Maxim Integrated Power-Supply Tracker/Sequencer MAX6877 - Manual (Page 2)

Description
The MAX6877 voltage tracker/sequencer can track or sequence up to three voltages. Use the
TRACK/SEQ pin to select tracking or sequencing. The device will either track or sequence the three
voltages, but it cannot track and sequence simultaneously. Power-up begins when EN/active-low UV is
asserted and all three input voltages are above their thresholds. In the example circuit in Figure 1,
EN/active-low UV is connected to the 12V bus, so the circuit will turn on when the bus exceeds its
threshold and turn off when it falls under its threshold.
To create a basic design with the MAX6877, such as the one in Figure 1, follow these steps:
1. Select the input voltage thresholds for IN1, IN2, and IN3. The input voltages must exceed these
thresholds for power-up to begin. Use the following formula to set the resistor-divider values for
IN1. (For IN2 or IN3, replace R
1
with R
3
, or R
5
and R
2
with R
4
or R
6
.)
2. Select the EN/active-low UV threshold using the following formula. EN/active-low UV can also be
used as a logic-level input to control power up. If not used, connect EN/active-low UV to ABP.
3. Set the voltage slew rate. (This is the rate at which the voltages at the outputs OUT1, OUT2, OUT3
ramp up and down.) Select capacitor C
SLEW
by using the following formula:
where C
SLEW
is in farads, and SR is in V/s. C
SLEW
must be in the range of 100pF < C
SLEW
<
1nF.
4. In sequencing-mode only, the voltages come up one after another, separated by a time delay which
is set by capacitor C
DELAY
using the following formula:
where C
DELAY
is in farads, and t
DELAY
is in seconds. t
DELAY
is also the delay time from when all
thresholds are exceeded to the start of sequencing or tracking. C
DELAY
can be safely left out of the
circuit, in which case t
DELAY
becomes the default 200µs.
5. Pick the MOSFETs for each channel. Ensure that the MAX6877 gate-drive circuit can drive the
gate capacitance of the MOSFET.
Multiple MAX6877 parts can be connected together by connecting the power-good output of the first
device to the enable input (EN/active-low UV) of the next device. For more information, see the data
sheet.
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