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ADM1087 View Datasheet(PDF) - Analog Devices

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ADM1087 Datasheet PDF : 16 Pages
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DUAL LOFO SEQUENCING
A power sequencing solution for a portable device, such as a
PDA, is shown in Figure 27. This solution requires that the
microprocessor’s power supply turn on before the LCD display
turns on, and that the LCD display power-down before the
microprocessor powers down. In other words, the last power
supply to turn on is the first one to turn off (LOFO).
An RC network connects the battery and the SD input of the
ADP3333 voltage regulator. This causes power-up and power-
down transients to appear at the SD input when the battery is
connected and disconnected. The 3.3 V microprocessor supply
turns on quickly on power-up and turns off slowly on power-
down. This is due to two factors: Capacitor C1 charges up to 9 V
on power-up and charges down from 9 V on power-down, and
the SD pin has logic-high and logic-low input levels of 2 V and
0.4 V.
For the display power sequencing, the ADM1085 is equipped
with capacitor C2, which creates the delay between the micro-
processor and display power turning on. When the system is
powered down, the ADM1085 turns off the display power
immediately, while the 3.3 V regulator waits for C1 to discharge
to 0.4 V before switching off.
SYSTEM
POWER SWITCH
9V
9V
SD ADP3333 2.5V
C1
3.3V
MICROPROCESSOR
POWER
9V
VIN
ENOUT
ADM1086
ENIN CEXT
C2
SD ADP3333 5V
DISPLAY
POWER
9V
SYSTEM
POWER
0V
9V
VC1
0V
2.5V
MICROPROCESSOR
POWER
0V
5V
DISPLAY
POWER 0V
Figure 27. Dual LOFO Power-Supply Sequencing
ADM1085/ADM1086/ADM1087/ADM1088
SIMULTANEOUS ENABLING
The enable output can drive multiple enable or shutdown
regulator inputs simultaneously.
12V
IN
SD ADP3333 OUT
3.3V
3.3V
3.3V
IN
SD ADP3333 OUT
2.5V
ENABLE
CONTROL
VCC
VIN
ENOUT
ADM1085
ENIN CEXT
12V
IN
SD ADP3333 OUT
1.8V
Figure 28. Enabling a Pair of Regulators from a Single ADM1085
POWER GOOD SIGNAL DELAYS
Sometimes sequencing is performed by asserting Power Good
signals when the voltage regulators are already on, rather than
sequencing the power supplies directly. In these scenarios, a
simple sequencer IC can provide variable delays so that
enabling separate circuit blocks can be staggered in time.
For example, in a notebook PC application, a dedicated
microcomputer asserts a Power Good signal for North Bridge™
and South Bridge™ ICs. The ADM1086 delays the south bridge’s
signal, so that it is enabled after the north bridge.
5V
POWER_GOOD
MICROCOMPUTER
5V
EN
NORTH
BRIDGE
IC
3.3V
VIN
ENOUT
ADM1086
ENIN CEXT
5V
EN
SOUTH
BRIDGE
IC
Figure 29. Power Good Delay
Rev. 0 | Page 13 of 16
 

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