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CIRCUIT MEANS FOR PROVIDING AN ACCURATE CURRENT RATIO DURING VOLTAGE DROPOUT

IP.com Disclosure Number: IPCOM000008280D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2002-Jun-03
Document File: 1 page(s) / 65K

Publishing Venue

Motorola

Related People

Paul T. Bennett: AUTHOR [+2]

Abstract

This circuit provides current sources having a well controlled ratio even during conditions of low supply current availability. The current soumes have been used in a current limit circuit for a power TMOS current source having gate voltage supplied by a charge pump. The charge pump converts the input supply voltage to a higher voltage needed to supply the gate of the TMOS operating as a high side current source. The current limit circuit uses a 'delta phi' design that depends on the accurate ratio of two currents. The charge pump has limited current capability and under low supply voltages is unable to supply the nominal currents required by the current limit circuit. The standard circuit fails to maintain the ratio of the currents under these conditions. This circuit provides a means of reduc- ing the current drain on the charge pump while maintaining the ratio of currents supplied to the current limit circuit, the current limit is then well controlled over the operating voltage range.

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CIRCUIT MEANS FOR PROVIDING AN ACCURATE CURRENT RATIO DURING VOLTAGE DROPOUT

Paul 1. Bennett and Robert P. Dixon

  This circuit provides current sources having a well controlled ratio even during conditions of low supply current availability. The current soumes have been used in a current limit circuit for a power TMOS current source having gate voltage supplied by a charge pump. The charge pump converts the input supply voltage to a higher voltage needed to supply the gate of the TMOS operating as a high side current source. The current limit circuit uses a 'delta phi' design that depends on the accurate ratio of two currents. The charge pump has limited current capability and under low supply voltages is unable to supply the nominal currents required by the current limit circuit. The standard circuit fails to maintain the ratio of the currents under these conditions. This circuit provides a means of reduc- ing the current drain on the charge pump while maintaining the ratio of currents supplied to the current limit circuit, the current limit is then well controlled over the operating voltage range.

  The standard current mirror design is modified by adding a single transistor which acts as a satura- tion detector for the current mirror, reducing the currents to maintain an out of saturation conditions. The current ratios are only maintained when the

mirror is not saturated.

  Figure 1 shows a simplified schematic of the design. The circuit...