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Bootstrap Charger for a High Side Switch

IP.com Disclosure Number: IPCOM000171349D
Original Publication Date: 2008-Jun-25
Included in the Prior Art Database: 2008-Jun-25
Document File: 4 page(s) / 134K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

In a NMOS High Side Switch (Negative Metal Oxide Semiconductor High Side Switch) a bootstrap cap is needed to generate the necessary overdrive voltage for the transistor during the switch on phase. Typically, a bootstrap circuit is used to charge this cap. For current technologies, fast and accurate bootstrap chargers are required. The main requirements for such circuits are working at low battery voltage, being fast during the charging phase and being precise.

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Bootstrap Charger for a High Side Switch

Idea: Christian Garbossa, IT-Padova; Marco Piselli, IT-Padova; Andrea Vecchiato, IT-Padova;

Emanuele Bodano, IT-Padova; Andrea Morra, IT-Padova

In a NMOS High Side Switch (Negative Metal Oxide Semiconductor High Side Switch) a bootstrap cap

is needed to generate the necessary overdrive voltage for the transistor during the switch on phase.

Typically, a bootstrap circuit is used to charge this cap. For current technologies, fast and accurate

bootstrap chargers are required. The main requirements for such circuits are working at low battery

voltage, being fast during the charging phase and being precise.

Currently, there are different possibilities to achieve these requirements. The simplest way to charge

the floating capacitance is to add a diode between the supply voltage and the bootstrap cap. The main

disadvantage of this embodiment is the need for a dedicated supply line, which has to be regulated

exactly to the voltage that should charge the bootstrap capacitance. Examples of this technique are

shown in Figure 1 and 2. The high voltage drop is defined by the V_charge value, which must be

higher than the sum of the single voltage values of the circuit elements. Another disadvantage is the

imprecise control of bootstrap voltage. Moreover, since this circuit is only useful for V_charge voltages

higher than the SW voltage, it is not capable for a wide range of applications. At last, the circuit has a

slow response during switching. This problem is affected by the constant current that has to charge a

parasitic capacitance. An alternative implementation of the bootstrap circuit is the use of a Dynamic

Regulator, as shown in Figure 3. This concept is based on a high side regulator that activates a switch

in series to a diode to charge the flying cap. The main drawback of this implementation is that the

charger is only effective during switching off. To drive the charging switch a minimum T_off (the off

time for the high side switch), and a minimum headroom, i.e. the voltage between the charging supply

and the SW voltage, are necessary. Another implementation can be achieved by the use of a charge

pump, which creates a voltage higher than the battery voltage capable of charging at any condition the

flying cap. The main disadvantage of this circuit is the strong noise emittance and the requirement for

high currents.

To solve the problems mentioned above, a Bootstrap Charger for High Side Switch is proposed. The

main goal of the present circuit is to create a flexible and complete system capable of working over a

wide range of operating conditions. The idea is to combine a Bootstrap Charger with a Charge Pump

Charger in a way to optimize the charger of the cap. In Figure 4, a Bootstrap Charger and the Charge

Pump Charger can be seen. Both chargers take the power from V...