Load-switch gate-protection circuit
Gate-protection circuitry protects a transistor, such as a MOSFET, from large gate-to-source voltage differentials that can permanently damage the transistor's gate-oxide layer. A source-voltage detector selectively enables the gate-protection circuitry based on a source voltage of the transistor. The gate-protection circuit is implemented without any Zener diodes. The transistor may be a load switch that is selectively controlled to apply a supply voltage to a load.
1. A load-switch circuit, comprising:
a main transistor having a gate terminal, a drain terminal, and a source terminal;
a turn-on circuit having an input configured to receive a control signal and an output connected to the gate terminal of the main transistor;
gate-protection circuitry connected between the gate and source terminals of the main transistor, wherein the gate-protection circuitry limits the gate-to-source voltage differential of the main transistor; and
a source-voltage detector having an input connected to the source terminal of the main transistor and an output connected to the gate-protection circuitry,
wherein the source-voltage detector disables the gate-protection circuitry based on a voltage level at the source terminal of the main transistor;
wherein the gate-protection circuitry comprises:
a pull-up transistor having a drain terminal connected to the source terminal of the main transistor;
a diode connected between a source terminal of the pull-up transistor and the gate terminal of the main transistor;
a disabling transistor having a gate terminal connected to the output of the source-voltage detector, a source terminal connected to ground, and a drain terminal connected to a gate terminal of the pull-up transistor; and
a first resistor connected between the source terminal of the main transistor and a node between the gate terminal of the pull-up transistor and the drain terminal of the disabling transistor.
2. The load-switch circuit of claim 1 , further comprising:
a pull-down transistor having a drain connected to the gate terminal of the main transistor, a source terminal connected to ground, and a gate terminal; and
a pull-down driver having an input configured to receive a pull-down control signal and an output connected to the gate terminal of the pull-down transistor,
wherein the pull-down driver controls the pull-down transistor based on the pull-down control signal.
3. The load-switch circuit of claim 2 ,
wherein the pulldown transistor is an n-type device.
4. The load-switch circuit of claim 1 ,
wherein the gate-protection circuitry further comprises a stabilizing capacitance connected in parallel with the first resistor between the source terminal of the main transistor and the gate terminal of the pull-up transistor.
5. The load-switch circuit of claim 1 ,
wherein the source-voltage detector comprises a comparator configured to compare a reference voltage to a feedback voltage based on the voltage level at the source terminal of the main transistor.
6. The load-switch circuit of claim 5 ,
wherein the source-voltage detector further comprises a voltage divider connected between the source terminal of the main transistor and ground and configured to generate the feedback voltage.
7. The load-switch circuit of claim 6 ,
wherein the voltage divider comprises at least two resistors connected in series.
8. The load-switch circuit of claim 6 ,
wherein the voltage divider comprises at least two diodes connected in series.
9. The load-switch circuit of claim 1 ,
wherein:
the drain terminal of the pull-up transistor is connected to the source terminal of the main transistor;
the source terminal of the pull-up transistor is connected to the anode of the diode; and
the cathode of the diode is connected to the gate terminal of the main transistor.
10. The load-switch circuit of claim 1 ,
wherein the pullup transistor and the disabling transistor are n-type devices.
11. The load-switch circuit of claim 1 ,
wherein the gate-protection circuitry is implemented without any Zener diodes.
12. The load-switch circuit of claim 1 , wherein the main transistor is a MOSFET transistor.
13. The load-switch circuit of claim 12 ,
wherein: the MOSFET transistor is a load switch having its drain terminal configured to be connected to a power supply and its source terminal configured to be connected to a load; and
the load-switch circuit further comprises:
an n-type pull-down transistor having a drain connected to the gate terminal of the load switch, a source terminal connected to ground, and a gate terminal; and
a pull-down driver having an input configured to receive a pull-down control signal and an output connected to the gate terminal of the pull-down transistor,
wherein: the pull-down driver is configured to control the pull-down transistor based on the pull-down control signal such that the pull-down driver turns on the pull-down transistor when the pull-down control signal indicates that the load switch is being turned off;
the gate-protection circuitry comprises:
a pull-up transistor having a gate terminal and connected in series to a diode between the source terminal of the load switch and the gate terminal of the load switch; and
a disabling transistor having a gate terminal connected to the output of the source-voltage detector,
wherein the pull-down transistor is connected in series with a first resistor at the gate terminal of the pull-up transistor;
the source-voltage detector comprises:
a comparator configured to compare a reference voltage to a feedback voltage based on the voltage level at the source terminal of the load switch; and
a voltage divider connected between the source terminal of the load switch and ground and configured to generate the feedback voltage;
the drain terminal of the pull-up transistor is connected to the source terminal of the load switch;
the source terminal of the pull-up transistor is connected to the anode of the diode;
the cathode of the diode is connected to the gate terminal of the load switch;
the pull-up transistor and the disabling transistor are n-type devices; and the gate-protection circuitry is implemented without any Zener diodes.
14. The load-switch circuit of claim 13 ,
wherein the gate-protection circuitry further comprises a stabilizing capacitance connected in parallel with the first resistor between the source terminal of the load switch and the gate terminal of the pull-up transistor.
15. The load-switch circuit of claim 13 ,
wherein the voltage divider comprises resistors connected in series.
16. The load-switch circuit of claim 13 ,
wherein the voltage divider comprises diodes connected in series.
17. A load-switch circuit, comprising:
a main transistor having a gate terminal, a drain terminal, and a source terminal;
a turn-on circuit having an input configured to receive a control signal and an output connected to the gate terminal of the main transistor;
gate-protection circuitry connected between the gate and source terminals of the main transistor, wherein the gate-protection circuitry limits the gate-to-source voltage differential of the main transistor; and
a source-voltage detector having an input connected to the source terminal of the main transistor and an output connected to the gate-protection circuitry,
wherein the source-voltage detector disables the gate-protection circuitry based on a voltage level at the source terminal of the main transistor;
a pull-down transistor having a drain connected to the gate terminal of the main transistor, a source terminal connected to ground, and a gate terminal; and
a pull-down driver having an input configured to receive a pull-down control signal and an output connected to the gate terminal of the pull-down transistor;
wherein the pull-down driver controls the pull-down transistor based on the pull-down control signal.
18. The load-switch circuit of claim 17 ,
wherein the source-voltage detector comprises a comparator configured to compare a reference voltage to a feedback voltage based on the voltage level at the source terminal of the main transistor.
19. The load-switch circuit of claim 18 ,
wherein the source-voltage detector further comprises a voltage divider connected between the source terminal of the main transistor and ground and configured to generate the feedback voltage.
20. The load-switch circuit of claim 17 ,
wherein the main transistor is a MOSFET transistor.