MIC1557BM5 MICREL [Micrel Semiconductor], MIC1557BM5 Datasheet - Page 5

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

Functional Diagrams
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) con-
nection 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
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 1 A. 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 (
flip-flop is set, making the output low. THR is dominant over
TRG.
October 1998
MIC1555/1557
Standby
MIC1555 Block Diagram with External Components
Trigger
<100µs
TRG
THR
(Monostable Configuration)
MIC1555
V
1
⁄
SUPPLY
Bias
3
V
VS
GND
S
S
R
and
Q
2
⁄
3
V
2
⁄
3
S
V
voltage for trigger
S
) is reached, the
OUT
100µs
1k
0.1µF
Output
5
Trigger Comparator
The trigger comparator is connected to R (reset) on the RS
flip-flop. When TRG (trigger) goes below the trigger voltage
(
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 V
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 V
V
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 V
age), the output is switched off, slowly discharging the
capacitor. When V
output is switched on, causing V
the cycle.
1
⁄
S
3
MIC1557 Block Diagram with External Components
V
, the output is switched off, discharging the capacitor.
S
), the flip-flop resets, making the output high.
T/T
CS
1M
MIC1557
(Astable Configuration)
T/T
decreases to
V
SUPPLY
Bias
THR
T/T
VS
GND
S
R
(threshold voltage) reaches
reaches
Q
T/T
S
.
1
⁄
3
to rise again, repeating
V
S
2
(trigger voltage), the
⁄
3
V
OUT
S
(threshold volt-
MIC1555/1557
8kHz
1k
0.1µF
Output
Micrel
2
⁄
3