DN466 - Hot Swap Controller, MOSFET and Sense Resistor Are Integrated in a 5mm × 3mm DFN for Accurate Current Limit and Load Current Monitoring in Tight Spaces

Hot Swap Controller, MOSFET and Sense Resistor Are Integrated in
a 5mm × 3mm DFN for Accurate Current Limit and Load Current
Monitoring in Tight Spaces – Design Note 466
Vladimir Ostrerov
Introduction
In general, a Hot Swap™ controller provides two important
functions to boards that can be plugged and unplugged
from a live backplane:
• It limits the potentially destructive inrush current
when a board is plugged in.
• It acts as a circuit breaker, with the maximum
current and maximum time at that current factored
into the breaker function.
Two of the components required to implement these
functions, the power MOSFET and sense resistor, tend
to dominate the board real estate taken by the Hot Swap
circuit. The LTC®4217 saves space by combining these two
components with a Hot Swap controller in a 16-pin 5mm ×
3mm DFN package (or 20-lead TSSOP). This 2A integrated
Hot Swap controller fits easily onto boards operating in
the voltage range from 2.9V to 26.5V. A dedicated 12V
version, LTC4217-12, is also available, which contains
preset 12V specific thresholds. Figure 1 shows how little
space is required for a complete Hot Swap circuit.
LTC4217 Features
Figure 2 shows a simplified block diagram of the LTC4217.
The controller provides inrush current control and a 5%
accurate 2A current limit with foldback. For soft-start, an
internal current source charges the gate of the N-channel
MOSFET with 300V/s slew rate. Lower soft-start output
voltage slew rates can be set by adding an external gate
capacitor.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
10mm
SENSE
RESISTOR
7.5mΩ
VOUT
VIN
VDD
25mΩ MOSFET OUT
FB
10mm
ISET
+
–
C1
GATE
20k
IMON
Figure 1. Tiny Integrated Controller
Package Results in a Small Footprint
+
DN466 F01
–
GND
INTVCC
CURRENT
AMP
PG
OV
MOSFET CONTROL
AND LOGIC
UV
GND
FLT
TIMER
DN466 F02
Figure 2. Simplified Block Diagram of the LTC4217
06/09/466
the MOSFET temperature. If the die temperature exceeds
145°C, the MOSFET is turned off. It is turned on again
when the temperature drops below 125°C.
Integrated MOSFET and Sense Resistor
The LTC4217 integrates a 25mΩ MOSFET and 7.5mΩ
current sense resistor. The default value of the active
current limit is 2A, which can be adjusted to a lower
value by adding an external resistor. Using an external
analog switch for the connection of this resistor allows
a start-up current to be larger than the maximum load
current in steady state operation.
Voltage and Current Monitoring
The LTC4217 protects the load from overvoltage and
undervoltage conditions with a 2% accurate comparator threshold. The LTC4217 also features an adjustable
current limit timer, a current monitor output and a fault
output.
Adjustable Current Limit
The voltage at the ISET pin determines the active current limit, which is 2A by default. This pin is driven by
a 0.618mV voltage reference through a 20k resistor. An
external resistor placed between the ISET pin and ground
forms a resistive divider with the internal 20k resistor.
The divider acts to lower the voltage at the ISET pin and
therefore lower the current limit threshold.
The adjustable current limit timer sets the time duration
for current limit before the MOSFET is turned off. The
current monitor produces a voltage signal scaled to the
load current. The fault output is an open drain that pulls
low when an overcurrent fault has occurred and the
circuit breaker trips.
Typical Application
The LTC4217 application circuit shown in Figure 3 operates
with a 100ms auto-retry time and a 2ms overcurrent condition when the load current reaches 2A. It also produces a
voltage signal for an ADC to monitor load current.
The ISET pin voltage increases linearly with temperature,
with a slope of 3.2V/°C when no external resistor is present. This compensates for the temperature coefficient of
the sense resistor—important for applications that must
maintain monitoring accuracy over a wide temperature
range. This also provides a convenient means to monitor
VDD
12V
OUT
CL
330μF
UV
AUTO
RETRY
LTC4217DHC-12
FLT
VOUT
12V
IIN
0.1A/DIV
R1
10k
PG
VOUT
10V/DIV
TIMER
INTVCC
C1
0.1μF
IMON
GND
VIN
10V/DIV
ADC
PG
10V/DIV
R2
20k
DN466 F03a
25ms/DIV
DN466 F03b
Figure 3. Typical Application with Current Monitored by an ADC
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn466f LT/TP 0609 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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