Internal Enable Signal

9V

7V

5.25V

LM5574

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SNVS478E – JANUARY 2007 – REVISED MARCH 2009

cycles necessary in high input voltage applications. The operating frequency is user programmable from 50kHz

to 500kHz. An oscillator synchronization pin allows multiple LM5574 regulators to self synchronize or be

synchronized to an external clock. The output voltage can be set as low as 1.225V. Fault protection features

include, current limiting, thermal shutdown and remote shutdown capability. The device is available in the

TSSOP-16 package.

The functional block diagram and typical application of the LM5574 are shown in Figure 2. The LM5574 can be

applied in numerous applications to efficiently step-down a high, unregulated input voltage. The device is well

suited for telecom, industrial and automotive power bus voltage ranges.

High Voltage Start-Up Regulator

The LM5574 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 6V to 75V is achieved through the use of this dual mode regulator.

The output of the Vcc regulator is current limited to 25mA. 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 5.35V

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 5.0V 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 shut off, 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 3. Vin and Vcc Sequencing

Shutdown / Standby

The LM5574 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.

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