`
`Simple Soft-Start Circuitry Provides Long Startup TimesSimple Soft-Start Circuitry Provides Long Startup Times
`
`Electronic Design
`Contributing Author
`Contributing Author
`Mon, 1998-06-22 (All day)
`
`Many dc-dc converters require relatively large amounts of input current at startup (when input power is first
`applied). A soft-start function minimizes this large input current by gradually increasing the switch-current
`limit at startup, slowing the rate of rise of the output voltage and reducing the peak current required when
`starting up.
`
`Some switching-regulator ICs include a short soft-start period (100 µs to 3 ms), but in some instances the start-
`up currents are still large enough to cause problems. The major problem occurs when the input power source to
`the switching regulator is current-limited or has poor load regulation, both of which will cause the input voltage
`to drop when large startup currents are required.
`
`Because of the negative resistance characteristic associated with the inputs of many switchers, insufficient soft-
`start time will result in high input currents, causing the input voltage source to be pulled down. With a lower
`input voltage, the switching regulator requires even more input current to correctly start up. This, in turn, will
`pull the input voltage lower still, resulting in a latch condition. When in a latch condition, the input current is
`relatively high, the input voltage is pulled low, and the switching regulator’s output never reaches its regulated
`value. Because of the charging of the output capacitor, this high startup current can exist even without a load on
`the switcher’s output.
`
`Besides minimizing the high startup current associated with switching regulators, a soft-start function also can
`be used for power supply sequencing. In some situations where multiple regulated voltages are present, it may
`be necessary for one or more voltages to come up after the main power has come up and stabilized.
`
`Netlist Ex 2022-p. 1
`Samsung v Netlist
`IPR2022-00996
`
`
`
`The circuit in the dashed box of Figure 1 can be applied to many different types of switching regulators,
`including buck, boost, flyback, and SEPIC. It provides a soft-start function by slowly increasing the switch
`current limit, thus controlling the output voltage slew rate. At startup, the output voltage increases linearly, with
`ramp times ranging from milliseconds to seconds, depending on the capacitor value. The compensation pin (VC
`pin) in many current-mode switching-regulator ICs is the output of the error amplifier, which directly controls
`the maximum current through the switch. By limiting the voltage at the VC pin, the current can be controlled.
`
`Referring to Figure 1, the circuit operates as follows. At startup, when the input voltage is first applied or when
`the shutdown input goes high, the regulator output voltage begins to rise. The soft-start capacitor (CSS) begins
`charging through the emitter-base of Q1, causing the collector to begin pulling down on the compensation (VC)
`pin (pin 6), thus lowering the switch current limit. Because of this feedback loop, the charge current for the soft-
`start capacitor is a constant 10 µA, which is determined by R1 (68k) and the VBE (≈680 mV) of Q1.
`
`When the regulator’s output voltage has stabilized and the soft-start capacitor is charged, the transistor turns
`off and the switching-regulator circuit operates normally. R2 limits the discharge current of CSS at powerdown
`to protect the emitter-base of Q1. The following formula can be used to determine the total startup time for
`various output voltages and soft-start capacitor values:
`
`t = CSS * VOUT /10 µA
`
`The two photos in Figure 2 show the input current and the output voltage of a buck switching-regulator circuit
`at startup.
`
`Netlist Ex 2022-p. 2
`Samsung v Netlist
`IPR2022-00996
`
`
`
`The left photo is the regulator’s performance without the external soft-start circuitry, whereas the one on the
`right shows the effect of the external soft-start circuit of Figure 1. As the right photo illustrates, the 3.8-A input
`surge current is completely eliminated, while the output voltage increases linearly in approximately 5.5 ms.
`Other capacitor values can be used if faster or slower rise times are desired. Although a stepdown (buck)
`regulator configuration was used for the photos, other topologies will provide similar results.
`
`Source URL: http://electronicdesign.com/power/simple-soft-start-circuitry-provides-long-startup-times
`
`Netlist Ex 2022-p. 3
`Samsung v Netlist
`IPR2022-00996
`
`