ML4810 Ver la hoja de datos (PDF) - Micro Linear Corporation
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ML4810 Datasheet PDF : 12 Pages
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FUNCTIONAL DESCRIPTION
SOFT START AND CURRENT LIMIT
The ML4810/11 offers a unique system of fault detection
and reset. Most PWM controllers use a two threshold
method which relies on the buildup of current in the
output inductor during a fault. This buildup occurs
because:
1. Inductor di/dt is a small number when the switch is off
under load fault (short circuit) conditions, since VL is
small.
2. Some energy is delivered to the inductor since the IC
must first detect the over-current because there is a
finite delay before the output switch can turn off.
TPD
TPD
TPD
VTH2
VTH1
SWITCH CURRENT
DIODE CURRENT
Figure 1. Current Waveforms for Slow Turn-Off System
with Load Fault
This scheme was adequate for controllers with longer
comparator propagation delays and turn-off delays than is
desirable in a high frequency system. For systems with
low propagation delays, very little energy will be
delivered to the inductor and the current “ratcheting”
described above will not occur. This results in the
controller never detecting the load fault and continuing to
pump full current to the load indefinitely, causing heating
in the output rectifiers and inductor.
TPD
TPD
TPD
VTH2
VTH1
SWITCH CURRENT
DIODE CURRENT
Figure 2. Current Waveforms for High Speed System
with Load Fault
ML4810, ML4811
A method of circumventing this problem involves
“counting” the number of times the controller terminates
the PWM cycle due to the cycle by cycle current limit.
When the switch current crosses the 1.1V threshold A1
signals the F1 to terminate the cycle and sets F3, which is
reset at the beginning of the PWM cycle. The output of F3
turns on a current source to charge C2. When, after
several cycles, C2 has charged to 2.45V, A5 turns on F2 to
discharge soft start capacitor C1. Charge is short lived (for
instance a disk drive start-up or a board being plugged
into a live rack) the control can “ride out” the surge with
the switch protected by the cycle by cycle limit. R1 and
C1 can be selected to track diode heating, or to ride out
various system surge requirements as required.
If the high current demand is caused by a short circuit, the
duty cycle will be short and the output diodes will carry
the current for the majority of PWM cycle. C2 charges
fastest for low duty cycles (since F3 will be on for a longer
time) providing for quicker shutdown during short-circuit
when the output diodes are being maximally stressed.
RESET DELAY
20
V+ ERROR
AMP
QI
A3
FROM OSC.
I(1)
I(1)
SOFT START
9
C1
1.1V
2.45V
RCRESET
4
–
– A4
+
–
A5
+
Q2
R
F2
Q
C2
I(2)
S
4UL0
R1
1.5V
–
A2
+
ILIM/SD
12
1.1V
+
A1
–
QR
F3
S
R Q G2
F1
S
FROM PWM
COMPARATOR
QI
G1
FROM OSC.
Figure 3. Integrating Soft Start Reset
5
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