Datasheet 2

AN120 – UNDERSTANDING MP6500 CURRENT CONTROL
AN120 Rev. 1.0 www.MonolithicPower.com 7
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I
OUT
I
TRIP
Level
Slow Decay During t
OFF
Unless I
OUT
> I
TRIP
at end of t
OFF
t
OFF
Fast Decay
I
TRIP
Change
Step Pulse
t
OFF
Slow Decay
Figure 6: MP6500 Automatic Decay during Step Transition
In cases where the supply voltage is high, the inductance is low or the desired current regulation level is
very low, so it is possible that the current will increase above the desired regulation level significantly.
This occurs during the blanking time, which results in a minimum PWM on time at the beginning of the
PWM cycle. This causes many legacy stepper motor drivers to lose control of the winding current. In the
MP6500, if this occurs, fast decay cycles are used to drive the current below the desired level (see Figure
7).
I
TRIP
Level
Fast
Decay
Slow
Decay
t
OFF
Current Regulation of Low Current / Low Inductance
t
ON_MIN
t
OFF
I
OUT
Figure 7: MP6500 Auto-Decay at Low Current
A small shift in the average current can still be seen compared to the regulation using only slow decay.
Since fast decay is only used long enough to drive the current below the desired level, the error is much
less than if fast decay were to be used for the entire PWM off time.