6
DEMO MANUAL DC116
HOT SWAP CONTROLLER
The demonstration board set, DC116, is easily set up for
evaluation of the LTC1421 IC. Please follow the proce-
dure outlined below for error-free operation.
• Connect the positive power supply with the lower
potential to V
CC
(TP9) and the higher one to V
DD
(TP8)
on DC116B. Connect the ground lead(s) to TP6. Do
not exceed 12.6V for either supply to ensure proper
operation of the LTC1421.
• Connect the negative power supply to V
EE
(TP7)
and GND (TP6) on DC116B. Do not exceed –12.6V
on V
EE
.
QUICK START GUIDE
• Connect the loads to outputs V
CC2
(TP8), V
DD2
(TP3)
and V
EE2
(TP2) on the DC116A board. Connect the
return side of the lead(s) to TP12 or TP16 (GND).
• Select the proper dip switches to monitor the outputs
and set the trip level for the RESET output.
• Apply power and insert the daughter board (DC116A)
into the system backplane (DC116B). Rock the daugh-
ter board back and forth to ensure good connection.
• The LEDs will turn on to indicate the status of the input
and output supplies as well as the circuit breaker.
OPERATIO
U
The circuit shown in the Schematic Diagram allows three
power supplies, V
DD
, V
CC
and V
EE
, to be safely inserted
into and removed from the backplane. V
CC
and V
DD
are
both positive supplies, whereas V
EE
is a negative supply.
The main supply to the LTC1421 is from V
CC
; its voltage
should be less than or equal to that of V
DD
. The nominal
voltages are 5V for V
CC
, 12V for V
DD
and –12V for V
EE
.
A 0.005
Ω
sense resistor (R1) sets the current limit to 10A
at the V
CC
output; R3 (0.025
Ω
) sets the current limit at V
DD
to 2A. The LTC1421 monitors the voltage across the sense
resistors. When either voltage is greater than 50mV, the
internal charge pump is turned off immediately and both
the gates and voltage outputs are actively pulled to ground.
The circuit breaker function remains active until the push-
button switch (S2) is pressed and released or the power is
cycled.
The LTC1421 limits the inrush current through the
N-channel pass transistor by increasing the voltage on the
gate in a controlled manner. The transient surge current
(I = C
OUT
• dV
OUT
/dt) drawn from the main backplane
power supply can then be limited to a safe value. The ramp
slope is determined by a fixed internal current source
(20
µ
A) and a ramp capacitor (C9) connected between the
RAMP and GATEHI pins. The voltage at GATEHI rises with
a slope equal to 20
µ
A/C
RAMP
. The voltage at the GATELO
pin is clamped one Schottky diode drop below GATEHI.
The negative supply voltage can be controlled using the
CPON pin. When the board makes a connection, the
N-channel pass transistor, Q2, is turned off by R4. CPON
is also pulled down to V
EE1
. When the charge pump is
turned on, CPON is pulled to V
CC1
and the gate of Q2 ramps
up with a time constant determined by R4, R5 and C2.
When the charge pump is turned off, CPON goes into high
impedance state; the gate of Q2 is then discharged to V
EE1
with a time constant determined by R4 and C2 and then Q2
turns off. There is no circuit breaker or current-limit
feature on the negative supply.
Power N-Channel MOSFET
External N-channel pass transistors are used to route the
power from the system power supply to the plug-in board.
An MTB50N06E from Motorola is used for the V
CC
output
and two 8-lead, surface mounted NFETs from Siliconix
(Si4410DY) are used for V
DD
and V
EE
. A number of similar
N-channel MOSFETs, available from different manufactur-
ers, are also well-suited for this type of application. As a
general rule, select the MOSFET with the lowest R
DS(ON)
to
get the smallest voltage drop across it at the maximum
output load.
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