A4935 datasheet(18/24 Pages) ALLEGRO

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Automotive 3-Phase MOSFET Driver

A4935

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

is set to zero by connecting RDEAD to AGND, the fault blank

time typically defaults to 2 μs.

Cross-Conduction

In some circumstances it is desirable to allow activation of both

high-side and low-side FETs in a single phase of the power

bridge. This can be used, with care, to reduce diode conduction

during synchronous rectification, which improves overall effi-

ciency and reduces electromagnetic emissions.

This mode of operation is also useful for independently control-

ling each phase of a VR motor or to turn on all high-side and low-

side FETs together, to cause a supply short circuit for blowing a

safety fuse.

Cross-conduction can occur only when the internally-generated

dead time is set to zero by connecting RDEAD to AGND and, at

the same time, CCEN is high. If zero internally-generated dead

time is required, but cross-conduction is to be prevented, then

CCEN can be tied to AGND. When the dead time is set to zero

it is still possible for some overlap to be present at the switching

instants due to the relative switching time of the FETs.

Bootstrap Capacitor Selection

The bootstrap capacitors, CBOOTx, must be correctly selected to

ensure proper operation of the A4935. If the capacitances are too

high, time will be wasted charging the capacitor, resulting in a

limit on the maximum duty cycle and the PWM frequency. If the

capacitances are too low, there can be a large voltage drop at the

time the charge is transferred from CBOOTx to the FET gate, due

to charge sharing.

To keep this voltage drop small, the charge in the bootstrap

capacitor, QBOOT, should be much larger than the charge required

by the gate of the FET, QGATE. A factor of 20 is a reasonable

value, and the following formula can be used to calculate the

value for CBOOT:

QBOOT

CBOOT × VBOOT = QGATE × 20 ,

therefore:

QGATE × 20

VBOOT

CBOOT =

(3)

where VBOOT is the voltage across the bootstrap capacitor.

The voltage drop across the bootstrap capacitor as the FET is

being turned on, ∆V, can be approximated by:

QGATE

CBOOT

∆V ≈

(4)

So, for a factor of 20, ∆V would be approximately 5% of VBOOT.

The maximum voltage across the bootstrap capacitor under

normal operating conditions is VREG(max). However, in some

circumstances the voltage may transiently reach 18 V, the clamp

voltage of the Zener diodes between the Cx and Sx pins. In most

applications, with a good ceramic capacitor the working voltage

can be limited to 16 V.

Bootstrap Charging

It is good practice to ensure the high-side bootstrap capacitor is

completely charged before a high-side PWM cycle is requested.

The time required to charge the capacitor, tCHARGE (μs), is

approximated by:

CBOOT × ∆V

100

tCHARGE

(5)

where CBOOT is the value of the bootstrap capacitor, in nF, and

∆V is the required voltage of the bootstrap capacitor.

At power-up and when the drives have been disabled for a long

time, the bootstrap capacitor can be completely discharged. In

this case ∆V can be considered to be the full high-side drive

voltage, 12 V. Otherwise, ∆V is the amount of voltage dropped

during the charge transfer, which should be 400 mV or less.

The capacitor is charged whenever the Sx pin is pulled low and

current flows from VREG through the internal bootstrap diode

circuit to CBOOT.

Bootstrap Charge Management

The A4935 provides automatic bootstrap capacitor charge

management. The bootstrap capacitor voltage for each phase

is continuously checked to ensure that it is above the bootstrap

under-voltage threshold, VBOOTUV. If the bootstrap capacitor volt-

age drops below this threshold, the A4935 will turn on the neces-

sary low-side FET, and continue charging until the bootstrap

capacitor exceeds the undervoltage threshold plus the hysteresis,

VBOOTUV + VBOOTUVhys. The minimum charge time is typically

7 μs, but may be longer for very large values of bootstrap capaci-

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