V
IN
V
CC
Internal Enable Signal
9V
7V
6.3V
LM5005
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SNVS397D –SEPTEMBER 2005–REVISED MARCH 2013
High Voltage Start-Up Regulator
The LM5005 contains a dual mode internal high voltage startup regulator that provides the Vcc bias supply for
the PWM controller and boot-strap MOSFET gate driver. The input pin (Vin) can be connected directly to the
input voltage, as high as 75 Volts. For input voltages below 9V, a low dropout switch connects Vcc directly to
Vin. In this supply range, Vcc is approximately equal to Vin. For Vin voltage greater than 9V, the low dropout
switch is disabled and the Vcc regulator is enabled to maintain Vcc at approximately 7V. The wide operating
range of 7 to 75V is achieved through the use of this dual mode regulator.
The output of the Vcc regulator is current limited to 20mA. Upon power up, the regulator sources current into the
capacitor connected to the Vcc pin. When the voltage at the Vcc pin exceeds the Vcc UVLO threshold of 6.3V
and the SD pin is greater than 1.225V, the output switch is enabled and a soft-start sequence begins. The output
switch remains enabled until Vcc falls below 5.3V or the SD pin falls below 1.125V.
An auxiliary supply voltage can be applied to the Vcc pin to reduce the IC power dissipation. If the auxiliary
voltage is greater than 7.3V, the internal regulator will essentially shutoff, reducing the IC power dissipation. The
Vcc regulator series pass transistor includes a diode between Vcc and Vin that should not be forward biased in
normal operation. Therefore the auxiliary Vcc voltage should never exceed the Vin voltage.
In high voltage applications extra care should be taken to ensure the Vin pin does not exceed the absolute
maximum voltage rating of 76V. During line or load transients, voltage ringing on the Vin line that exceeds the
Absolute Maximum Ratings can damage the IC. Both careful PC board layout and the use of quality bypass
capacitors located close to the Vin and GND pins are essential.
Figure 10. Vin and Vcc Sequencing
Shutdown / Standby
The LM5005 contains a dual level Shutdown (SD) circuit. When the SD pin voltage is below 0.7V, the regulator is
in a low current shutdown mode. When the SD pin voltage is greater than 0.7V but less than 1.225V, the
regulator is in standby mode. In standby mode the Vcc regulator is active but the output switch is disabled. When
the SD pin voltage exceeds 1.225V, the output switch is enabled and normal operation begins. An internal 5µA
pull-up current source configures the regulator to be fully operational if the SD pin is left open.
An external set-point voltage divider from Vin to GND can be used to set the operational input range of the
regulator. The divider must be designed such that the voltage at the SD pin will be greater than 1.225V when Vin
is in the desired operating range. The internal 5µA pull-up current source must be included in calculations of the
external set-point divider. Hysteresis of 0.1V is included for both the shutdown and standby thresholds. The
voltage at the SD pin should never exceed 7V. When using an external set-point divider, it may be necessary to
clamp the SD pin to limit its voltage at high input voltage conditions.
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