MIC4604 datasheet(12/18 Pages) MICREL

12 / 18 page

Micrel, Inc.

MIC4604

June 25, 2013

Revision 1.0

The die temperature can be calculated after the total

power dissipation is known.

total

Pdiss

Eq. 11

Where:

TA = maximum ambient temperature

TJ = junction temperature (°C)

Pdisstotal = power dissipation of the MIC4604

JA = thermal resistance from junction to ambient air

Propagation Delay and Other Timing Considerations

Propagation

delay

and

signal

timing

are

important

considerations. Many power supply topologies use two

switching MOSFETs operating 180° out of phase from

each other. These MOSFETs must not be on at the same

time or a short circuit will occur, causing high peak

currents and higher power dissipation in the MOSFETs.

The MIC4604 output gate drivers are not designed with

anti-shoot-through protection circuitry. The output drive

signals simply follow the inputs. The power supply design

must include timing delays (dead-time) between the input

signals to prevent shoot-through.

Make sure the input signal pulse width is greater than the

minimum specified pulse width. An input signal that is less

than the minimum pulse width may result in no output

pulse or an output pulse whose width is significantly less

than the input.

The maximum duty cycle (ratio of high side on-time to

switching period) is controlled by the minimum pulse width

of the low side and by the time required for the CB

capacitor to charge during the off-time. Adequate time

must be allowed for the CB capacitor to charge up before

the high-side driver is turned on.

Decoupling and Bootstrap Capacitor Selection

Decoupling capacitors are required for both the low side

(VDD) and high side (HB) supply pins. These capacitors

supply the charge necessary to drive the external

MOSFETs and also minimize the voltage ripple on these

pins. The capacitor from HB to HS has two functions: it

provides decoupling for the high-side circuitry and also

provides current to the high-side circuit while the high-side

external

MOSFET

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 0.1µF

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

the

section

Grounding,

Component Placement and Circuit Layout 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 versus

Vgs

voltage.

Based

this

information

and

recommended

ΔV

HB of less than 0.1V, the minimum value

of bootstrap capacitance is calculated as:

gate

∆

≥

Eq. 12

Where:

Qgate = total gate charge at VHB

∆V

HB = 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.

Grounding, Component Placement and Circuit Layout

Nanosecond switching speeds and ampere peak currents

in and around the MIC4604 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 8 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 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 internal driver, into the MOSFET gate, and out the

source. The return connection back to the decoupling

capacitor

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 when it should be turned on.

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