LM25005EVAL National Semiconductor, LM25005EVAL Datasheet - Page 13

• Manufacturer Part Number: LM25005EVAL
• Description: BOARD EVALUATION LM25005
• Manufacturer: National Semiconductor

Specifications of LM25005EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 2.5A
• Voltage - Input: 7 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25005
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Current Limit
The LM25005 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 current 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 runaway condi-
tions due to propagation delays or inductor saturation 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 LM25005 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 ca-
pacitor 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 temper-
ature is exceeded. When activated, typically at 165 degrees
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 accidental
device overheating.
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