LM5005MH National Semiconductor, LM5005MH Datasheet - Page 14

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Current Limit
The LM5005 contains a unique current monitoring scheme
for control and over-current protection. When set correctly,
the emulated current sense signal provides a signal which is
proportional to the buck switch current with a scale factor of
0.5 V / A. The emulated ramp signal is applied to the current
limit comparator. If the emulated ramp signal exceeds 1.75V
(3.5A) the present cycle is terminated (cycle-by-cycle current
limiting). In applications with small output inductance and
high input voltage the switch current may overshoot due to
the propagation delay of the current limit comparator. If an
overshoot should occur, the diode current sampling circuit
will detect the excess inductor current during the off-time of
the buck switch. If the Sample and Hold DC Level exceeds
the 1.75V current limit threshold, the buck switch will be
disabled and skip pulses until the diode current sampling
circuit detects the inductor current has decayed below the
current limit threshold. This approach prevents current run-
away conditions due to propagation delays or inductor satu-
ration since the inductor current is forced to decay following
any current overshoot.
Soft-Start
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
stresses and surges. The internal soft-start current source,
set to 10µA, gradually increases the voltage of an external
soft-start capacitor connected to the SS pin. The soft-start
capacitor voltage is connected to the reference input of the
error amplifier. Various sequencing and tracking schemes
can be implemented using external circuits that limit or
clamp the voltage level of the SS pin.
In the event a fault is detected (over-temperature, Vcc
UVLO, SD) the soft-start capacitor will be discharged. When
the fault condition is no longer present a new soft-start
sequence will commence.
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Boost Pin
The LM5005 integrates an N-Channel buck switch and as-
sociated floating high voltage level shift / gate driver. This
gate driver circuit works in conjunction with an internal diode
and an external bootstrap capacitor. A 0.022µF ceramic
capacitor, connected with short traces between the BST pin
and SW pin, is recommended. During the off time of the buck
switch, the SW pin voltage is approximately - 0.5V and the
bootstrap capacitor is charged from Vcc through the internal
bootstrap diode. When operating with a high PWM duty
cycle, the buck switch will be forced off each cycle for 500ns
to ensure that the bootstrap capacitor is recharged.
Under very light load conditions or when the output voltage is
pre-charged, the SW voltage will not remain low during the
off-time of the buck switch. If the inductor current falls to zero
and the SW pin rises, the bootstrap capacitor will not receive
sufficient voltage to operate the buck switch gate driver. For
these applications, the PRE pin can be connected to the SW
pin to pre-charge the bootstrap capacitor. The internal pre-
charge MOSFET and diode connected between the PRE pin
and PGND turns on each cycle for 250ns just prior to the
onset of a new switching cycle. If the SW pin is at a normal
negative voltage level (continuous conduction mode), then
no current will flow through the pre-charge MOSFET/diode.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction tem-
perature is exceeded. When activated, typically at 165 de-
grees Celsius, the controller is forced into a low power reset
state, disabling the output driver and the bias regulator. This
feature is provided to prevent catastrophic failures from ac-
cidental device overheating.