LM25037EVAL National Semiconductor, LM25037EVAL Datasheet - Page 10

• Manufacturer Part Number: LM25037EVAL
• Description: BOARD EVAL FOR LM25037
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM25037EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 5V
• Current - Output: 10A
• Voltage - Input: 16 ~ 32V
• Regulator Topology: Buck
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25037
• Silicon Manufacturer: National
• Silicon Core Number: LM25037
• Kit Application Type: Power Management - Voltage Regulator
• Application Sub Type: PWM Controllers
• Kit Contents: Board
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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type of resistor is required. When designing with a current
sense resistor, all the noise sensitive, low power ground con-
nections should be connected together near the AGND pin,
and a single connection should be made to the power ground
(sense resistor ground point).
OVERLOAD PROTECTION TIMER
The LM25037provides a current limit restart timer to disable
the outputs and force a delayed restart (hiccup mode) if a
current limit condition is repeatedly sensed. The number of
cycle-by-cycle current limit events required to trigger the
restart is programmed by the external capacitor at the RES
pin. During each PWM cycle, the LM25037 either sources to
or sinks current from the RES pin capacitor. If no current limit
is detected during a cycle, a 8 µA discharge current sink is
enabled to pull the RES pin towards ground. If a current limit
is detected, the 8 µA sink current is disabled and an 18 µA
current source causes the voltage at the RES pin to gradually
increase. The LM25037 protects the converter with cycle-by-
cycle current limiting while the voltage at RES pin increases.
If the RES voltage reaches the 2.0V threshold, the following
restart sequence occurs (also see
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The RES capacitor and SS capacitors are fully discharged.
The soft-start current source is reduced from 100 µA to 1
µA.
The SS capacitor voltage slowly increases. When the SS
voltage reaches
the first narrow output pulse. After the first pulse occurs,
the SS source current reverts to the normal 100 µA level.
The SS voltage increases at its normal rate, gradually
increasing the duty cycle of the output drivers.
If the overload condition persists after restart, cycle-by-
cycle current limiting will begin to increase the voltage on
the RES capacitor again, repeating the hiccup mode
sequence.
≊
1V, the PWM comparator will produce
Figure
2):
FIGURE 2. Current Limit Restart Circuit
10
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The overload timer function is very versatile and can be con-
figured for the following modes of protection:
1.
2.
3.
4.
If the overload condition no longer exists after restart, the
RES pin will be held at ground by the 8 µA current sink and
normal operation resumes.
Cycle-by-cycle only: The hiccup mode can be
completely disabled by connecting a zero to 50 kΩ
resistor from the RES pin to AGND. In this configuration,
the cycle-by-cycle protection will limit the output current
indefinitely and no hiccup sequences will occur.
Hiccup only: The timer can be configured for immediate
activation of a hiccup sequence upon detection of an
overload by leaving the RES pin open circuit. In this
configuration, the first detection of current limit condition
by the CS pin comparator will initiate a hiccup cycle with
SS capacitor fully discharged and a delayed restart.
Delayed Hiccup: Connecting a capacitor to the RES pin
provides a programmed interval of cycle-by-cycle limiting
before initiating a hiccup mode restart, as previously
described. The dual advantages of this configuration are
that a short term overload will not cause a hiccup mode
restart but during extended overload conditions, the
average dissipation of the power converter will be very
low.
Externally Controlled Hiccup: The RES pin can also
be used as an input. By externally driving the pin to a level
greater than the 2.0V hiccup threshold, the controller will
be forced into the delayed restart sequence. For
example, the external trigger for a delayed restart
sequence could come from an over-temperature
protection circuit or an output over-voltage sensor
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