MIC1557BM5 MICREL [Micrel Semiconductor], MIC1557BM5 Datasheet - Page 7

• Manufacturer Part Number: MIC1557BM5
• Manufacturer: MICREL [Micrel Semiconductor]

To use the MIC1555 as an oscillator, connect TRG to THR.
The MIC1557 features a CS input. When logic-low, CS
places the MIC1557 into a <1 A shutdown state. If unused,
the MIC1557 CS input on must be pulled up.
Falling-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled falling-
edge, as shown in figure 5. The RC time constant of the
input capacitor and pull-up resistor should be less than the
output pulse time, to prevent multiple output pulses. A
diode across the timing resistor provides a fast reset at the
end of the positive timing pulse.
Rising-Edge Triggered Monostable Circuit
The MIC1555 may be triggered by an ac-coupled rising-
edge, as shown in figure 6. The pulse begins when the ac-
coupled input rises, and a diode from the output holds the
THR input low until TRG discharges to
provides a low-going output pulse.
October 1998
MIC1555/1557
Figure 5. Falling-Edge Trigger Configuration
Figure 6. Rising-Edge Trigger Configuration
Figure 3. MIC1555 Oscillator Configuration
Figure 4. MIC1557 Oscillator Configuration
Input
+5V
+5V
+5V
1
4
2
1M
1M
1
4
2
VS
TRG
GND
+5V
VS
TRG
GND
4
3
2
MIC1555
VS
CS
GND
MIC1555
1
4
2
MIC1557
VS
TRG
GND
OUT
THR
MIC1555
OUT
THR
OUT
T/T
3
5
1k
OUT
THR
3
5
5
1
0.1µF
3
5
8kHz
1k
100µs
1k
1N4148
1k
8kHz
0.1µF
0.1µF
Output
1
Output
100µs
Output
⁄
0.1µF
3
V
Input
1N4148
S
. This circuit
Output
7
Accuracy
The two comparators in the MIC1555/7 use a resistor voltage
divider to set the threshold and trigger trip points to approxi-
mately
charge and discharge rates of an RC circuit are dependent on
the applied voltage, the timing remains constant if the input
voltage varies. If a duty cycle of exactly 50% (or any other
value from 1 to 99%), two resistors (or a variable resistor) and
two diodes are needed to vary the charge and discharge
times. The forward voltage of diodes varies with temperature,
so some change in frequency will be seen with temperature
extremes, but the duty cycle should track. For absolute timing
accuracy, the MIC1555/7 output could be used to control
constant current sources to linearly charge and discharge the
capacitor, at the expense of added components and board
space.
Long Time Delays
Timing resistors larger than 1M
10 F are not recommended due to leakage current inaccura-
cies. Time delays greater than 10 seconds are more accu-
rately produced by dividing the output of an oscillator by a
chain of flip-flop counter stages. To produce an accurate
one-hour delay, for example, divide an 4.55Hz MIC1557
oscillator by 16,384 (4000
divider. 4.5Hz may be generated with a 1 F C
mately 156k .
Inverting Schmitt Trigger
Refer to figure 7. The trip points of the MIC1555/7 are defined
as
signal conditioning inverter, with hysteresis. A slowly chang-
ing input on T/T will be converted to a fast rise or fall-time
opposite direction rail-to-rail output voltage. This output may
be used to directly drive the gate of a logic-level P-channel
MOSFET with a gate pull-up resistor. This is an inverted logic
low-side logic level MOSFET driver. A standard N-channel
MOSFET may be driven by a second MIC1555/7, powered by
12V to 15V, to level-shift the input.
Charge-Pump Low-Side MOSFET Drivers
A standard MOSFET requires approximately 15V to fully
enhance the gate for minimum R
level MOSFET reduces the required gate voltage, allowing
an MIC1557 to be used as an inverting Schmitt Trigger,
described above. An MIC1557 may be configured as a
voltage quadrupler to boost a 5V input to over 15V to fully
enhance an N-channel MOSFET which may have its drain
1
/
3
and
2
⁄
3
OFF
ON
and
2
/
3
V
3.3V
1.6V
1
S
⁄
3
, which allows either device to be used as a
of the input voltage, respectively. Since the
Figure 7. Schmitt trigger
hex
, 2
5
4
2
THR
TRG
GND
14
DS(on)
MIC1555
) using a CD4020 CMOS
or capacitors larger than
. Substituting a logic-
OUT
VS
+5V +12V
1
3
MIC1555/1557
T
and approxi-
R
L
Micrel