REV. 0
ADM1070
–13–
CURRENT FAULT PLOTS
Some timing waveforms associated with current over faults are
shown in the following figures. Figure 9 shows how a current
glitch (of approximately 500 µs) is dealt with when the output is
shorted after power-up. The gate voltage is at a constant 12.3 V
before the glitch occurs. When the short circuit occurs, the
sense voltage rises sharply as the load current ramps up quickly.
When the sense voltage reaches 100 mV (V
ACL
), the ADM1070
reduces the gate voltage to stop the load current from increasing
any further. When V
SENSE
drops back below V
ACL
, the gate
voltage is increased again.
GATE
SENSE
V
OUT
CH3
10.00VCH1
20.00V
CH2
100mV M 500s CH2
34mV
T
T
T
Figure 9. Timing Waveforms Associated with a
Current Glitch
The plots shown illustrate the operation of the ADM1070’s
unique limited consecutive retry function. Figure 10 highlights
what happens when a current fault occurs for more than 14 ms
(default t
LIMITON
when TIMER Pin tied to V
EE
) and a current
fault is registered. In this case, gate is previously low and
the
part is being powered up into a current fault situation
(shorted
load). When power is applied, gate is allowed to ramp until
sense reaches 100 mV. gate is then held constant to keep sense
at this level. After t
ON
, the PWM cycle begins and gate is
reduced to zero.
5.00V CH2 100mV
M 5.00s
CH1 1.4VCH1
GATE
SENSE
14ms
Figure 10. Timing Waveforms Associated with a
Current Fault
Figure 11 shows a current fault on a wider timebase. The first
spike on the sense line represents the first current fault. The
sense voltage is allowed to ramp up to 100 mV before the gate
voltage is reduced to compensate. The gate and sense voltages
remain at these levels until the t
ON
time has expired. A current
fault is then registered and the gate voltage, and therefore the
sense voltage, are then both held low for the time period t
OFF
.
Note that the PWM ratio (t
ON
/t
OFF
) is equal to 3%. The cycle
then restarts and the sense voltage is free to ramp up to 100 mV
again (it will if the fault is still present). This cycle repeats itself
a total of seven times. Figure 12 shows the seven consecutive
faults occurring on an even wider timebase. If the ADM1070
detects seven consecutive current faults, the part then latches off
(after a total time t
SHORT
).
GATE
SENSE
5.00V
CH2
1.00VCH3
100mV M 100ms
CH1
t
ON
t
OFF
Figure 11. Illustration of the PWM Ratio (t
ON
/t
OFF
)
GATE
SENSE
5.00V
CH2
1.00VCH3
100mV M 100ms
CH1
t
SHORT
Figure 12. Illustration of the Limited Consecutive
Retry Function (Seven Retries and Latch Off)
