DESIGN IDEAS
48V Hot Swap Circuit Blocks
Reverse Battery Voltage
Hot Swap controllers guard against
inrush current and short circuits,
but reverse battery installation is
another matter. In central office applications, OR-ing diodes block
reversed input voltages. In systems
with a single power source OR-ing
diodes become unnecessary; although
a single diode could be retained for
reverse conditions, its forward drop
loss is a significant penalty.
The circuit shown in Figure 1 eliminates the loss associated with a
blocking diode. The LT1641 and Q1
handle familiar hot swap chores of
undervoltage lockout, inrush control,
short circuit protection and system
reset, while Q2 and Q3 handle reverse
input situations.
Under positive input conditions,
Q2’s body diode is forward biased and
power reaches the LT1641 and Q1,
allowing them to function in the normal manner. When the LT1641 is
commanded to turn on, GATE (pin 6)
and R7 slowly charge C1, thereby
limiting the inrush current to CLOAD.
The LT1641 drives both Q1 and Q2
fully into enhancement mode, minimizing losses and eliminating the drop
in Q2’s body diode.
Q3 is included as part of the circuitry that blocks reverse inputs, yet
it must “get out of the way” when
positive inputs are present. With a
positive input, Q3’s emitter is pulled
up, dragging along its base via D2.
Since the base is slightly negative
with respect to the emitter, Q3 is off.
A small current flows through resistor R9 to ground, but this is of no
consequence. If the LT1641 is in the
off state, GATE pulls low. Current
flows from the forward-biased collector -base junction of Q3 to the
LT1641’s GATE pin, but is limited by
R7. When the LT1641 turns on, GATE
pulls up above the input supply and
D3
*
36V–72V
INPUT
LONG
RSNUB
100Ω
D5
R7 1M
C1, 10nF/160V
R9
100k
SHORT
D4
INTERNATIONAL RECTIFIER IRF530 (310) 322-3331
ON SEMICONDUCTOR MPSA 42
(602) 244-6600
1N5245, 15V
1N4148
DIODES INC. SMAT70A
(805) 446-4800
BAV21
BODY DIODE
R1
35.7k
1%
R6
1k
8
7
6
VCC
SENSE
GATE
PWRGD
1
ON
3
LT1641
R2
1.24k
1%
C3
100nF
LONG
CLOAD
100µF, TYP
D1
Q3
D2
Q1, Q2:
Q3:
D1:
D2:
D3:
D4–D5:
*
OUTPUT
Q1
R5
10Ω
R8
10Ω
CSNUB
10nF
the collector-base junction of Q3
becomes reverse-biased. Q3 is off, so
no collector current flows and the
LT1641’s GATE output is not loaded
by Q3.
If a reverse input polarity is applied,
Q3 goes to work and ensures that Q2
is held off. Negative inputs pull the
emitter of Q3 below ground. Q3 turns
on, biased by R9, and the collector
effectively shorts Q2’s gate and source.
With Q2 in the off state, no current
can flow into RS, the LT1641 or Q1.
The circuit can take up to –100V in
this condition. Of course, some leakage current does flow in Q2; this is
absorbed by D3.
This circuit can handle instantaneous steps from zero to ±75V. As
shown, short circuit protection limits
the output to 2.5A. Q1, Q2, and RS
can be scaled to handle loads in excess
of 1kW.
*
RS
0.02Ω
Q2
by Mitchell Lee
FB
GND
TIMER
4
5
RESET
R3
35.7k
1%
2
R4
1.21k
1%
C2
1µF
10V
RTN
Figure 1. Reverse-battery protected LT1641 48V Hot Swap circuit
30
Linear Technology Magazine • February 2001