Datasheet

11Maxim Integrated
1V to 16V, Single-Channel, Hot-Swap Controllers
with Precision Current-Sensing Output
MAX5977A/MAX5977B
An output resistor, R
CSOUT
, must be connected between
the transconductance current-sense amplifier output
CSOUT and AGND. The transconductance G
M
, of the
amplifier is typically 2500FS:
CSOUT M SENSE, FS
R G V 2.5V×× ≤
n-Channel MOSFET Selection
Select the external n-channel MOSFET according to the
application’s current level. The MOSFET’s on-resistance
(R
DS(ON)
) should be chosen low enough to have a mini-
mum voltage drop at full load to limit the MOSFET power
dissipation. High R
DS(ON)
causes output ripple if there
is a pulsating load. Determine the device power rating to
accommodate a short-circuit condition on the board at
startup and when the device is in automatic-retry mode
(see the MOSFET Thermal Considerations section).
The MAX5977A’s fault latch allows the use of MOSFETs
with lower power ratings. A MOSFET typically withstands
single-shot pulses with higher dissipation than the speci-
fied package rating.
MOSFET Thermal Considerations
During normal operation, the external MOSFETs dis-
sipate little power. The MOSFET R
DS(ON)
is low when
the MOSFET is fully enhanced. The power dissipated in
normal operation is P
D
= I
LOAD
2
x R
DS(ON).
The most
power dissipation occurs during the turn-on and turn-off
transients when the MOSFETs are in their linear regions.
Take into consideration the worst-case scenario of a
continuous short-circuit fault; consider these two cases:
1) The single turn-on with the device latched after a fault
(MAX5977A).
2) The continuous automatic retry after a fault
(MAX5977B).
MOSFET manufacturers typically include the package
thermal resistance from junction to ambient (R
BJA
) and
thermal resistance from junction to case (R
BJC
), which
determine the startup time and the retry duty cycle
(d = t
SU
/(t
SU
+ t
RETRY
)). Calculate the required transient
thermal resistance with the following equation:
JMAX A
JA(MAX)
IN INRUSH
TT
Z
VI
θ
×
≤
×
Layout Considerations
To take full advantage of the switch response time to an
output fault condition, it is important to keep all traces
as short as possible and to maximize the high-current
trace dimensions to reduce the effect of undesirable
parasitic resistance and inductance. Place the devices
close to the card’s connector, and a 0.01FF capacitor to
GND should be placed as close as possible to V
IN
. Use
a ground plane to minimize impedance and inductance.
Minimize the current-sense resistor trace length and
ensure accurate current sensing with Kelvin connec-
tions.
When the output is short circuited, the voltage drop
across the external MOSFET becomes large. Hence, the
power dissipation across the switch increases, as does
the die temperature. An efficient way to achieve good
power dissipation on a surface-mount package is to lay
out two copper pads directly under the MOSFET pack-
age on both sides of the board. Connect the two pads to
the ground plane through vias, and use enlarged copper
mounting pads on the top side of the board.
Related Parts
PART DESCRIPTION
MAX5970
0 to 16V, Dual Hot-Swap Controller with
a 10-Bit Current and Voltage Monitor and
Four LED Drivers
MAX5978
0 to 16V, Single Hot-Swap Controller with
a 10-Bit Current and Voltage Monitor Plus
Four LED Drivers