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Maxim > Design Support > Technical Documents > Reference Designs > Hot-Swap and Power Switching Circuits > APP 4163
Keywords: telecom,hot-swap,16ms dropout,150V transient,MAX5921,ATCA,transient immune, dropout immune,80
Watt,holdup
REFERENCE DESIGN 4163 INCLUDES: Tested Circuit Schematic BOM Description Test Data Layout
Telecom Hot-Swap Reference Design Is Immune to
Overvoltage and Brownout Input Transients
Mar 26, 2008
Abstract:
This 80W telecom
hot-swap reference design is immune to 150V input-voltage transients and 16ms power
dropouts. Additional features include -32V to -72V operation, two-diode OR-ing input, and an inrush current that is
≤ 1.5 times the full-load current. Included are a detailed schematic, representative PCB layout, bill of materials,
waveforms detailing actual performance at startup, -150V input transient, and full-load holdup at power loss.
Introduction
Telecom systems typically must provide full-power operation from either of two voltage sources that may range
between -36V and -72V. Protection must additionally be provided against lightning-induced, higher-voltage pulses
and occasional power brownout conditions where the input voltage may drop to 0V for several milliseconds. A plug-
in line card may include a high-voltage to low-voltage DC-DC converter preceded by a hot-swap circuit. This circuit
limits inrush current to little more than full-load current when the line card is hot-plugged into a live backplane. The
hot-swap circuit must provide a power-good (active-low PGOOD) signal to activate the following DC-DC converter
after on-card filter capacitors have reached full charge. A hot-swap circuit meeting all requirements is detailed in
this document. Most requirements are similar to those of an AdvancedTCA® (ATCA®) first-level hot-swap circuit.
Typical Telecom System Requirements
1. Input Voltage: -43V to -72V Operation
2. Two-Diode OR-ing at Input
3. -32V to -36V Turn-Off (at the Power Source Before the Diode)
4. 80W (max) Input Power, C
LOAD
= 680µF
5. Withstand -150V, 1ms Overvoltage Pulse¹
6. Ride-Through, 0V, 16ms Brownout Beginning at -43V Input Voltage; Active-Low PGOOD Must Remain
Asserted Throughout this Period
7. Inrush Current ≤ 1.5x Full-Load Current at 43V Startup Voltage
Reference Design Features and Considerations
1. The undervoltage-lockout (UVLO) increasing voltage is set to ~43V. The overvoltage-lockout (OVLO)
increasing voltage is set to ≥ 72V.
2. Two 100V Schottky diodes installed at the input allow OR-ing between two discrete sources.
3. The UVLO decreasing voltage is set to ~32V. The design must satisfy Telecom System Requirement 6 while
maintaining normal operation with active-low PGOOD active down to the UVLO decreasing voltage.
4. P
IN
= 80W at V
IN
= 43V indicates an input current of 1.86A.
5. A 70V TVS diode clamps any input-voltage pulse to a safe level for U1, C3, C4, and Q2. This diode may be
placed either directly across the input terminals or across the V
DD
–V
EE
pins of the MAX5921 hot-swap
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