Western Digital caviar blue xl500s Technical Reference Manual page 30

Product Features XL500S
3.11.1.1.2 Precharge Resistors
Current limiting series precharge resistors on designated power pins make contact before
hard power connection is made, providing controlled partial charge of the disk drive's
decoupling capacitors. This partial charge is due to IR drop across the precharge resistor
caused by disk drive DC loads at partial power. A second current surge occurs when final
power connections are made, completing charge of the decoupling capacitors. Use of
precharge resistors requires three stages of contact sequencing on the mating connector.
The first stage makes initial ground contact, establishing a ground reference between the disk
drive and backplane. It also discharges any ESD voltage between the two devices. The
second stage contacts connect the precharge resistors, supplying limited current to the
inserting disk drive's voltage busses. This allows partial charging of decoupling capacitors on
the disk drive. Stage three contacts make hard power and signals connections. The ideal
value for precharge resistors is where peak inrush current for both the precharge resistor
connections and final hard power connections are similar in amplitude. Even with precharge
resistors, there may be some momentary droop of the backplane voltage busses. This residual
droop needs to be eliminated or at least reduced to a very small value because most power
distribution budgets do not provide allowances for this droop. Low ESR bulk capacitors
installed on the backplane voltage busses for each hot plug connector can minimize this
droop. Organic dielectric aluminum electrolytic's like OSCON capacitors are a good choice.
3.11.1.2 Capacitor Inrush Current Issues
Tantalum decoupling capacitors can be damaged if inrush current is excessive. Inrush current
issues with Tantalum capacitors are complex, but manufactures recommend, as a general rule,
1 ohm of resistance for every volt being switched or a maximum inrush current of 1 amp. See
"Surge in solid Tantalum Capacitors" by John Gill of AVX Corporation. This limits the lowest
value of precharge resistance for each voltage bus used to a resistance in ohms equal to the
bus voltage in volts.
3.11.1.3 Connector Inrush Current Issues
The SATA specification requires 1.5 amps current capacity for every power connector pin.
This is well above the initial precharge resistor inrush current limit set by tantalum capacitor
requirements. The second inrush current spike when hard power connection is made along
with whatever DC current has been established at that time could exceed 1.5 amps but for too
short a period of time (<1 ms) to have any affect on connector reliability.
3.11.1.4 Disk Drive Hot Plug Insertion Velocity
The delay between precharge resistor connections and final power connections must be
sufficient to allow precharge resistor charging of the disk drives power busses to at least 90%
of maximum value the precharge resistors are capable of. The following compares calculated
maximum allowable insertion velocity with SATA specification insertion velocity analysis.
3.11.1.4.1 Calculated Velocity
The minimum SATA mating connector staggered pin spacing is 0.35 millimeters. Disk drives
typically have around 20mF input capacitance on power busses. Assuming 20mF total
decoupling capacitance and a 10-ohm series precharge resistor, the precharge time constant
is 0.2 milliseconds. From this, the maximum total precharge time to 90% of full charge is
around 2.2 ´ 0.2 milliseconds or 0.44 milliseconds. Therefore, the maximum insertion velocity
allowed is .035 centimeters divided by 0.44 milliseconds giving a maximum insertion velocity
of 80 centimeters per second.
3.11.1.4.2 SATA Insertion Velocity Analysis
The SATA specification has done tests with hot plug insertion speed. The fastest insertion
velocities achieved produced a staggered contact delay of 3 milliseconds, well within the
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