lm5034mtcx National Semiconductor Corporation, lm5034mtcx Datasheet - Page 20

• Manufacturer Part Number: lm5034mtcx
• Description: High Voltage Dual Interleaved Current Mode Controller With Active Clamp
• Manufacturer: National Semiconductor Corporation

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Applications Information
VIN
The voltage applied to the VIN pin, normally the same as the
system voltage applied to the power transformer’s primary
(V
capability to 105V. The current into VIN depends primarily on
the output driver capacitive loads, the switching frequency,
and any external loads on the VCC pins. If the power dissi-
pation associated with the VIN current exceeds the package
capability, an external voltage should be applied to the VCC
pins (see Figure 4 & Figure 5) to reduce power in the internal
start-up regulator. It is recommended the circuit of Figure 24
be used to suppress transients which may occur at the input
supply, in particular where VIN is operated close to the
maximum operating rating of the LM5034.
When all internal bias currents for the LM5034 and output
driver currents are supplied through VIN and the internal V
regulator, the required input current (I
& Figure 5. In most applications, upon turn-on, I
with V
reached. After the outputs are enabled and the external VCC
supply voltage is active, the current into VIN then drops to a
nominal 120 µA.
FOR APPLICATIONS
For applications where the system input voltage (V
exceeds 100V, VIN can be powered from an external
start-up regulator as shown in Figure 25, or from any other
low voltage source as shown in Figure 26. Connecting VIN
and the VCC together allows the LM5034 to be operated
with VIN below 13V. The voltage at the VCC pins must not
exceed 15V. The voltage source at the right side of Figure 25
is typically derived from the power stage, and becomes
active once the LM5034’s outputs are active.
PWR
FIGURE 25. Start-up Regulator for V
), can vary in the range of 13 to 100V with transient
IN
FIGURE 24. Input Transient Production
as shown in Figure 4 until the UVLO threshold is
>
100V
IN
) is shown in Figure 4
PWR
20136827
IN
>
100V
increases
20136847
PWR
CC
)
20
UVLO
The under-voltage lockout threshold (UVLO) is internally set
at 1.25V at the UVLO pin. With two external resistors as
shown in Figure 27, the LM5034 is enabled when V
exceeds the programmed threshold voltage. When V
above the threshold, the internal 20 µA current source is
enabled to raise the voltage at the UVLO pin, providing
hysteresis. R1 and R2 are determined from the following
equations:
where V
V
example, if the LM5034 is to be enabled when V
20V, and disabled when V
calculates to 150 kΩ, and R2 calculates to 10 kΩ. The
voltage at UVLO should not exceed 6V at any time.
The LM5034 can be remotely shutdown by taking the UVLO
pin below 1.25V with an external open collector or open
drain device, as shown in Figure 28. The outputs, and the
V
power mode. To shut down one regulator without affecting
the other, see the Soft-start section.
FIGURE 26. Bypassing the Internal Start-up Regulator
PWR
CC
regulator, are disabled, and the LM5034 enters a low
in the second equation is the turn-on voltage. For
HYS
is the desired UVLO hysteresis at V
FIGURE 27. UVLO Circuit
R1 = V
PWR
HYS
/20 µA
is decreased to 17V, R1
PWR
PWR
20136828
reaches
PWR
20136848
, and
PWR
is