MIC4102 Datasheet, PDF(19/28 Page) Micrel Semiconductor – 100V Half Bridge MOSFET Driver with Anti-Shoot Through Protection

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MIC4102 Datasheet, PDF (19/28 Pages) Micrel Semiconductor – 100V Half Bridge MOSFET Driver with Anti-Shoot Through Protection

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temperature and voltage. A minimum value of 0.1 ÂµF is

required for each of the capacitors, regardless of the

MOSFETs being driven. Larger MOSFETs 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 HB supply pin must be located as

close as possible between the HB and HS 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 section on

layout and component placement 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

MOSFET. Most MOSFET specifications specify gate

charge vs. VGS voltage. Based on this information and

a recommended âVHB of less than 0.1V, the minimum

value of bootstrap capacitance is calculated as:

EQUATION 6-16:

Where:

âVHB

ï³

----Q----G-----

ïVHB

Total Gate Charge at VHB

Voltage Drop at the HB Pin

The decoupling capacitor for the VDD input may be

calculated in with the same formula; however, the two

capacitors are usually equal in value.

6.9 Grounding, Component

Placement, and Circuit Layout

Nanosecond switching speeds and ampere peak

currents in and around the MIC4102 driver require

proper placement and trace routing of all components.

Improper placement may cause degraded noise

immunity, false switching, excessive ringing, or circuit

latch-up.

Figure 6-5 shows the critical current paths when the

driver outputs go high and turn on the external

MOSFETs. It also shows the need for a low impedance

ground plane. The charge needed to turn-on the

MOSFET gates comes from the decoupling capacitors

CVDD and CB. Current in the low-side gate driver flows

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MIC4102

from CVDD through the internal 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 MOSFET. This voltage works

against the gate voltage and can either slow down or

turn off the MOSFET during the period where it should

be turned on.

Current in the high-side driver is sourced from

capacitor CB, flows into the HB pin, and out the HO 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

MOSFET source and the decoupling capacitor causes

negative voltage feedback that fights the turn-on of the

MOSFET.

It is important to note that capacitor CB must be placed

close to the HB and HS pins. This capacitor not only

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

HB pin noise and ripple low for proper operation of the

high-side drive circuitry.

LOW-SIDE DRIVE

TURN-ON

CURRENT PATH

VDD

C VDD

GND

PLANE

HIGH-SIDE DRIVE

TURN-ON

CURRENT PATH

LEVEL

SHIFT

VSS

GND

PLANE

PWM

FIGURE 6-5:

Turn-On Current Paths.

Figure 6-6 shows the critical current paths when the

driver outputs go low and turn off the external

MOSFETs. Short, low impedance connections are

important during turn-off for the same reasons given in

the turn-on explanation. Current flowing through the

internal diode replenishes charge in the bootstrap

capacitor, CB.

DS20005575A-page 19

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