Load switch having current-ramping soft start
A load switch with a soft start feature operates an MOS power device as a current source having a limited current capability. During a soft start ramp period, the power device is driven under feedback control such that a gradually-increasing servo current through a reference device of the same type and driven by the same gate drive voltage follows a ramp signal. The ramp signal may be generated using a current source to charge a soft start capacitor. Substantially matched load switches connected in parallel, each incorporating such a ramp generator, may share a single soft start capacitor to synchronize their ramp signals and balance power dissipation during soft start as well as during continuous operation. Load switch integrated circuits and coordinated systems of load switches are also provided, as are methods of performing soft start and coordinating the soft start of a system of load switches.
1 . A load switch connecting a power supply to a load and having current-ramping soft start, comprising:
a power device controlling a load current from the power supply to the load, comprising a type of MOS transistor having a gate driven by a gate drive voltage, and having a rated current capability for continuous operation and an instantaneous current capability;
a reference device of the same type as the power device and having a gate driven by the gate drive voltage, the reference device configured to pass a servo current proportional to the instantaneous current capability of the power device;
a gate control circuit accepting an enable signal that can be inactive or active, and also accepting an endpoint voltage selected to correspond to a target current capability of the power device, comprising
a ramp generator generating a ramp signal comprising a ramp voltage at a ramp voltage node that is reset to a starting voltage corresponding to the power device being fully off when the enable signal is inactive, and that varies gradually from the starting voltage to the endpoint voltage during a soft start ramp period that starts when the enable signal changes from inactive to active and that ends when the power device instantaneous current capability reaches the target current capability, and
an endpoint evaluation circuit generating a DONE signal that is inactive while the enable signal is inactive as well as during the soft start ramp period and that transitions from inactive to active at the end of the soft start ramp period; and
a gate drive circuit providing the gate drive voltage and having inputs accepting the enable signal, the ramp signal, and the DONE signal, comprising a feedback circuit and configured to
turn the power device fully off when the enable signal is inactive,
turn the power device fully on when both the enable signal and the DONE signal are active, and
during the soft start ramp period, control the servo current to track the ramp voltage using the feedback circuit, so as to gradually increase the instantaneous current capability of the power device from fully off to the target current capability,
whereby the power device is operated as a current source having a limited current capability that gradually increases during the soft start ramp period, charging a capacitance of the load with a limited inrush current before switching to drive the power device fully on for continuous operation.
2 . The load switch of claim 1 , wherein the reference device is sized to have a lower current capability than the power device for the same gate drive voltage.
3 . The load switch of claim 1 , wherein the power device and reference device are PMOS transistors.
4 . The load switch of claim 1 , wherein the ramp generator comprises a current source that charges a soft start capacitor connected to the ramp voltage node.
5 . The load switch of claim 1 , wherein the ramp voltage varies nonlinearly with time.
6 . The load switch of claim 1 , wherein the endpoint evaluation circuit compares the ramp voltage to the endpoint voltage to generate the DONE signal, which transitions from inactive to active when the ramp signal reaches the endpoint voltage.
7 . The load switch of claim 6 , wherein the target current capability exceeds the rated current capability by at least 20 percent.
8 . The load switch of claim 1 , wherein the target current capability is greater than the rated current capability.
9 . The load switch of claim 1 , wherein the length of the soft start ramp period is between approximately one hundred microseconds and approximately ten milliseconds.
10 . A load switch integrated circuit having current-ramping soft start, comprising:
a semiconductor substrate having a surface; and
a load switch circuit formed on the surface, wherein the load switch circuit comprises
a power supply connection,
a ground connection,
a load connection,
an enable connection accepting an enable signal that can be inactive or active,
a soft start connection connected to a ramp voltage node,
a power device controlling a load current from the power supply to the load, comprising a type of MOS transistor having a gate driven by a gate drive voltage, and having a rated current capability for continuous operation and an instantaneous current capability,
a reference device of the same type as the power device and having a gate driven by the gate drive voltage, the reference device configured to pass a servo current proportional to the instantaneous current capability of the power device,
a gate control circuit accepting the enable signal, an endpoint voltage selected to correspond to a target current capability of the power device, and a ramp signal from the ramp voltage node,
wherein the ramp signal comprises a ramp voltage that is reset to a starting voltage corresponding to the power device being fully off when the enable signal is inactive, and varies gradually from the starting voltage to the endpoint voltage during a soft start ramp period that starts when the enable signal changes from inactive to active and that ends when when the power device instantaneous current capability reaches the target current capability,
the gate control circuit comprising an endpoint evaluation circuit generating a DONE signal that is inactive while the enable signal is inactive as well as during the soft start ramp period and that transitions from inactive to active at the end of the soft start ramp period; and
a gate drive circuit providing the gate drive voltage and having inputs accepting the enable signal, the ramp signal, and the DONE signal, comprising a feedback circuit and configured to
turn the power device fully off when the enable signal is inactive,
turn the power device fully on when both the enable signal and the DONE signal are active, and
during the soft start ramp period, control the servo current to track the ramp voltage using the feedback circuit, so as to gradually increase the instantaneous current capability of the power device from fully off to the target current capability,
whereby the power device is operated as a current source having a limited current capability that gradually increases during the soft start ramp period, charging a capacitance of the load with a limited inrush current before switching to drive the power device fully on for continuous operation.
11 . The load switch integrated circuit of claim 10 , wherein the reference device is sized to have a lower current capability than the power device for the same gate drive voltage.
12 . The load switch integrated circuit of claim 10 , wherein the power device and reference device are PMOS transistors.
13 . The load switch integrated circuit of claim 10 , wherein the endpoint evaluation circuit compares the ramp voltage to the endpoint voltage to generate the DONE signal, which transitions from inactive to active when the ramp signal reaches the endpoint voltage.
14 . The load switch integrated circuit of claim 10 , wherein the target current capability is greater than the rated current capability.
15 . The load switch integrated circuit of claim 14 , wherein the target current capability exceeds the rated current capability by at least 20 percent.
16 . The load switch integrated circuit of claim 10 , the load switch circuit further comprising a ramp generator that accepts the enable signal and generates the ramp signal at the ramp voltage node.
17 . The load switch integrated circuit of claim 16 , wherein the ramp generator comprises a current source charging a capacitance at the ramp voltage node.
18 . The load switch integrated circuit of claim 17 , further comprising an integrated soft start capacitor connected to the ramp voltage node.
19 . The load switch integrated circuit of claim 10 , further comprising an integrated circuit package encapsulating the semiconductor substrate and having terminals connected to at least the power supply connection, the ground connection, the load connection, the enable connection, and the soft start connection on the load switch circuit.
20 . A coordinated system of load switches having current-ramping soft start, comprising:
a plurality of load switches connected in parallel between a power supply and a load and all connected to an enable signal, each load switch comprising
a soft start connection, connecting a ramp signal to a ramp voltage node in the load switch,
a power device controlling a load current from the power supply to the load, comprising a type of MOS transistor having a gate driven by a gate drive voltage, and having a rated current capability for continuous operation and an instantaneous current capability,
a reference device of the same type as the power device and having a gate driven by the gate drive voltage, the reference device configured to pass a servo current proportional to the instantaneous current capability of the power device,
the ramp signal comprising a ramp voltage that is reset to a starting voltage corresponding to the power device being fully off when the enable signal is inactive, and varies gradually from the starting voltage to an endpoint voltage selected to correspond to a target current capability of the power device during a soft start ramp period that starts when the enable signal changes from inactive to active and that ends when the power device instantaneous current capability reaches the target current capability,
an endpoint evaluation circuit that accepts the ramp signal and the endpoint voltage and generates a DONE signal that is inactive while the enable signal is inactive as well as during the soft start ramp period and that transitions from inactive to active at the end of the soft start ramp period, and
a gate drive circuit providing the gate drive voltage and having inputs accepting the enable signal, the ramp signal, and the DONE signal, comprising a feedback circuit and configured to
turn the power device fully off when the enable signal is inactive,
turn the power device fully on when both the enable signal and the DONE signal are active, and
during the soft start ramp period, control the servo current to track the ramp voltage using the feedback circuit, so as to gradually increase the instantaneous current capability of the power device from fully off to the target current capability,
wherein parameters of at least the power devices, the reference devices, the ramp signals, the endpoint evaluation circuits, and the gate drive circuits are matched between the load switches within a set of tolerances sufficient to balance a power dissipation substantially evenly between the load switches within predetermined tolerances during the soft start ramp period as well as during continuous operation.
21 . The system of claim 20 , further comprising a ramp generator that accepts the enable signal and provides the ramp signal to the soft start connections of all the load switches.
22 . The system of claim 20 , each load switch circuit further comprising a ramp generator that accepts the enable signal and provides the ramp signal to the endpoint evaluation circuit and the gate drive circuit, wherein parameters of the ramp generators and the endpoint evaluation circuits are matched to within sufficiently tight tolerances so as to generate substantially matched and synchronized ramp signals and DONE signals.
23 . The system of claim 22 , wherein the ramp generator comprises a current source charging a capacitance at the ramp voltage node to generate the ramp signal.
24 . The system of claim 23 , wherein one soft start capacitor connected to the soft start connections is shared by all of the load switches.