MIC1555 View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MIC1555 Datasheet PDF : 10 Pages
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Micrel, Inc.
Functional Description
Refer to the block diagrams.
The MIC1555/7 provides the logic for creating simple RC
timer or oscillator circuits.
The MIC1555 has separate THR (threshold) and TRG
(trigger) connections for monostable (one-shot) or
astable (oscillator) operation.
The MIC1557 has a single T/T (threshold and trigger)
connection for astable (oscillator) operation only. The
MIC1557 includes a CS (chip select/reset) control.
Supply
VS (supply) is rated for +2.7V to +18V. An external
capacitor is recommended to decouple noise.
Resistive Divider
The resistive voltage divider is constructed of three
equal value resistors to produce 1/3VS and 2/3VS voltage
for trigger and threshold reference voltages.
Chip Select/Reset (MIC1557 only)
CS (chip select/reset) controls the bias supply to the
oscillator’s internal circuitry. CS must be connected to
CMOS logic-high or logic-low levels. Floating CS will
result in unpredictable operation. When the chip is
deselected, the supply current is less than 1µA. Forcing
CS low resets the MIC1557 by setting the flip flop,
forcing the output low.
Threshold Comparator
The threshold comparator is connected to S (set) on the
RS flip-flop. When the threshold voltage (2/3VS) is
reached, the flip-flop is set, making the output low. THR
is dominant over TRG.
MIC1555/1557
Trigger Comparator
The trigger comparator is connected to R (reset) on the
RS flip-flop. When TRG (trigger) goes below the trigger
voltage (1/3VS), the flip-flop resets, making the output
high.
Flip-Flop and Output
A reset signal causes Q to go low, turning on the P-
channel MOSFET and turning off the N-channel
MOSFET. This makes the output rise to nearly VS.
A set signal causes Q to go high, turning off the P-
channel MOSFET, and turning on the N-channel
MOSFET, grounding OUT.
Basic Monostable Operation
Refer to the MIC1555 functional diagram.
A momentary low signal applied to TRG causes the
output to go high. The external capacitor charges slowly
through the external resistor. When VTHR (threshold
voltage) reaches 2/3VS, the output is switched off,
discharging the capacitor. During power-on, a single
pulse may be generated.
Basic Astable Operation
Refer to the MIC1557 functional diagram.
The MIC1557 starts with T/T low, causing the output to
go high. The external capacitor charges slowly through
the external resistor. When VT/T reaches 2/3VS
(threshold voltage), the output is switched off, slowly
discharging the capacitor. When VT/T decreases to
1/3VS (trigger voltage), the output is switched on,
causing VT/T to rise again, repeating the cycle.
May 2010
6
M9999-052610-D
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