MIC4100 datasheet(15/18 Pages) MICREL

15 / 18 page

Micrel, Inc.

MIC4100/1

March 2006

M9999-031506

The bypass

he MOSFET.

capacitors are recommended for most applications. The

minimum capacitance value should be increased if low

voltage capacitors are use since 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.

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 t

Most MOSFET specifications specify gate charge vs. Vgs

voltage. Based on this information and a recommended

∆VHB of less than 0.1V, the minimum value of bootstrap

capacitance is calculated as:

at the

drop

Voltage

∆V

Charge

Gate

Total

where

gate

∆

≥

gate

The decoupling capacitor for the Vdd input may be

calculated in with the same formula; however, the two

capacitors are usually equal in value.

pere peak currents

round

the current

ust also keep HB

Grounding, Component Placement and Circuit Layout

Nanosecond switching speeds and am

in and around the MIC4100 and MIC4101 drivers 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 9 shows the critical current paths when the driver

outputs go high and turn on the external MOSFETs. It also

helps demonstrate the need for a low impedance g

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 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 drive

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

and flows into the HB pin and out the HO pin, into the gate

of the high side MOSFET. The return path for

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

which 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 m

pin noise and ripple low for proper operation of the high-

side drive circuitry.

Vdd

Level

shift

Vss

Vdd

gnd

plane

gnd

plane

Low-side drive turn-on

current path

High-side drive turn-on

current path

Turn-On Current Paths

Figure 9

Figure 10 shows the critical cu

t paths when the driver

o low and turn

ernal MOSFETs. Short,

w impedance connections are important during turn-off

rren

off the ext

outputs g

for the same reasons given in the turn-on explanation.

Current flowing through the internal diode replenishes

charge in the bootstrap capacitor, CB.

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