Simple circuit allows long PWM soft starts

-February 01, 2007

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 CSS, drives Q1’s base from IC1’s internally generated, 5V precision-reference source.

When the external power-supply voltage, VDD, exceeds IC1’s internally preset UVLO (undervoltage-lockout) threshold, the 5V reference source switches on. The voltage on CSS ramps upward toward 5V at a rate that the time constant, τ, of R1×CSS 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.

The simple circuit in Figure 1 satisfies the requirements of many soft-start applications. To obtain longer soft starts, you can increase CSS or increase R1 to decrease CSS’s charging current. However, increasing either component can cause problems. Depending on the construction of capacitor CSS, its leakage current may be significant. Also, you can no longer ignore Q1’s base current. For example, a survey of PWM-control-IC designs shows that the COMP pin typically sources an output current of 1 mA. If Q1, a 2N3906, provides a minimum beta of 80, Q1’s base draws a minimum current of 12.5 µA. The base current flows from the base pin of Q1 and adds to CSS’s charging current. If the circuit in Figure 1 uses a 1-µF capacitor for CSS and a 1-MΩ resistor for R1, you would expect a nominal 1-second charging-time constant and an average charging-current flow of 2.5 µA through R1. However, the charging current actually totals 15 µA—the sum of the 2.5-µA charging current plus Q1’s 12.5-µA base current, and the soft-start time falls considerably short of the nominal value.

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.

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