MIC4607 datasheet(28/42 Pages) MICROCHIP

28 / 42 page

MIC4607

DS20005610A-page 28

 2016 Microchip Technology Inc.

two functions: it provides decoupling for the high-side

circuitry and also provides current to the high-side cir-

cuit while the high-side external MOSFET is on.

Ceramic capacitors are recommended because of their

low impedance and small size. Z5U type ceramic

capacitor dielectrics are not recommended because of

the large change in capacitance over temperature and

voltage. A minimum value of 0.1 μF is required for CB

(xHB to xHS capacitors) and 1 μF for the VDD capacitor,

regardless of the MOSFETs being driven. Larger MOS-

FETs may require larger capacitance values for proper

operation. The voltage rating of the capacitors depends

on the supply voltage, ambient temperature and the

voltage derating used for reliability. 25V rated X5R or

X7R ceramic capacitors are recommended for most

applications. The minimum capacitance value should

be increased if low voltage capacitors are used

because even good quality dielectric capacitors, such

as X5R, will lose 40% to 70% of their capacitance value

at the rated voltage.

Placement of the decoupling capacitors is critical. The

bypass capacitor for VDD should be placed as close as

possible between the VDD and VSS pins. The bypass

capacitor (CB) for the xHB supply pin must be located

as close as possible between the xHB and xHS pins.

The etch connections must be short, wide, and direct.

The use of a ground plane to minimize connection

impedance is recommended. Refer to the “Grounding,

Component Placement and Circuit Layout” sub-section

for more information.

The voltage on the bootstrap capacitor drops each time

it delivers charge to turn on the MOSFET. The voltage

drop depends on the gate charge required by the MOS-

FET. Most MOSFET specifications specify gate charge

versus VGS voltage. Based on this information and a

recommended ΔVHB of less than 0.1V, the minimum

value of bootstrap capacitance is calculated as:

EQUATION 6-13:

Where:

QGATE = Total gate charge at VHB.

VHB = Voltage drop at the HB pin.

If the high-side MOSFET is not switched but held in an

on state, the voltage in the bootstrap capacitor will drop

due to leakage current that flows from the HB pin to

ground. This current is specified in the Electrical Char-

acteristics table. In this case, the value of CB is calcu-

lated as:

EQUATION 6-14:

Where:

IHBS = Maximum xHB pin leakage current.

tON = maximum high-side FET on-time.

The larger value of CB from Equation 6-13 or

Equation 6-14 should be used.

6.10

Grounding, Component

Placement and Circuit Layout

Nanosecond switching speeds and ampere peak cur-

rents in and around the MIC4607 driver require proper

placement and trace routing of all components.

Improper placement may cause degraded noise immu-

nity, false switching, excessive ringing, or circuit

latch-up.

Figure 6-9 shows the critical current paths of the high-

and low-side driver when their outputs go high and turn

on the external MOSFETs. It also helps demonstrate

the need for a low impedance ground plane. Charge

needed to turn-on the MOSFET gates comes from the

decoupling capacitors CVDD and CB. Current in the

low-side gate driver flows from CVDD through the inter-

nal driver, into the MOSFET gate, and out the source.

The return connection back to the decoupling capacitor

is made through the ground plane. Any inductance or

resistance in the ground return path causes a voltage

spike or ringing to appear on the source of the MOS-

FET. This voltage works against the gate drive voltage

and can either slow down or turn off the MOSFET

during the period when it should be turned on.

Current in the high-side driver is sourced from capaci-

tor CB and flows into the xHB pin and out the xHO pin,

into the gate of the high side MOSFET. The return path

for the current is from the source of the MOSFET and

back to capacitor CB. The high-side circuit return path

usually does not have a low-impedance ground plane

so the etch connections in this critical path should be

short and wide to minimize parasitic inductance. As

with the low-side circuit, impedance between the MOS-

FET source and the decoupling capacitor causes neg-

ative voltage feedback that fights the turn-on of the

MOSFET.

It is important to note that capacitor CB must be placed

close to the xHB and xHS pins. This capacitor not only

provides all the energy for turn-on but it must also keep

xHB pin noise and ripple low for proper operation of the

high-side drive circuitry.

GATE

V

-----------------



HBS



V

----------------------------



MIC4607 数据表(PDF) 28 Page - Microchip Technology

MIC4607 数据表(HTML) 28 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

数据表下载