max5905 Maxim Integrated Products, Inc., max5905 Datasheet - Page 18

• Manufacturer Part Number: max5905
• Description: Low-voltage, Dual Hot-swap Controllers/power Sequencers
• Manufacturer: Maxim Integrated Products, Inc.

Q
I
In this case, the inrush current depends on the MOSFET
gate-to-drain capacitance (C
capacitance from gate to GND (C
load current (I
Example: Charging and discharging times using the
Fairchild FDB7030L MOSFET
If V
V
data sheet specifies that the FDB7030L has approxi-
mately 60nC of gate charge and C
MAX5904–MAX5909 have a 100µA gate-charging cur-
rent and a 100µA weak discharging current or 3mA
strong discharging current.
C
during the startup period.
With no gate capacitor the inrush current, charge, and
discharge times are:
With a 22nF gate capacitor the inrush current, charge,
and discharge times are:
Low-Voltage, Dual Hot-Swap Controllers/Power
Sequencers
18
GATE
DRIVE
GATE
BOARD
I
t
t
t
INRUSH
DISCHARGE SLOW
DISCHARGE FAST
CHARGE
IN1
I
t
t
t
______________________________________________________________________________________
INRUSH
CHARGE
DISCHARGE SLOW
DISCHARGE FAST
is the gate charging/discharging current
), therefore ∆V
is the MOSFET total gate charge
= 5V then GATE1 charges up to 10.4V (V
I
INRUSH
= 6µF and the load does not draw any current
=
=
600
=
22
LOAD
=
_
_
600
pF
nF
0 10 4
=
_
_
6
6
×
µ
pF
C
µ
+
×
) present during the startup period.
F
F
100
rss
10 4
22
100
C
=
GATE
+
.
=
BOARD
nF
.
22
0
+
V
=
µ
22
=
µ
V
×
C
A
+
nF
0 10 4
A
×
100
nF
0 10 4
+
GATE
60
×
= 10.4V. The manufacturer’s
100
×
60
×
×
nC
10 4
µ + =
100
nC
10 4
3
rss
.
100
A
3
µ + =
mA
.
.
mA
A
×
V
=
.
) plus any additional
V
V
=
µ
I
GATE
0 6
+
µ
0 1
V
A
+
2 89
+
.
A
0 26 5
GATE
60
+
.
60
60
rss
ms
60
nC
A
nC
nC
ms
+
nC
= 600pF. The
), and on any
I
.
LOAD
=
=
=
mA
0 02
=
0 6
0 096
.
2 89
.
.
.
ms
ms
ms
IN1
ms
+
In applications where the board capacitance (C
is high, the inrush current causes a voltage drop across
R
threshold. The fast comparator regulates the voltage
across the sense resistor to V
regulates the inrush current during startup. In this case,
the current charging C
stant and the turn-on time is:
The maximum inrush current in this case is:
Figures 2–8 show the waveforms and timing diagrams
for a startup transient with current regulation. (See the
Typical Operating Characteristics.) When operating
under this condition, an external gate capacitor is not
required.
The ON comparator controls the on/off function of the
MAX5904–MAX5909. ON is the input to a precision
three-level voltage comparator that allows individual
control over channel 1 and channel 2. Drive ON high
(> 2.025V) to enable channel 1 and channel 2. Pull ON
low (<0.4V) to disable both channels. To enable chan-
nel 1 only, V
threshold (0.825V) and the channel 2 ON threshold
(2.025V). The device can be turned off slowly, reducing
inductive kickback, by forcing ON between 0.4V and
0.825V until the gates are discharged. The ON com-
parator is ideal for power sequencing (Figure 11).
Note that a minimum RC time delay of 20µs is applied
to the steeply rising voltage at IN_ before the input volt-
age reaches the ON_ terminal. This allows internal cir-
cuits to stabilize prior to the signal arriving at the ON_
terminal.
The MAX5906–MAX5909 feature an uncommitted com-
parator that increases system flexibility. This compara-
tor can be used for voltage monitoring, or for
generating a power-on reset signal for on-card micro-
processors (Figure 12).
The uncommitted comparator output (OUTC) is open
drain and is pulled low when the comparator input volt-
age (V
SENSE
INC+
that exceeds the startup fast-comparator
Case B: Fast Turn-On (With Current Limit)
t
) is below its threshold voltage (1.236V).
ON
ON
=
I
must be between the channel 1 ON
INRUSH
C
BOARD
Uncommitted Comparator
BOARD
=
V
×
R
SU TH
V
V
SENSE
SU TH
IN
,
can be considered con-
,
×
SU,TH
R
SENSE
ON Comparator
. This effectively
BOARD
)