ML4863 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4863 Datasheet PDF : 10 Pages
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ML4863
FUNCTIONAL DESCRIPTION (Continued)
SYNCHRONOUS RECTIFIER CONTROL
The control circuitry for the synchronous rectifier does not
influence the operation of the main controller. The
synchronous rectifier is turned on during the minimum off
time, or whenever the SENSE pin is less than –18mV.
During transitions where the primary switch is turned on
before the voltage on the SENSE pin goes above –18mV,
the gate of the synchronous rectifier is discharged softly to
avoid accidently triggering the current-mode comparator
with the gate discharge spike on the sense resistor.
The part will also operate with a Schottky diode in place
of the synchronous rectifier, but the conversion efficiency
will suffer.
CURRENT LIMIT AND MODES OF OPERATION
The normal operating range and current limit point are
determined by the current programming comparator. They
are dependent on the value of the synchronous rectifier
current sense resistor (RSENSE), the nominal transformer
primary inductance (LP), and the input voltage.
RSENSE can be calculated by:
0 5 0 5 0 05 5 0 5 RSENSE
=
VIN MIN
VOUT + VIN
´
150mV
IOUT MAX
+
VIN MIN
20 ´ VIN MAX ´ IOUT MAX
´ h (1)
where h = converter efficiency.
Once RSENSE has been determined, LP can be found:
0 5 LP = (25 × 10−6 ) × VIN MAX × RSENSE
(2)
Three operational modes are defined by the voltage at the
SENSE pin at the end of the off-time: discontinuous mode,
continuous mode, and current limit. The following values
can be used to determine the current levels of each mode:
VSENSE < 0V: discontinuous mode
0V < VSENSE < 160mV: continuous mode
160mV < VSENSE < 235mV: current limit
Inserting the maximum value of VSENSE for each
operational mode into the following equation will
determine the maximum current levels for each
operational mode:
IOUT
=
VIN
VOUT + VIN
×
VSENSE + t ON × VIN
RSENSE
2 × LP
×η
(3)
6
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