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参数资料
| 型号： | LM5025CSDX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, DSO16 |
| 封装： | LLP-16 |
| 文件页数： | 3/16页 |
| 文件大小： | 362K |
| 代理商： | LM5025CSDX |
charging the SS capacitor protects a continuously over-load-
ed converter with a low duty cycle hiccup mode.
These two modes of over-current protection allow the user
great flexibility to configure the system behavior in over-load
conditions. If it is desired for the system to act as a current
source during an over-load, then the CS1 cycle-by-cycle cur-
rent limiting should be used. In this case the current sense
signal should be applied to the CS1 input and the CS2 input
should be grounded. If during an overload condition it is de-
sired for the system to briefly shutdown, followed by softstart
retry, then the CS2 hiccup current limiting mode should be
used. In this case the current sense signal should be applied
to the CS2 input and the CS1 input should be grounded. This
shutdown / soft-start retry will repeat indefinitely while the
over-load condition remains. The hiccup mode will greatly re-
duce the thermal stresses to the system during heavy over-
loads. The cycle-by-cycle mode will have higher system
thermal dissipations during heavy overloads, but provides the
advantage of continuous operation for short duration overload
conditions.
It is possible to utilize both over-current modes concurrently,
whereby slight overload conditions activate the CS1 cycle-by-
cycle mode while more severe overloading activates the CS2
hiccup mode. Generally the CS1 input will always be config-
ured to monitor the main switch FET current each cycle. The
CS2 input can be configured in several different ways de-
pending upon the system requirements.
a) The CS2 input can also be set to monitor the main switch
FET current except scaled to a higher threshold than CS1.
b) An external over-current timer can be configured which
trips after a pre-determined over-current time, driving the CS2
input high, initiating a hiccup event.
c) In a closed loop voltage regulaton system, the COMP input
will rise to saturation when the cycle-by-cycle current limit is
active. An external filter/delay timer and voltage divider can
be configured between the COMP pin and the CS2 pin to
scale and delay the COMP voltage. If the CS2 pin voltage
reaches 0.5V a hiccup event will initiate.
A small RC filter, located near the controller, is recommended
for each of the CS pins. The CS1 input has an internal FET
which discharges the current sense filter capacitor at the con-
clusion of every cycle, to improve dynamic performance. This
same FET remains on an additional 50ns at the start of each
main switch cycle to attenuate the leading edge spike in the
current sense signal. The CS2 discharge FET only operates
following a CS2 event, UVLO and thermal shutdown.
The LM5025C CS comparators are very fast and may re-
spond to short duration noise pulses. Layout considerations
are critical for the current sense filter and sense resistor. The
capacitor associated with the CS filter must be placed very
close to the device and connected directly to the pins of the
IC (CS and GND). If a current sense transformer is used, both
leads of the transformer secondary should be routed to the
filter network , which should be located close to the IC. If a
sense resistor in the source of the main switch MOSFET is
used for current sensing, a low inductance type of resistor is
required. When designing with a current sense resistor, all of
the noise sensitive low power ground connections should be
connected together near the IC GND and a single connection
should be made to the power ground (sense resistor ground
point).
Oscillator and Sync Capability
The LM5025C oscillator is set by a single external resistor
connected between the RT pin and GND. To set a desired
oscillator frequency (F), the necessary RT resistor can be
calculated from:
RT = (6002/F)1.0192
where F is in kHz and RT in k
.
The RT resistor should be located very close to the device
and connected directly to the pins of the IC (RT and GND).
A unique feature of LM5025C is the ability to synchronize the
oscillator to an external clock with a frequency that is either
higher or lower than the frequency of the internal oscillator.
The lower frequency sync frequency range is 91% of the free
running internal oscillator frequency. There is no constraint
on the maximum SYNC frequency. A minimum pulse width of
100 ns is required for the synchronization clock . If the syn-
chronization feature is not required, the SYNC pin should be
connected to GND to prevent any abnormal interference . The
internal oscillator can be completely disabled by connecting
the RT pin to REF. Once disabled, the sync signal will act
directly as the master clock for the controller. Both the fre-
quency and the maximum duty cycle of the PWM controller
can be controlled by the SYNC signal (within the limitations
of the Volt x Second Clamp). The maximum duty cycle (D) will
be (1-D) of the SYNC signal.
30058914
11
www.national.com
LM5025C
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