PRELIMINARY
FUNCTIONAL DESCRIPTION
SOFT START
The voltage at SOFT START (pin 19) sets the current limit
threshold. The ML4435 has an internal voltage divider
with a 1.1V supply voltage. This circuit is shown in Figure
3. The divider consists of two 225k
Ω
resistors setting
the current limit threshold to approximately 0.55V. An
external voltage divider off of VCC or an external reference
can be used to override the default setting of SOFT START
by using a divider with 10 times the current draw of the
internal divider.
RSENSE
ML4435
The function of RSENSE is to provide a voltage propor-
tional to the motor current, for current limiting. The default
trip voltage across RSENSE is 0.6V as set by the SOFT
START (pin 19). The current sense resistor should be a
low inductance resistor such as a carbon composition. For
resistors in the milli ohms range wire wound resistors tend
to have low values of inductance. RSENSE can be selected
using Figure 4. The power rating of RSENSE should be
sized to handle the power dissipation (I MAX squared
times RSENSE) seen at maximum current.
ISENSE FILTER
. V
225kΩ
SOFT START
9
CSOFT START
0.5V
225kΩ
An RC lowpass filter is required at the ISENSE input pin
to remove the voltage spike on the leading edge of the
current sense signal caused by the diode reverse recovery
shoot through current. Absent the filter, false triggering of
the current limit could occur.
The recommended starting values for this circuit are
R = 1KΩ and C = 1000pF a configuration that will filter
out spikes less than 1µs long. It is recommended that the
capacitor value not be increased beyond 330pF.
Pulse-By-Pulse Current Limiting
Figure 3. SOFT START Function
A capacitor to ground on the SOFT START pin can be used
to provide a soft ramping of the current limit on power up.
The ramp time can be selected using Figure 4.
When current limit is activated by the voltage on ISENSE
exceeding the voltage on SOFT START the current limit is
tripped, turning off LA, LB, and LC for the remainder of
the PWM period.
COMMUTATION CONTROL
A 3 phase Brushless DC motor requires electronic com-
30
RAMP TIME s
5
BEFORE FILTERIN
0
0
50
C SOFT START
00
F
AFTER FILTERIN
Figure 4. SOFT START Ramp Time vs. CSOFTSTART
Figure 5. ISENSE Filter Wave Forms
6
PRELIMINARY DATASHEET
May, 2000
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
