Low-power floating-rail reference generator
A floating-rail reference generator and method of operating the same are provided. Generally, the generator includes a tracking current source coupled in series with a current scaling resistor between an input voltage (V BAT ) and ground. The tracking current source is operable to receive a reference voltage and couple a tracking current through the resistor to produce a floating-rail reference voltage (V SSHV_REF ) at an output between the tracking current source and scaling resistor, wherein: V SSHV_REF =((V BAT −V GS )/k)·1/R·k·R, where V GS is a desired constant potential difference between V BAT and V SSHV_REF , k is a voltage scaling ratio, and R is a resistance of the current scaling resistor. In some embodiments, the tracking current source includes a transistor coupled between V BAT and the output, and controlled by a differential amplifier.
1. A floating-rail reference generator comprising a tracking current source coupled in series with a current scaling resistor between an input voltage (V BAT ) and ground, the tracking current source operable to receive a reference voltage and generate a tracking current (Isource) through the current scaling resistor to produce a floating-rail reference voltage (V SSHV_REF ) at an output between the tracking current source current and scaling resistor,
wherein:
V
SSHV
_
REF
=
(
V
BAT
k
-
V
GS
k
)
·
1
R
·
k
·
R
where V GS is a constant potential difference between V BAT and V SSHV_REF , k is a voltage scaling ratio, and R is a resistance of the current scaling resistor; and
wherein the tracking current source comprises a pair of MOS transistors including a first transistor coupled between the input voltage (V BAT ) and the output and a second transistor, and a differential amplifier having an output coupled to gates of the first and second transistors and operable to control the first and second transistors, the differential amplifier comprising:
an inverting input coupled to the input voltage (V BAT ) through a first resistor (R 1 ) of a voltage divider and to ground through a second resistor (R 2 ) of the voltage divider; and
a non-inverting input coupled to a drain of the second transistor and to ground through a third resistor (R 3 ).
2. The floating-rail reference generator of claim 1 wherein V GS equals 1.8V, and V SSHV_REF equals V BAT −1.8 V for V BAT between 1.8 V and 4.8 V, and V SSHV_REF equals 0 V for V BAT less than 1.8 V.
3. The floating-rail reference generator of claim 2 wherein a total current through the floating-rail reference generator is less than 100 nano-amperes (nA).
4. The floating-rail reference generator of claim 1 wherein V GS is a preselected maximum gate-source voltage for a fabrication process, the voltage scaling ratio (k) is equal to a resistance of the second resistor (R 2 ) divided by a sum of resistances of the first resistor (R 1 ) and second resistor (R 2 ), and a resistance of the current scaling resistor is equal to a product of the resistance of the second resistor (R 2 ) and a resistance of the third resistor (R 3 ) divided by a sum of resistances of the first resistor (R 1 ) and second resistor (R 2 ).
5. The floating-rail reference generator of claim 4 wherein the third resistor (R 3 ) is coupled to ground through a voltage source (V 1 ) and wherein:
V
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS the preselected maximum gate-source voltage, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
6. The floating-rail reference generator of claim 5 wherein a total current through the floating-rail reference generator is less than 100 nano-amperes (nA).
7. The floating-rail reference generator of claim 4 further comprising a reference current source (I 1 ) through which the non-inverting input, drain of the second transistor and the third resistor (R 3 ) are coupled to the input voltage (V BAT ), and wherein:
I
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS is the preselected maximum gate-source voltage, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
8. The floating-rail reference generator of claim 7 wherein a total current through the floating-rail reference generator is less than 100 nano-amperes (nA).
9. A method of generating a floating-rail reference voltage comprising:
generating a reference voltage;
generating a tracking current (Isource) from the reference voltage; and
coupling the tracking current (Isource) into a current scaling resistor having a resistance (R), to generate a floating-rail reference voltage (V SSHV_REF ) equal to Isource·R at an output,
wherein generating the tracking current comprises controlling a pair of MOS transistors including a first transistor coupled between an input voltage (V BAT ) and the output, and a second transistor, using a differential amplifier having an output coupled to gates of the first and second transistors, and wherein the differential amplifier comprises an inverting input coupled to the input voltage (V BAT ) through a first resistor (R 1 ) of a voltage divider and to ground through a second resistor (R 2 ) of the voltage divider, and a non-inverting input coupled to the reference voltage.
10. The method of claim 9 wherein generating the reference voltage comprises coupling the non-inverting input of the differential amplifier to a drain of the second transistor and to ground through a third resistor (R 3 ) and a voltage source (V 1 ) coupled between the third resistor and ground, wherein:
V
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS is a preselected maximum gate-source voltage for a fabrication process, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
11. The method of claim 9 wherein generating the reference voltage comprises coupling the non-inverting input of the differential amplifier to a drain of the second transistor and to ground through a third resistor (R 3 ), and to the input voltage (V BAT ) through a reference current source (I 1 ) coupled between the third resistor and the input voltage (V BAT ), wherein:
I
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS is a preselected maximum gate-source voltage for a fabrication process, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
12. The method of claim 9 wherein:
V
SSHV
_
REF
=
(
V
BAT
k
-
V
GS
k
)
·
1
R
·
k
·
R
where V GS is a preselected maximum gate-source voltage for a fabrication process, R is a resistance of the current scaling resistor, and k is a voltage scaling ratio equal to a resistance of the second resistor (R 2 ) divided by a sum of resistances of the first resistor (R 1 ) and second resistor (R 2 ).
13. The method of claim 9 wherein a maximum gate-source voltage (V GS ) for the first transistor when fabricated using a 22 nm fabrication process is 1.8V, and wherein generating the floating-rail reference voltage comprises generating a V SSHV_REF equal to V BAT −V GS for V BAT between 1.8 V and 4.8 V, and a V SSHV_REF equal to 0 V for V BAT less than 1.8 V.
14. A floating-rail reference generator comprising:
a current scaling resistor coupled between an output and ground, wherein a floating-rail reference voltage (V SSHV_REF ) at the output:
V
SSHV
_
REF
=
(
V
BAT
k
-
V
GS
k
)
·
1
R
·
k
·
R
where V GS is a preselected maximum gate-source voltage for a fabrication process, R is a resistance of the current scaling resistor, and k is a voltage scaling ratio equal to a resistance of the second resistor (R 2 ) divided by a sum of resistances of the first resistor (R 1 ) and second resistor (R 2 );
a pair of MOS transistors including a first transistor having a source coupled to an input voltage (V BAT ) and a drain coupled to the output, and a second transistor having a source coupled to the input voltage (V BAT );
a differential amplifier having an output coupled to gates of the first and second transistors and operable to control the first and second transistors, the differential amplifier comprising:
an inverting input coupled to the input voltage (V BAT ) through a first resistor (R 1 ) of a voltage divider and to ground through a second resistor (R 2 ) of the voltage divider; and
a non-inverting input coupled to a drain of the second transistor and to ground through a third resistor (R 3 ).
15. The floating-rail reference generator of claim 14 wherein the third resistor (R 3 ) is coupled to ground through a voltage source (V 1 ) and wherein:
V
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS the preselected maximum gate-source voltage for the fabrication process, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
16. The floating-rail reference generator of claim 15 further comprising a reference current source (I 1 ) through which the non-inverting input, drain of the second transistor and the third resistor (R 3 ) are coupled to the input voltage (V BAT ), and wherein:
I
1
=
V
GS
(
R
1
+
R
2
R
2
)
where V GS the preselected maximum gate-source voltage for the fabrication process, R 1 is the resistance of the first resistor, and R 2 is the resistance of the second resistor.
17. The floating-rail reference generator of claim 15 wherein V GS is 1.8V, and wherein generating the floating-rail reference voltage comprises generating a V SSHV_REF equal to V BAT −V GS for V BAT between 1.8 V and 4.8 V, and a V SSHV_REF equal to 0 V for V BAT less than 1.8 V.
18. The floating-rail reference generator of claim 17 wherein a total current through the floating-rail reference generator is less than 100 nano-amperes (nA).