 nA. Figure 3 compares the responses of both circuits. The dark-green trace shows that the circuit of Figure 2 produces the expected 1-second soft-start time interval, and the light-green trace illustrates Figure 1’s too-brief start-up time. Although the circuit of Figure 2 yields a more accurate soft-start ramp, it also allows the use of smaller capacitors, such as multilayer ceramics, to reduce pc-board area and component cost.

Although a Darlington-connected transistor pair would also provide high current gain, its output transistor cannot saturate—a prerequisite for keeping the off-state voltage at IC₁’s COMP pin below 1V. The PNP transistor, Q₁ in the PNP-NPN compound connection in Figure 2 can saturate, and the NPN transistor, Q₂, maintains its voltage-controlled saturation voltage at significantly less than 1V over the circuit’s operating-temperature range.