Denon DRA-F107 Service Manual page 42

the Current-Limit Comparator cannot switch off the
gate drive.
3.5.2 Propagation Delay Compensation
In case of overcurrent detection by I
of CoolMOS™ is delayed due to the propagation delay
of the circuit. This delay causes an overshoot of the
peak current I which depends on the ratio of dI/dt of
peak
the peak current (see Figure 17).
.
Signal1
I
Sense
I
I
Overshoot2
peak2
I
peak1
I
Limit
Figure 17 Current Limiting
The overshoot of Signal2 is bigger than of Signal1 due
to the steeper rising waveform.
A propagation delay compensation is integrated to
bound the overshoot dependent on dI/dt of the rising
primary current. That means the propagation delay
time between exceeding the current sense threshold
V and the switch off of CoolMOS™ is compensated
csth
over temperature within a range of at least.
dI dV
peak
0 R
----------- - ---------------
Sense
dt
V
OSC
max. Duty Cycle
V
Sense
V
csth
Signal1
Figure 18 Dynamic Voltage Threshold V
the shut down
Limit
Signal2
t
Propagation Delay
I
Overshoot1
t
Sense
dt
off time
Propagation Delay
Signal2
csth
DRA-F107 / DRA-F107DAB
The propagation delay compensation is done by
means of a dynamic threshold voltage V
18). In case of a steeper slope the switch off of the
driver is earlier to compensate the delay.
E.g. I = 0.5A with R
peak
delay compensation the current sense threshold is set
to a static voltage level V
dI/dt = 0.4A/μs, that means dV
propagation delay time of i.e. t
leads then to a I
peak
propagation delay compensation the overshoot is only
about 2% (see Figure 19).
with compensation
V
1.3
1.25
1.2
1.15
1.1
1.05
1
0.95
0.9
0 0.2 0.4 0.6
Figure 19 Overcurrent Shutdown
3.6 PWM-Latch
The oscillator clock output applies a set pulse to the
PWM-Latch when initiating CoolMOS™ conduction.
After setting the PWM-Latch can be reset by the PWM-
OP, the Soft-Start-Comparator, the Current-Limit-
Comparator, Comparator C3 or the Error-Latch of the
Protection Unit. In case of resetting the driver is shut
down immediately.
3.7 Driver
The driver-stage drives the gate of the CoolMOS™ and
is optimized to minimize EMI and to provide high circuit
t
efficiency. This is done by reducing the switch on slope
when reaching the CoolMOS™ threshold. This is
achieved by a slope control of the rising edge at the
driver's output (see Figure 20) to the CoolMOS™ gate.
Thus the leading switch on spike is minimized. When
CoolMOS™ is switched off, the falling shape of the
driver is slowed down when reaching 2V to prevent an
t
overshoot below ground. Furthermore the driver circuit
is designed to eliminate cross conduction of the output
stage. At voltages below the undervoltage lockout
threshold V
the gate drive is active low.
VCCoff
42
Functional Description
(see Figure
csth
= 2. Without propagation
Sense
=1V. A current ramp of
csth
/dt = 0.8V/μs, and a
Sense
Propagation Delay
overshoot of 14.4%. By means of
without compensation
0.8 1 1.2 1.4 1.6 1.8
dV
Sense
dt
=180ns
V/us
2