DRV8844 Datasheet, PDF (8/17 Pages) Texas Instruments – QUAD 1/2-H-BRIDGE DRIVER IC

DRV8844
SLVSBA2 – JULY 2012
Table 2. H-Bridge Logic
INx
ENx
OUTx
X
0
Z
0
1
L
1
1
H
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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:
IN1
PWM
0
1
0
EN1
1
1
PWM
PWM
Table 3. PWM Function
IN2
0
PWM
0
1
EN2
1
1
PWM
PWM
FUNCTION
Forward PWM, slow decay
Forward PWM, slow decay
Reverse PWM, fast decay
Reverse PWM, fast decay
The drawings below show the current paths in different drive and decay modes:
VM
VM
OUT1
1 Forward drive
1
2 Fast decay
OUT2 _{3 Slow decay}
OUT1
2
3
1 Reverse drive
1
2 Fast decay
OUT2
3 Slow decay
2
3
FORWARD
REVERSE
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.
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