AN1625 STMicroelectronics, AN1625 Datasheet - Page 5

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

2.3 Choosing the Bulk Capacitor
Since the bulk capacitor, placed between V
AC current capability must be greater than the r.m.s. value of the charge/discharge current. This current flows
from the capacitor to the IC during the on-time (t
power supply during off-time) to the capacitor during the off-time (t
into the bulk capacitor depends on peak output current, output current ripple, switching frequency, duty-cycle.
It also depends on power supply characteristics. 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; r.m.s. current in the capacitor, how-
ever, does not exceed the r.m.s. output current. Bulk capacitor value (C) and the ESR determine the amount of
voltage ripple on the capacitor itself and on the IC. Neglecting the 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
where I
supply voltage ripple can be estimated as
For Example, if a maximum ripple of 500 mV is allowed and I
Note that additional ripple is due to parasitic inductances on V
Range section).
Actually, current sunk by V
charge of internal freewheeling diodes. Duration of these peaks is, tough, very short (100÷200 ns) and can be
filtered using a small value (100÷200 nF), good quality ceramic capacitor, connected as close as possible to the
V
than the maximum supply voltage, considering also power supply tolerances. For example, with a 48 V nominal
power supply, with 5% tolerance, maximum voltage is 50.4 V, then operating voltage for the capacitor should
be at least 63 V.
2.4 Layout Considerations
Working with devices that combine high power switches and control logic in the same IC careful attention has
to be paid to the PCB layout. In extreme cases, Power DMOS commutation can induce noises that could cause
improper operation in the logic section of the device. Noise can be radiated by high dv/dt nodes or high di/dt
paths, or conducted through GND or Supply connections. Logic connections, especially high-impedance nodes
(actually all logic inputs, see further), must be kept far from switching nodes and paths. With the L6235, in par-
ticular, external components for the charge pump circuitry should be connected together through short paths,
since these components are subject to voltage and current switching at relatively high frequency (600 kHz). Pri-
mary mean in minimizing conducted noise is working on a good GND layout (see Figure 5).
SA
, V
SB
OUT
and GND pins of the IC. Bulk capacitor will be chosen with maximum operating voltage 25% greater
is the output current. Usually (if C>100 µF) the capacitance role is much less than the ESR, then
SA
and V
SB
pins of the device is subject to higher peaks due to reverse recovery
S
V
ESR
and GND pins, is charged and discharged during IC operation, its
S
–
I
O UT
ON
I
O UT
0.5V
------------
2 A
) and from the IC (during some phase changes; from the


ESR
ESR
=
250m
OUT
+
S
t
-------- -
O N
PCB tracks (see Voltage Ratings and Operating
C
= 2 A, the capacitor ESR should be lower than


OFF
). The r.m.s. value of the current flowing
AN1625 APPLICATION NOTE
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