GENERAL DESCRIPTION

The ADM1085/ADM1086/ADM1087 are simple sequencing circuits that provide a time delay between the enabling of voltage regulators and/or dc-dc converters at power-up in multiple supply systems. When the output voltage of the first power module reaches a preset threshold, a time delay is initiated before an enable signal allows subsequent regulators to power up. Any number of these devices can be cascaded with regulators to allow sequencing of multiple power supplies.

Threshold levels can be set with a pair of external resistors in a voltage divider configuration. With appropriate resistor values, the threshold can be adjusted to monitor voltages as low as 0.6V.

The ADM1085 has an active-high (ENOUT) logic output; the ADM1087 has an active-low (ENOUT) output. Both the ADM1085 and ADM1087 have open-drain output stages that can be pulled up to voltage levels as high as 22 V through an external resistor. This level-shifting property ensures compatibility with enable input logic levels of different regulators and converters.

All four models have a dedicated enable input pin that allows the output signal to the regulator to be controlled externally. This is an active high input (ENIN) for the ADM1085 and ADM1086,and an active low input (ENIN) for the ADM1087.

The Simple Sequencers are specified over the extended −40°C to +125°C temperature range. With low current consumption of 15 μA (typical) and 6-lead SC70 packaging, the parts are suitable for low-power portable applications.

 

DUAL LOFO SEQUENCING

A power sequencing solution for a portable device, such as a PDA. This solution requires that the microprocessor 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 powerdown 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 powerdown. 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 to create the delay between the microprocessor 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.

 

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.

 

QUAD-SUPPLY POWER GOOD INDICATOR

The enable output of the Simple Sequencers is equivalent to an AND function of VIN and ENIN. ENOUT is high only when the voltage at VIN is above the threshold and the enable input (ENIN) is high as well. Although ENIN is a digital input, it can tolerate voltages as high as 22 V and can detect if a supply is present. Therefore, a simple sequencer can monitor two supplies and assert what can be interpreted as a power good signal when both supplies are present. The outputs of two ADM1085s can be wire-AND’ed together to make a quadsupply power good indicator.