參數(shù)資料
型號(hào)： LM5026SD
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： Active Clamp Current Mode PWM Controller
中文描述： 3 A SWITCHING CONTROLLER, 660 kHz SWITCHING FREQ-MAX, DSO16
封裝： 5 X 5 MM, EXPOSED PAD, LLP-16
文件頁(yè)數(shù)： 16/23頁(yè)
文件大小： 936K
代理商： LM5026SD
Oscillator and Sync Capability
(Continued)
Multiple LM5026 devices can be synchronized together sim-
ply by connecting the devices SYNC pins together as shown
in
Figure 9
. Care should be taken to ensure the ground
potential differences between devices are minimized. In this
configuration all of the devices will be synchronized to the
highest frequency device. The internal block diagram of the
oscillator and synchronization circuit is shown in
Figure 10
.
The SYNC I/O pin is a CMOS buffer with pull-up current
limited to 200 μA. If an external device forces the SYNC pin
low before the internal oscillator ramp completes its charging
cycle, the ramp will be reset and another cycle begins. If the
SYNC pins of multiple LM5026 devices are connected to-
gether, the first SYNC pin that pulls low will reset the oscil-
lator RAMP of all other devices. All controllers will operate in
phase when synchronized using the SYNC I/O feature. Up to
five LM5026 devices can be synchronized using this tech-
nique.
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 165C,
the controller is forced into a low power standby state with
the output drivers and the bias regulator disabled. The de-
vice will restart after the thermal hysteresis (typically 25C).
During thermal shutdown, the soft-start capacitor is fully
discharged and the controller follows a normal start-up se-
quence after the junction temperature falls to the operating
level.
Applications Information
LINE INPUT (VIN)
The LM5026 contains an internal high voltage start-up regu-
lator that allows the input pin (VIN) to be connected directly
to a nominal 48V line voltage. The voltage applied to the VIN
pin can vary in the range of 13 to 100V with transient
capability to 105V. When power is applied and the UVLO pin
potential is greater than 0.4V, the VCC regulator is enabled
and sources current into an external capacitor connected to
the VCC pin. When the voltage on the VCC pin reaches the
regulation point of 7.7V, the internal voltage reference (REF)
is enabled. The reference regulation set point is 5V. The
controller outputs are enabled when the UVLO pin potential
is greater than 1.25V. In typical applications, an auxiliary
transformer winding is connected through a diode to the
VCC pin. This winding must raise the VCC voltage above 8V
to shut off the internal start-up regulator. It is recommended
a filtering circuit shown in
Figure 11
be used to suppress
transients, which may occur at the input supply, in particular
when VIN is operated close to the maximum operating rat-
ing.
FOR APPLICATION
>
100V
For applications where the system input voltage exceed
100V or IC power dissipation is a concern, the LM5026 can
be powered from an external start-up regulator as shown in
Figure 12
. In this configuration, the VIN and the VCC pins
should be connected together, which allows the LM5026 to
be operated below 13V. The voltage at the VCC pin must be
greater than 8V yet not exceed 15V.An auxiliary winding can
be used to reduce the dissipation in the external regulator
once the power converter is active.
20147920
FIGURE 9. Sync from Multiple Devices
20147921
FIGURE 10. Oscillator Sync I/O Block Diagram
20147922
FIGURE 11. Input Transient Protection
L
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