Applications Information

The MAX6820 features a capacitor adjustable

sequence delay. The adjustable delay provides power

sequencing for a wide range of devices with different

power-supply delay requirements. Connect a capacitor

time (Figure 2). Calculate the sequence delay time as

follows:

Setting Threshold Voltage at SETV

higher than the threshold voltage, connect external

resistors as a voltage-divider to SETV, and calculate the

Since SETV input current is 10nA (typ), high value resis-

tors can be used.

Selecting the Pass MOSFET

The external pass MOSFET is connected in series with

the sequenced power-supply source. Since the load

current and the MOSFET drain-to-source impedance

of the MOSFET affect the load supply accuracy. The

MAX6819/MAX6820 fully enhance the external MOSFET

out of its linear range to ensure the lowest drain-to-

source on impedance. For highest supply accuracy/

lowest voltage drop, select a MOSFET with an appropri-

ate drain-to-source on impedance for a gate-to-source

bias of 4.5V to 6.0V.

Gate Drive Characteristics

The MAX6819/MAX6820 internal charge pump drives

the N-channel MOSFET with a gate-to-source voltage

capacitive load of a MOSFET gate input.

Loading the GATE output resistively adds load current

and reduces gate drive capability. The internal charge

pump does not require external capacitors.

The external pass MOSFET is disabled, and charge

are above the 1.875V UVLO or EN is low.

Logic Driven Supply Sequencing

The MAX6819 offers a logic-compatible enable input

(EN) that allows digital devices to control sequencing.

When the TTL/CMOS-compatible EN input is logic low,

the GATE output is low. When the EN input is logic high

(and SETV is above the monitor threshold), the GATE

output is enabled after an internally fixed 200ms delay.

For a logic controlled sequencer when voltage monitor-

0.62V (Figure 3).

Sequencing Three or More Supplies

Cascade multiple MAX6819/MAX6820 to sequence

more than two supplies. Daisy-chaining devices allows

one sequencer to monitor the passed voltage of an

upstream sequencer through the SETV comparator

inputs. EN allows any sequencer to be shut down inde-

pendent of the SETV levels. Figure 4 shows an example

of a three-supply system in which the first supply must

come up before the second supply and the third supply

must yield for both supplies.

Negative-Going Voltage

Transient Immunity

The MAX6819/MAX6820 power-supply voltage sequen-

cers are relatively immune to short-duration (pulse

width), negative-going voltage transients (Figure 5).

However, the amplitude of the transient is inversely pro-

portional to its pulse width.

Chip Information

TRANSISTOR COUNT: 638

PROCESS: BiCMOS

SOT23 Power-Supply Sequencers

6

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SETD

SETV

R1

R2

MAX6820