MIC4607 datasheet(24/42 Pages) MICROCHIP

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MIC4607

DS20005610A-page 24

 2016 Microchip Technology Inc.

turn-off. The xLO driver turns on after a short delay

(tLOON). Once the xLO driver is turned on, it is latched

on until the xPWM signal goes high. This prevents any

ringing or oscillations on the switch node or xHS pin

from turning off the xLO driver. If the xPWM pin goes

low and the voltage on the xHS pin does not cross the

VSWTH threshold, the xLO pin will be forced high after

a short delay (tSWTO), ensuring proper operation.

The internal logic circuits also ensure a “first on” priority

at the inputs. If the xHO output is high, the xLI pin is

inhibited. A high signal or noise glitch on the xLI pin has

no effect on the xHO or xLO outputs until the xHI pin

goes low. Similarly, xLO being high holds xHO low until

xLI and xLO are low.

Fast propagation delay between the input and output

drive waveform is desirable. It improves overcurrent

protection by decreasing the response time between

the control signal and the MOSFET gate drive. Minimiz-

ing propagation delay also minimizes phase shift errors

in power supplies with wide bandwidth control loops.

Care must be taken to ensure that the input signal

pulse width is greater than the minimum specified pulse

width. An input signal that is less than the minimum

pulse width can 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 determined by the time required for

the CB capacitor to charge during the off-time. Ade-

quate time must be allowed for the CB capacitor to

charge up before the high-side driver is turned back on.

Although the adaptive dead-time circuit in the MIC4607

prevents the driver from turning both MOSFETs on at

the same time, other factors outside of the

anti-shoot-through

circuit’s

control

can

cause

shoot-through. Other factors include ringing on the gate

drive node and capacitive coupling of the switching

node voltage on the gate of the low-side MOSFET.

The scope photo in Figure 6-4 shows the dead time

(<20 ns) between the high- and low-side MOSFET

transitions as the low-side driver switches off while the

high-side driver transitions from off to on.

FIGURE 6-4:

Adaptive Dead-Time LO

(LOW) to HO (HIGH).

Table 6-1 contains truth tables for the MIC4607-1 (inde-

pendent TTL inputs) and Table 6-2 is for the

MIC4607-2 (PWM inputs) that details the “first on” pri-

ority as well as the failsafe delay (tSWTO).

6.2

HS Node Clamp

A resistor/diode clamp between the switching node and

the HS pin is necessary to clamp large negative

glitches or pulses on the HS pin.

Figure 6-5 shows the Phase A section high-side and

low-side MOSFETs connected to one phase of the

three phase motor. There is a brief period of time (dead

time) between switching to prevent both MOSFETs

from being on at the same time. When the high-side

MOSFET is conducting during the on-time state, cur-

rent flows into the motor. After the high-side MOSFET

turns off, but before the low-side MOSFET turns on,

current from the motor flows through the body diode in

parallel with the low-side MOSFET. Depending upon

the turn-on time of the body diode, the motor current,

and circuit parasitics, the initial negative voltage on the

switch node can be several volts or more. The forward

voltage drop of the body diode can be several volts,

depending on the body diode characteristics and motor

current.

Even though the HS pin is rated for negative voltage, it

is good practice to clamp the negative voltage on the

HS pin with a resistor and possibly a diode to prevent

excessive negative voltage from damaging the driver.

Depending upon the application and amount of nega-

tive voltage on the switch node, a 3Ω resistor is recom-

TABLE 6-1:

MIC4607-1 TRUTH TABLE

xLI

xHI

xLO

xHO

Comments

Both outputs off.

xHO will not go HIGH

until xLO falls below

1.9V.

xLO will be delayed an

extra 250 ns if xHS

never falls below 2.2V.

First ON stays on until

input of same goes

LOW.

TABLE 6-2:

MIC4607-2 TRUTH TABLE

xPWM

xLO

xHO

Comments

xLO will be delayed an

extra 250 ns if xHS

never falls below 2.2V.

xHO will not go HIGH

until xLO falls below

1.9V.

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