12
LTC1645
1645fa
nearly unlimited current flow. The LTC1645 discharges
the GATE pin in a few microseconds, but during this
discharge time current on the order of 150 amperes flows
from the V
CC
power supply. This current spike glitches the
power supply, causing V
CC
to dip (Figure 8a and 8b).
On recovery from overload, some supplies may over-
shoot. Other devices attached to this supply may reset or
malfunction and the overshoot may also damage some
components. An inductor (1µH to 10µH) in series with the
FET’s source limits the short-circuit di/dt, thereby limiting
the peak current and the supply glitch (Figure 8c and 8d).
Additional power supply bypass capacitance also reduces
the magnitude of the V
CC
glitch.
Reset
The LTC1645 uses an internal 1.238V bandgap reference,
a precision voltage comparator, and a resistive divider to
monitor the output supply voltage (Figure 9).
Whenever the voltage at the FB pin rises above its reset
threshold (1.238V), the comparator output goes high, and
a timing cycle starts (see Figure 10, time points 1 and 4).
After a complete timing cycle, RESET is released. An
external pull-up is required for the RESET pin to rise to a
logic high.
When the voltage at the FB pin drops below its reset
threshold, the comparator output goes low. After passing
through a glitch filter, RESET is pulled low (time point 2).
If the FB pin rises above the reset threshold for less than
a timing cycle, the RESET output remains low (time
point 3).
APPLICATIO S I FOR ATIO
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Glitch Filter
The LTC1645 has a glitch filter to prevent RESET from
generating a spurious system reset in the presence of
transients on the FB pin. The filter is 20µs for large
transients (greater than 150mV) and up to 80µs for
smaller transients. The relationship between glitch filter
time and the transient voltage is shown in Typical Perfor-
mance Characteristics: Glitch Filter Time vs Feedback
Transient.
Timer
The system timing for the LTC1645 is generated by the
circuitry shown in Figure 11. The timer is used to set the
turn-on delay after the ON pin goes high. It also sets the
delay before the RESET pin goes high after the FB pin
exceeds 1.238V.
Whenever the timer is off, the internal N-channel shorts
the TIMER pin to ground (Figure 11). Activating the timer
connects a 2µA current from V
CC1
to the TIMER pin and the
Figure 10. Supply Monitor Waveforms
Figure 9. Supply Monitor Block Diagram
Figure 11. System Timing Block Diagram
–
+
COMP
TIMER
C
TIMER
FB
RESET
V
OUT
ON
TIMER
µP
10k
1645 F09
RESET
LOGIC
1.238V
REFERENCE
V2
1
1.23V
1.23V
1645 F10
23 4
V1
V
OUT
TIMER
RESET
V2 V1 V2
–
+
COMP
TIMER
2µA
C
TIMER
ON
1.23V
SUPPLY
MONITOR
1645 F11
LOGIC
