Browse Prior Art Database

BOOTSTRAPPED CHARGE PUMP CIRCUIT

IP.com Disclosure Number: IPCOM000006786D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2002-Feb-01
Document File: 1 page(s) / 59K

Publishing Venue

Motorola

Related People

John Qualich: AUTHOR [+2]

Abstract

Charge pumps are used to provide a voltage higher than battery voltage to the gate of an N-channel MOSFET in automotive high-side-drive applications. A limitation of this method, especially if the charge pump uses a small IC capacitor, is slow turn-on of the FET-the gate charge of the FET is so large and the current sourcing capability of the charge pump so limited that it takes too long to develop sufficient gate voltage to turn the FET completely 'on? This problem prevents switching a large FET using an IC based charge pump (such as might be used in an automotive Solid State Relay) at speeds approaching 2OkHz. Boot-strap circuits are used to turn a FET quickly 'on: but they cannot be used to hold a FET 'on' for any length of time, due to non-zero gate leakage currents ofreal-world FETs.

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MOlOROLA INC. Technical Developments Volume 18 March 1993

BOOTSTRAPPED CHARGE PUMP ClRdlJlT

by John Qualich and Jim Edwards

   Charge pumps are used to provide a voltage higher than battery voltage to the gate of an N-channel MOSFET in automotive high-side-drive applications. A limitation of this method, especially if the charge pump uses a small IC capacitor, is slow turn-on of the FET-the gate charge of the FET is so large and the current sourcing capability of the charge pump so limited that it takes too long to develop sufficient gate voltage to turn the FET completely 'on? This problem prevents switching a large FET using an IC based charge pump (such as might be used in an automotive Solid State Relay) at speeds approaching 2OkHz. Boot-strap circuits are used to turn a FET quickly 'on: but they cannot be used to hold a FET 'on' for any length of time, due to non-zero gate leakage currents ofreal-world FETs.

Our solution is to combine a bootstrap sub-circuit
(11) which turns the FET 'on' quickly, with a charge- pump sub-circuit (12) which maintains the FET in-the 'on' condition. Transistor Q2 (5) joins the two sub-circuits. Transistor Q3 (10) is used to turn the FET 'off: All of the circuitry shown, with the exceptiori of C2 (6) and

the FET (8) can be fabricated on an IC.

  The output transistor @), an N-channel TMOS FET device, is in the 'off state vihen the CTRL signal is high because transistor 43 (10) is 'on:

  When the CTRL sign@ is brought low, current wi...