Datasheet

Setting the Startup Period, R
TIM
The startup period (t
START
) is adjustable from 0.45ms to
50ms. The adjustable startup period feature allows sys-
tems to be customized for MOSFET gate capacitance
and board capacitance (C
BOARD
). The startup period is
adjusted with the resistance connected from TIM to GND
(R
TIM
). R
TIM
must be between 4kΩ and 500kΩ. The
startup period has a default value of 9ms when TIM is left
floating. Calculate R
TIM
with the following equation:
where t
START
is the desired startup period.
Startup Sequence
There are two ways of completing the startup sequence.
Case A describes a startup sequence that slowly turns
on the MOSFETs by limiting the gate charge. Case B
uses the current-limiting feature and turns on the
MOSFETs as fast as possible while still preventing a high
inrush current. The output voltage ramp-up time (t
ON
) is
determined by the longer of the two timings, case A and
case B. Set the startup timer t
START
to be longer than t
ON
to guarantee enough time for the output voltage to settle.
Case A: Slow Turn-On (Without Current Limit)
There are two ways to turn on the MOSFETs without
reaching the fast-comparator current limit:
• If the board capacitance (C
BOARD
) is small, the
inrush current is low.
• If the gate capacitance is high, the MOSFETs turn
on slowly.
In both cases, the turn-on time is determined only by the
charge required to enhance the MOSFET. The small
gate-charging current of 100µA effectively limits the out-
put voltage dV/dt. Connecting an external capacitor
between GATE and GND extends turn-on time. The time
required to charge/discharge a MOSFET is as follows:
where:
C
GATE
is the external gate to ground capacitance
(Figure 4).
∆V
GATE
is the change in gate voltage.
Q
GATE
is the MOSFET total gate charge.
I
GATE
is the gate-charging/discharging current.
In this case, the inrush current depends on the MOSFET
gate-to-drain capacitance (C
rss
) plus any additional
capacitance from GATE to GND (C
GATE
), and on any
load current (I
LOAD
) present during the startup period.
Example: Charging and Discharging Times Using
the Fairchild FDB7030L MOSFET
If V
IN1
= 5V then GATE1 charges up to 10.4V (V
IN1
+
V
DRIVE
); therefore ∆V
GATE
= 10.4V. The manufacturer’s
data sheet specifies that the FDB7030L has approxi-
mately 60nC of gate charge and C
rss
= 600pF. The
MAX5955/MAX5956 have a 100µA gate-charging cur-
rent and a 3mA strong discharging current.
I
C
CC
II
INRUSH
BOARD
rss GATE
GATE LOAD
=
+
×+
t
CVQ
I
GATE GATE GATE
GATE
=
×∆ +
R
t
pF
TIM
START
=
×128 800
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
______________________________________________________________________________________ 11
COMPONENT MANUFACTURER PHONE WEBSITE
Dale-Vishay 402-564-3131 www.vishay.com
Sense Resistors
IRC 704-264-8861 www.irctt.com
Fairchild 888-522-5372 www.fairchildsemi.com
International Rectifier 310-233-3331 www.irf.com
MOSFETs
Motorola 602-244-3576 www.mot-sps.com/ppd
Table 3. Component Manufacturers
GATE
SENSE
GND
ON_
*
* REQUIRED COMPONENTS. SEE THE ON_ COMPARATORS SECTION.
R
SENSE
V
OUT
C
GATE
C
BOARD
V
IN
IN_
R
PULLUP
PGOOD_
MAX5955
MAX5956
0.1µF
Figure 4. Operating with an External Gate Capacitor