MIC4604 datasheet(9/18 Pages) MICREL

9 / 18 page

Micrel, Inc.

MIC4604

June 25, 2013

Revision 1.0

The bootstrap circuit consists of an internal diode and

external capacitor, CB. In a typical application, such as the

synchronous buck converter shown in Figure 4, the HS pin

is at ground potential while the low-side MOSFET is on.

The internal diode allows capacitor CB to charge up to

VDD-VF during this time (where VF is the forward voltage

drop of the internal diode). After the low-side MOSFET is

turned off and the HO pin turns on, the voltage across

capacitor CB is applied to the gate of the upper external

MOSFET. As the upper MOSFET turns on, voltage on the

HS pin rises with the source of the high-side MOSFET until

it reaches VIN. As the HS and HB pin rise, the internal

diode is reverse biased preventing capacitor CB from

discharging.

Figure 4. High-Side Driver and Bootstrap Circuit Block

Diagram

Programmable Gate Drive

The MIC4604 offers programmable gate drive, which

means the MOSFET gate drive (gate to source voltage)

equals the VDD voltage. This feature offers designers

flexibility in driving the MOSFETs. Different MOSFETs

require different VGS characteristics for optimum RDSON

performance. Typically, the higher the gate voltage (up to

16V), the lower the RDSON achieved. For example, a 4899

MOSFET can be driven to the ON state at 4.5V gate

voltage but RDSON

is 7.5mΩ. If driven to 10V gate voltage,

RDSON

is 4.5mΩ. In low-current applications, the losses due

to RDSON are minimal, but in high-current applications such

as power hand tools, the difference in RDSON can cut into

the efficiency budget.

portable

hand

tools

and

other

battery-powered

applications, the MIC4604 offers the ability to drive motors

at a lower voltage compared to the traditional MOSFET

drivers because of the wide VDD range (5.5V to 16V).

Traditional MOSFET drivers typically require a VDD

greater than 9V. The MIC4604 drives a motor using only

two Li-ion batteries (total 7.2V) compared to traditional

MOSFET drivers which will require at least three cells

(total of 10.8V) to exceed the minimal VDD range. As an

additional benefit, the low 5.5V gate drive capability allows

a longer run time. This is because the Li-ion battery can

run down to 5.5V, which is just above its 4.8V minimum

recommended discharge voltage. This is also a benefit in

higher current power tools that use five or six cells. The

driver can be operated up to 16V to minimize the RDSON of

the MOSFETs and use as much of the discharge battery

pack as possible for a longer run time. For example, an

18V battery pack can be used to the lowest operating

discharge voltage of 13.5V.

Application Information

Power Dissipation Considerations

Power dissipation in the driver can be separated into three

areas:

•

Internal diode dissipation in the bootstrap circuit

•

Internal driver dissipation

•

Quiescent current dissipation used to supply the

internal logic and control functions.

Bootstrap Circuit Power Dissipation

Power dissipation of the internal bootstrap diode primarily

comes from the average charging current of the CB

capacitor multiplied by the forward voltage drop of the

diode. Secondary sources of diode power dissipation are

the reverse leakage current and reverse recovery effects

of the diode.

The average current drawn by repeated charging of the

high-side MOSFET is calculated by:

gate

)

AVE

(

Eq. 1

Where:

Qgate = total gate charge at VHB

fs = gate drive switching frequency

The average power dissipated by the forward voltage drop

of the diode equals:

)

AVE

(

fwd

Pdiode

Eq. 2

Where:

VF = diode forward voltage drop

The value of VF should be taken at the peak current

through the diode; however, this current is difficult to

calculate because of differences in source impedances.

The peak current can either be measured or the value of

VF at the average current can be used, which will yield a

good approximation of diode power dissipation.

The reverse leakage current of the internal bootstrap diode

is typically 2µA at a reverse voltage of 85V at 125C. Power

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