LM5068MM-2 Datasheet, PDF (11/33 Pages) Texas Instruments – LM5068 Negative Voltage Hot Swap Controller

LM5068
www.ti.com
SNVS254C – JANUARY 2004 – REVISED MARCH 2013
FUNCTION DESCRIPTION
The LM5068 is designed to facilitate the insertion and removal of circuit cards into live backplanes in a controlled
manner. Because the supply bypass capacitors on the circuit card can draw large transient currents, it is critical
to control the supply current during insertion to limit system power glitches and connector damage. Controlling in-
rush current prevents other boards in the system from resetting during board insertion. Load short-circuit
protection is accomplished by active current limiting of the load current. The topology of the LM5068 is illustrated
in the simplified application circuit shown in Figure 23.
VDD (GND)
LIVE
BACKPLANE
VEE (-48V)
PLUG-IN BOARD
LM5068
+
CL
Figure 23. LM5068 Topology
Start-Up Operation
The LM5068 resides on a removable circuit card. Power is applied to the load or power conversion circuitry
through an external N-Channel MOSFET switch and current sense resistor.
When power is initially applied to the card, the gate of the external MOSFET is held low. When certain interlock
conditions are met, a turn-on sequence begins and an internal 60 µA current source charges the gate of the
MOSFET. To initiate the start-up sequence, all of the following interlock conditions must be satisfied:
• The input voltage V_DD - V_EE exceeds 9V(V_UVS)
• The voltage at UV is above 2.5V (V_UV)
• The voltage at OV falls below 2.5V (V_OV)
• The voltage on the Timer capacitor (C_T) is less than 1V (V_TLVT)
• The GATE pin is below 0.5V (V_GLT)
When all of the interlock conditions are met, a 6 µA TIMER current source is enabled to charge the timer
capacitor C_T. During this initial timer sequence the GATE output is held low. When the C_T capacitor successfully
charges up to 4V, the TIMER circuit resets the timer capacitor to 1V and activates a 60 µA current source (I_GATE)
into the MOSFET gate.
VDD
(GND)
R1
R2
R3
VEE
(-48V)
RF
+ _CF
VDD PWRGD
UV
OV
TIMER
LM5068
VEE _{SENSE GATE}
CL ⁺
+ _CT
CC
RS
Q1
Figure 24. Hot Swap Controller
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