LM5069EVAL/NOPB National Semiconductor, LM5069EVAL/NOPB Datasheet - Page 2

• Manufacturer Part Number: LM5069EVAL/NOPB
• Description: BOARD EVALUATION LM5069
• Manufacturer: National Semiconductor

Specifications of LM5069EVAL/NOPB

• Main Purpose: Power Management, Hot Swap Controller
• Utilized Ic / Part: LM5069
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Other names: LM5069EVAL

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Theory of Operation
The LM5069 provides intelligent control of the power supply
connections of a load which is to be connected to a live power
source. The two primary functions of a hot swap circuit are in-
rush current limiting during turn-on, and monitoring of the load
current for faults during normal operation. Additional functions
include Under-Voltage Lock-Out (UVLO) and Over-Voltage
Lock-Out (OVLO) to ensure voltage is supplied to the load
only when the system input voltage is within a defined range,
power limiting in the series pass FET (Q1) during turn-on, and
a Power Good logic output (PGD) to indicate the circuit status.
Upon applying the input voltage to the LM5069 (e.g., SW1 is
switched on), Q1 is initially held off for the insertion delay
(
side. At the end of the insertion delay, if the input voltage at
VIN is between the UVLO and OVLO thresholds, Q1 is turned
on in a controlled manner to limit the in-rush current. If the in-
rush current were not limited during turn-on, the current would
be high (very high!) as the load capacitors (C3, C4) charge
up, limited only by the surge current capability of the voltage
source, C7’s characteristics, and the wiring resistance (a few
milliohms). That very high current could damage the edge
connector, PC board traces, and possibly the load capacitors
receiving the high current. Additionally, the dV/dt at the load’s
input is controlled to reduce possible EMI problems.
The LM5069 limits in-rush current to a safe level using a two
step process. In the first portion of the turn-on cycle, when the
voltage differential across Q1 is highest, Q1’s power dissipa-
tion is limited to a peak of 45W by monitoring its drain current
(the voltage across R10) and its drain-to-source voltage.
≊
600 ms) to allow ringing and transients at the input to sub-
FIGURE 2. Evaluation Board Schematic
2
Their product is maintained constant by controlling the drain
current as the drain-to-source voltage decreases (as the out-
put voltage increases). This is shown in the constant power
portion of Figure 3 where the drain current is increasing to
I
old (5.5 Amps), it is then maintained constant as the output
voltage continues to increase. When the output voltage
reaches the input voltage (V
drain current then reduces to a value determined by the load.
Q1’s gate-to-source voltage then increases to
the OUT voltage. The circuit is now in normal operation mode.
Monitoring of the load current for faults during normal opera-
tion is accomplished using the current limit circuit described
above. If the load current increases to 5.5 Amps (55 mV
across R10), Q1’s gate is controlled to prevent the current
from increasing further. When current limiting takes effect, the
fault timer limits the duration of the fault. At the end of the fault
timeout period (
load. The LM5069-2 then initiates a restart every 7.7 seconds.
The restart consists of turning on Q1 and monitoring the load
current to determine if the fault is still present. After the fault
is removed, the circuit powers up to normal operation at the
next restart.
In a sudden overload condition (e.g., the output is shorted to
ground), it is possible the current could increase faster than
the response time of the current limit circuit. In this case, the
circuit breaker sensor shuts off Q1’s gate rapidly when the
voltage across R10 reaches
duces to the current limit threshold, the current limit circuitry
then takes over.
LIM
. When the drain currrent reaches the current limit thresh-
≊
38 ms) Q1 is shut off, denying current to the
DS
≊
105 mV. When the current re-
decreases to near zero), the
≊
12V above
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