LTC2924 Linear Technology, LTC2924 Datasheet - Page 11

• Manufacturer Part Number: LTC2924
• Description: Quad Power Supply Sequencer
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Minimize stray capacitance on the ON and IN1-IN4 pins.
As a practical matter, lay out these resistors as close to
the LTC2924 as possible.
Details of Resistor Calculations
In this example, the voltage at the IN pins is 0.61V when the
LTC2924 detects that the power supply is On during a Power
On sequence or Off during a Power Off sequence.
The delta voltage, ΔV, represents the difference:
This delta voltage on R
hysteresis current I
The current I
During the Power On sequence, I
to I
V
Use caution if designs call for V
Many loads stop using significant current at this level, and
the power supply may take a long time to go below this
voltage. If V
necessary, consider adding an extra resistive load at the
output of the power supply to ensure it discharges in a
reasonable amount of time.
OFF
ΔV = 2.2V – 1V = 1.2V
R
R
RB
I
RB
B
A
Precaution
and R
=
=
=
I
66
HYS
∆
2 2
0 61
V
.
.
µ
A
OFF
RB
V
A
=
is:
24
– .
=
at the Power On voltage of 2.2V is:
50
voltages at or less than this voltage are
1 2
k
0 61
9 2
.
µ
.
U
HYS
V
A
k
V
=
and R
B
=
24
U
will be equal to the product of
66
k
Ω
µ
B
OFF
A
. Therefore:
voltages less than ~0.8V.
HYS
W
= 0, so I
FB
U
is equal
Selecting the Timing Capacitor
During the Power On sequence, the timer is used to cre-
ate a delay between the time one supply reaches the On
threshold and the next supply is enabled. During the Power
Off sequence, the timer is used to create a delay between
the time one supply reaches the Off threshold and the
next supply is disabled. Select the timing capacitor with
the following equation:
Leaving the TMR pin unconnected will generate the mini-
mum delay. The accuracy of the time delay will be affected
by the capacitor leakage (the nominal charge current is
5µA) and capacitor tolerance. A low leakage ceramic
capacitor is recommended.
Selecting the Power Good Timer (PGT) Capacitor
During the Power On sequence, the PGT can be used to
detect the failure of a power supply to reach the desired
On voltage. The PGT is enabled each time a power supply
is enabled by the OUT1-OUT4 pins. The PGT is reset each
time an IN1-IN4 pin detects that a power supply is at the
desired On voltage. Select the PGT timeout capacitor with
the following equation:
If no PGT is desired, the PGT pin must be shorted to
ground. The accuracy of the PGT timeout will be affected
by the capacitor leakage (the nominal charge current
is 5µA) and capacitor tolerance. A low leakage ceramic
capacitor is recommended.
C
C
TMR
PGT
[µF] = t
[µF] = t
PGT
DELAY
• 5µF/s
• 5µF/s
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LTC2924
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