Datasheet

MAX5915/MAX5915A/MAX5916/MAX5916A
Applications Information
Component Selection
External MOSFETs
Select the external n-channel MOSFETs according to
the application’s current requirement. Limit switch power
dissipation by choosing a MOSFET with an R
DS(ON)
low
enough to have a minimum voltage drop at full load.
High R
DS(ON)
causes output ripple if the board has
pulsing loads. High R
DS(ON)
can trigger an external
undervoltage fault at full load. Determine the MOSFET’s
power rating requirement to accommodate a short-cir-
cuit condition on the board during startup (see the
External MOSFET Thermal Considerations section).
Table 3 lists MOSFET and sense resistor manufacturers.
Sense Resistors
The overcurrent sense voltage threshold on the +3.3V
output is 46mV and 31mV on the +5V output. Choose a
sense resistor using the following equation:
R
SENSE
= (V-
ILIM
/ I
LOAD
)
where I
LOAD
is the brickwall current limit for the output.
Choose the sense resistors’ power rating to accommo-
date the overload current:
P
SENSE
= (I
LOAD
)
2
x R
SENSE
Additional External Gate Capacitance
Connecting an external capacitance from the gates of
the external MOSFETs to GND slows the turn on of the
+5V and +3.3V supplies.
Dual PCI 2.2 Hot-Swap Controllers
16 ______________________________________________________________________________________
t
DELAY
t
DELAY
t
DELAY
t
RESTART
t
RESTART
t
RESTART
ENABLE_+12VIN
STARTUP_+12VIN
OC_3.3VAUXO_
UV_3.3VAUXO_
RESET
RESET
RESET
DQ
V
Q
POSITIVE EDGE
TRIGGERED D FLIP-FLOP,
RESET IS ACTIVE LOW
OC_3.3VO_
OC_5VO_
OC_+12VO_
OC_-12VO_
UV_3.3VO_
UV_5VO_
UV_+12VO_
POSITIVE EDGE TRIGGERED D FLIP-FLOP,
RESET IS ACTIVE LOW
HIGH = MAIN SUPPLIES
ARE TURNED OFF
MAIN OFF
AUX OFF
HIGH = AUX SUPPLY
TURNED ON
ENABLE_3.3VAUXIN
STARTUP_3.3VAUXIN
V = INTERNAL SUPPLY LOGIC
GENERATED BY THE 12VIN OR THE
3.3VAUXIN, WHICHEVER IS HIGHER
RESET
DQ
V
Q
SEE TABLE 2 FOR EXPLANATION OF SIGNALS.
Figure 4. Main and Auxiliary Supply Shutdown Control Logic for MAX5916/MAX5916A