AN2839 Datasheet, PDF (6/40 Pages) STMicroelectronics – L6228 fully-integrated two-phase stepper motor driver

Designing an application with L6228
Figure 4. Voltage at the two outputs at the beginning of the off-time
AN2839
2XW
2XW
!-V
1.3
Choosing the bulk capacitor
Since the bulk capacitor, placed between the V_S and GND pins, is charged and discharged
during IC operation, its AC current capability must be greater than the RMS value of the
charge/discharge current. This current flows from the capacitor to the IC during the on time
(t_ON) and from the IC (in fast decay; from the power supply in slow decay) to the capacitor
during the off time (t_OFF). The RMS value of the current flowing into the bulk capacitor
depends on the peak output current, output current ripple, switching frequency, duty cycle
and chopping style. It also depends on the characteristics of the power supply. A power
supply with poor high-frequency performances (or long, inductive connections to the IC) will
cause the bulk capacitor to be recharged slowly: the higher the current control switching
frequency, the higher the current ripple in the capacitor; RMS current in the capacitor,
however, does not exceed the RMS output current. The bulk capacitor value (C) and the
ESR determine the amount of voltage ripple on the capacitor itself and on the IC. In slow
decay, neglecting the dead-time and output current ripple, and assuming that during the on
time the capacitor is not recharged by the power supply, the voltage at the end of the on time
is:
Equation 1
so the supply voltage ripple is:
VS – IOUT ⋅ _⎝⎛ESR + ^t--^O--_C----^N--_⎠⎞
Equation 2
IOUT ⋅ _⎝⎛ESR + ^t--^O--_C----^N--_⎠⎞
where I_OUT is the output current. In fast decay, instead, the recirculating current recharges
the capacitor, causing the supply voltage to exceed the nominal voltage. This can be very
dangerous if the nominal supply voltage is close to the maximum recommended supply
voltage (52 V).
In fast decay the supply voltage ripple is about:
6/40
Doc ID 15104 Rev 1