DRV8844_15 Datasheet, PDF (10/25 Pages) Texas Instruments – DRV8844 Quad ½-H-Bridge Driver IC

DRV8844
SLVSBA2C – JULY 2012 – REVISED MAY 2015
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Feature Description (continued)
8.3.3 Bridge Control
The INx input pins directly control the state (high or low) of the OUTx outputs; the ENx input pins enable or
disable the OUTx driver. Table 1 shows the logic.
Table 1. 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 2 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 2. PWM Function
IN2
0
PWM
0
1
EN2
1
1
PWM
PWM
FUNCTION
Forward PWM, slow decay
Reverse PWM, slow decay
Forward PWM, fast decay
Reverse PWM, fast decay
Figure 6 shows 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 6. Current Paths
8.3.4 Charge Pump
Because 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.
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