Datasheet

10 Maxim Integrated
1V to 16V, Single-Channel, Hot-Swap Controllers
with Precision Current-Sensing Output
MAX5977A/MAX5977B
Power-Good Output
The open-drain, active-high output PG indicates the
power-good status of the output. Once the input voltage
satisfies the undervoltage and overvoltage requirements
for startup and V
IN
- V
SOURCE
is less than 100mV and
the V
GATE
- V
SOURCE
> 4V, the PG timer is started. At
the expiration of the 50ms PG timer, PG is asserted high.
Applications Information
Undervoltage and Overvoltage Protection
The undervoltage and overvoltage protection is pro-
grammed with a voltage-divider formed by three resis-
tors (R1, R2, and R3) placed in series. The resistor
values should be selected such that the series current,
I
S
, is greater than 5FA. The resistor values are then cal-
culated using the following equations with the overvolt-
age threshold (V
OVR
), undervoltage threshold (V
UVR
),
and the overvoltage hysteresis (V
OVHYS
) obtained from
the Electrical Characteristics table:
( )
( )
OVR
S
IN,OV UVR
IN,UV OVR OVHYS
IN,UV
UVR
V
R3
I
VV
R2 1 R3
V VV
V
R1 1 R2 R3
V
−
−
=

×
= ×


×


= −× +


where V
IN,UV
and V
IN,OV
are the desired undervoltage
and overvoltage thresholds for the hot-swap input volt-
age IN.
Programmable Slow and Fast Current Limit
The slow and fast current-limit thresholds are pro-
grammed by connecting resistors between the high
side of R
SENSE
to SCOMP and FCOMP. The current-limit
thresholds are set using the following equations:
SENSE, SCOMP SENSE
SCOMP
IR
R
25 A
×
=
µ
and:
SENSE, FCOMP SENSE
FCOMP
IR
R
50 A
×
=
µ
where I
SENSE,_COMP
is the desired circuit-breaker cur-
rent limit for the slow or fast current limit.
Startup Sequence
When all conditions for channel turn-on are met, the
external n-channel MOSFET switch is fully enhanced
with a typical gate-to-source voltage of 5V to ensure
a low drain-to-source resistance. The charge pump at
GATE sources 5FA to control the output voltage turn-on
voltage slew rate. An external capacitor must be added
from GATE to ground to further reduce the voltage slew
rate. Placing a 1kI resistor in series with this capaci-
tance prevents the added capacitance from increasing
the gate turn-off time. Total inrush current is the load cur-
rent summed with the product of the gate voltage slew
rate dV/dt and the load capacitance.
To determine the output dV/dt during startup, divide the
GATE pullup current I
GATEPU
by the GATE to ground
capacitance. The voltage at the source of the external
MOSFET follows the gate voltage, so the load dV/dt is
the same as the gate dV/dt. Inrush current is the product
of the dV/dt and the load capacitance. The time to start
up t
SU
is the hot-swap voltage V
IN
divided by the output
dV/dt.
Be sure to choose an external MOSFET that can handle
the power dissipated during startup. The inrush cur-
rent is roughly constant during startup and the voltage
drop across the MOSFET (drain to source) decreases
linearly as the load capacitance charges. The resulting
power dissipation is therefore roughly equivalent to a
single pulse of magnitude (V
IN
x Inrush current)/2 and
duration t
SU
. Refer to the thermal resistance charts in
the MOSFET data sheet to determine the junction tem-
perature rise during startup, and ensure that this does
not exceed the maximum junction temperature for worst-
case ambient conditions.
Transconductance Current-Sense Amplifier
The current-sense resistor, R
SENSE
, must be connected
between IN and SENSE to sense the average current
into the load. The voltage drop across R
SENSE
should
be less than or equal to the slow current-limit threshold;
therefore, R
SENSE
should be selected based on the fol-
lowing equation:
SENSE SENSE,FS SCOMP
RI V
×≤
where I
SENSE
,
FS
is the full-scale current into the load
and V
SCOMP
is the slow current-limit threshold. A Kelvin
sense connection should be used to connect R
SENSE
to
IN and SENSE.