DRV8704 Datasheet, PDF (14/38 Pages) Texas Instruments – Dual H-Bridge PWM Gate Driver

DRV8704
SLVSD29 – OCTOBER 2015
VM
PWM
ON
Slow Decay
PWM OFF
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Fast Decay
1
1 Drive Current
xOUT1
xOUT2 ^{2 Fast decay (reverse)}
3
3 Slow decay (brake)
2
Mixed Decay
TBLANK
TDECAY
Itrip
TOFF
Figure 9. Decay Mode Current
Figure 10. Decay Mode Comparison
The DRV8704 supports fast decay and slow decay modes. In addition it supports fixed mixed decay and auto
mixed decay modes. Decay mode is selected by the DECMOD bits in the DECAY register.
Mixed decay mode begins as fast decay, but after a programmable period of time (set by the TDECAY bits in the
DECAY register) switches to slow decay mode for the remainder of the fixed off time.
Auto mixed decay mode samples the current level at the end of the blanking time, and if the current is above the
Itrip threshold, immediately changes the H-bridge to fast decay. During fast decay, the (negative) current is
monitored, and when it falls below the Itrip threshold (and another blanking time has passed), the bridge is
switched to slow decay. Once the fixed off time expires, a new cycle is started.
If the bridge is turned on and at the end of t_BLANK the current is below the Itrip threshold, the bridge remains on
until the current reaches Itrip. Then slow decay is entered for the fixed off time, and a new cycle begins.
Refer to Figure 11.
The upper waveform shows the behavior if I < Itrip at the end of t_BLANK. This is a stable, slow decay mode of
operation.
The lower waveform shows what happens when I > Itrip at the end of t_BLANK. Note that (at slow motor speeds,
where back EMF is not significant), the current increase during the ON phase is the same magnitude as the
current decrease in fast decay, since both times are controlled by t_BLANK, and the rate of change is the same (full
VM is applied to the load inductance in both cases, but in opposite directions). In this case, the current will
gradually be driven down until the peak current is just hitting Itrip at the end of the blanking time, after which
some cycles will be slow decay, and some will be mixed decay.
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