ML4425 Ver la hoja de datos (PDF) - Micro Linear Corporation
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ML4425 Datasheet PDF : 16 Pages
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ML4425
FUNCTIONAL DESCRIPTION (Continued)
CROSS CONDUCTION COMPARATOR
When the ML4425 goes from align mode into ramp mode,
there is a possibility of cross conduction in phase 3 of the
bridge power stage. This cross conduction can happen
when HC is on in the align mode shown as state R in
Table 1, and the controller transitions to state A in ramp
mode where HC is turned off and LC is turned on. Cross
conduction can appear due to the differences in turn on
and turn off times of the power devices. To solve this
problem, the LC output driver is gated off until the HC is
equal to VDD – 3V as shown in Figure 10.
BRAKING
When the BRAKE pin is pulled below 1.4V, the low side
output drivers LA, LB, and LC are turned on and the high
side output drivers HA, HB, HC are turned off. Braking
causes rapid deceleration of the motor and current
limiting is de-activated, and care should be taken when
using the BRAKE pin. BRAKE is has an internal 4kW pull-
up as shown in Figure 10, and can be driven by a switch
to ground, an open collector or drain logic signal, or a TTL
logic signal.
UNDERVOLTAGE LOCKOUT
Undervoltage lockout is used to protect the 3-phase
bridge power stage from a low VDD condition.
Undervoltage is triggered at VDD of 9.5V or less and is
indicated by a TTL low output on the UV FAULT pin.
Undervoltage lockout also turns off all output drivers (LA,
LB, LC, HA, HB, and HC). The comparator that triggers
undervoltage lockout has 150mV of hystresis.
DESIGN CONSIDERATIONS
INTERFACING TO A 3-PHASE BRIDGE POWER STAGE
The ML4425 output drivers are configured to drive a 3
phase bridge power stage. For applications with buss
voltages from 12V up to 80V, level shifting circuitry can
be used to drive higher voltage P-channel MOSFETS for
the high side switches as shown in Figure 11.
The most flexible configuration is to use high side drivers
to control N-Channel MOSFETs (or IGBTs) which allows
applications from less than 12V up to 600V. Figure 12
shows the interface between the ML4425 and IR2118 high
side drivers from International Rectifier. This configuration
is capable of driving motors from busses of up to 320V.
The BRAKE pin can be pulsed prior to startup with an RC
circuit. This charges the bootstrap capacitors (C19, C20,
and C21) for the three high side drivers, allowing the reset
phase to operate normally. These capacitors must be sized
so that they stay sufficiently charged during the align
mode. Refer to AN-43 for additional applications
information on the ML4425.
FROM
COMMUTATION
STATE MACHINE
FROM
SPEED CONTROL LOOP
& CURRENT LIMIT
–
1.4V
+
9.5V +
–
VDD
GATING
LOGIC
&
OUTPUT
DRIVERS
4kΩ
REFERENCE
HA
2
HB
3
HC
4
LA
9
LB
10
LC
11
UV FAULT
18
BRAKE
25
VDD
GND
RREF
VREF
14
28
27
7
Figure 10. Cross Conduction, Brake, and UVLO Circuits
12
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