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MAX34565
12V Hot-Plug Switch in TDFN Package
Output-Voltage Ramp (VRAMP)
The voltage ramp circuit uses an operational amplifier to
control the gate bias of the n-channel power MOSFET.
When the voltage on EN/FAULT is below V OLT , a FET
is used to keep C VRAMP discharged, which forces the
output voltage to GND. Once the voltage on EN/FAULT
is above V OHT , an internal current source begins to
charge the external capacitor, C VRAMP , connected to the
VRAMP pin. The amplifier controls the gate of the power
MOSFET so that the LOAD output voltage divided by
two tracks the rising voltage level of C VRAMP . The output
voltage continues to ramp until it reaches either the input
V CC level or the overvoltage clamp limits.
Overvoltage Limit
The overvoltage-limiting clamp monitors the VRAMP
voltage level compared to an internal voltage reference.
When the voltage on VRAMP exceeds V OVC /2, the gate
voltage of the n-channel power MOSFET is reduced,
limiting the voltage on LOAD to V OVC even as V CC
increases. If the device is in overvoltage for an extended
period of time, the device may overheat and enter ther-
Direct and Kelvin Current Sensing
The current limit circuit can be biased in one of two ways:
direct sensing or Kelvin sensing. Using direct sensing,
the sense resistor is connected between the ILIM pin and
the LOAD pins ( Figure 3 ). Because the bond wires of all
five LOAD pins are in parallel, the on-resistance is slightly
lower. Using Kelvin sensing, only one of the LOAD pins
is connected to the ILIM pin through the sense resistor
( Figure 4 ). Using this circuit, the voltage on the die is
sensed, negating any impact of the bond wire resistance.
Because four LOAD pins are connected in parallel, the
on-resistance is slightly higher.
Thermal Shutdown
The device enters a thermal shutdown state when the
temperature of the power MOSFET reaches or exceeds
T SHDN , approximately +155 N C. When T SHDN is exceed-
ed, the thermal-limiting circuitry disables the device
using the enable circuitry. The EN/FAULT pin is driven to
V FAULT . The device is offered in two different versions:
an autoretry version and a latchoff version.
mal shutdown. This is caused by the power created by
the voltage drop across the power MOSFET and the load
current.
LOAD
LOAD
I LOAD
TO APPLICATION
Current Limit
The load current is continuously monitored during the
initial voltage ramping (I SCL ) and during normal opera-
tion (I OVL ). If the current exceeds the current limit that is
set by the external resistance at ILIM, the gate voltage
of the power MOSFET is decreased, reducing the output
MAX34565
LOAD
LOAD
LOAD
ILIM
R LIM
current to the set current limit. Current is limited by the
device comparing the voltage difference between the
LOAD and ILIM pins to an internal reference voltage.
When the output power is initially ramping up, the current
Figure 3. Direct Sensing Circuit
limit is I SCL . Once the voltage ramping is complete, the
current limit is I OVL . The lower I SCL current limit protects
the source if there is a dead short on initial power-up. If
the high current is maintained, the device will heat up
LOAD
LOAD
LOAD
I LOAD
TO APPLICATION
and the internal temperature will eventually reach T SHDN .
The device act as a fuse and automatically disable the
current flowing to the load when the temperature of the
power MOSFET has exceeded the shutdown junction
temperature, T SHDN .
MAX34565
LOAD
LOAD
ILIM
R LIM
Figure 4. Kelvin Sensing Circuit
7
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