Datasheet

OUT1

OUT2

Forward drive

Slow decay

Fast decay

Reverse drive

Slow decay

Fast decay

OUT1 OUT2

FORWARD

REVERSE

DRV8844

SLVSBA2A –JULY 2012–REVISED OCTOBER 2012

www.ti.com

Table 2. H-Bridge Logic

INx ENx OUTx

X 0 Z

0 1 L

1 1 H

The inputs can also be used for PWM control of, for example, the speed of a DC motor. When controlling a

winding with PWM, when the drive current is interrupted, the inductive nature of the motor requires that the

current must continue to flow. This is called recirculation current. To handle this recirculation current, the H-

bridge can operate in two different states, fast decay or slow decay. In fast decay mode, the H-bridge is disabled

and recirculation current flows through the body diodes; in slow decay, the motor winding is shorted.

To PWM using fast decay, the PWM signal is applied to the ENx pin; to use slow decay, the PWM signal is

applied to the INx pin. Table 3 is an example of driving a DC motor using OUT1 and OUT2 as an H-bridge:

Table 3. PWM Function

IN1 EN1 IN2 EN2 FUNCTION

PWM 1 0 1 Forward PWM, slow decay

0 1 PWM 1 Reverse PWM, slow decay

1 PWM 0 PWM Forward PWM, fast decay

0 PWM 1 PWM Reverse PWM, fast decay

The drawings below show the current paths in different drive and decay modes:

Figure 3. Current Paths

Charge Pump

Since the output stages use N-channel FETs, a gate drive voltage higher than the VM power supply is needed to

fully enhance the high-side FETs. The DRV8844 integrates a charge pump circuit that generates a voltage above

the VM supply for this purpose.

The charge pump requires two external capacitors for operation. Refer to the block diagram and pin descriptions

for details on these capacitors (value, connection, etc.).

The charge pump is shut down when SLEEPn is active low.

Product Folder Links: DRV8844