Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With bootstrap circuit
Reexamination Certificate
2002-10-25
2004-11-02
Riley, Shawn (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With bootstrap circuit
C323S282000, C323S288000
Reexamination Certificate
active
06812782
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to electronic systems and in particular it relates to gate drivers for Buck mode converters.
BACKGROUND OF THE INVENTION
In conventional prior art buck mode (switch mode) converters employing MOS transistors as the high side and low side power switches, the high side switch is either a PMOS or NMOS. For a given design of buck mode converter, operating at some output voltage Vout, suppose that the input voltage drops. As the input voltage drops, the buck converter increases the duty cycle (D) (here, duty cycle D is defined as the on-time of the high side transistor multiplied by the frequency of switching of the high side transistor) Ultimately, there is some limitation on how large D can be. In the case of a buck converter where the high side transistor is a PMOS, the duty cycle can be increased all the way to 100%, since it is sufficient to simply pull the gate of the PMOS down to ground. However, PMOS output devices are larger than NMOS devices of comparable performance, so it is more expensive to use PMOS devices.
If an NMOS device is used, a method has to be found to drive the gate to a higher voltage than the source. In the prior art device shown in
FIG. 1
, this is done using a “bootstrap” capacitor and a diode or synchronous diode. The prior art circuit of
FIG. 1
includes high side driver
20
; low side driver
22
; high side switch
24
(transistor); low side switch
26
(transistor); diode
28
; bootstrap capacitor
30
; inductor
32
; output capacitance
34
; output load resistance
36
; and input voltage Vin. The disadvantage of this prior art system is that the bootstrap capacitor
30
depends on the converter maintaining less than 100% duty-cycle in order to recharge the bootstrap capacitor
30
. Consequently, it is not possible to run the part down to as low an input supply as would be possible with 100% duty cycle.
A high power charge pump is used in some prior art devices to supply the entire high side gate-drive whether the device is running 100% duty cycle or not. However, the high power charge pump is very expensive in silicon area and uses several additional external capacitors. This is more costly in the switching converter IC and externally in the system.
In another prior art solution, an additional supply rail is used to power the NMOS drive. However, an additional rail is not available in many cases.
SUMMARY OF THE INVENTION
A switch mode converter uses the bootstrap capacitor to operate all the way to 100% duty cycle by adding only a very small amount of low-area extra circuitry. The additional circuitry includes a charge pump and a duty cycle detect device. When the duty cycle detect device detects that the converter is attempting to operate in 100% duty cycle, the charge pump provides additional charge to the bootstrap capacitor to ensure that the 100% duty cycle is maintained. Performance into dropout (when the input supply is actually lower than the desired output) is improved. A significant advantage in some applications (notably battery operation) is provided.
REFERENCES:
patent: 5138200 (1992-08-01), Barsanti et al.
patent: 5373435 (1994-12-01), Jayaraman et al.
patent: 5627460 (1997-05-01), Bazinet et al.
patent: 5818209 (1998-10-01), Masini et al.
patent: 6201717 (2001-03-01), Grant
Riley Shawn
Stewart Alan K.
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