VIN

VCC

Internal Enable Signal

9V

7V

5.25V

LM5576Q0

SNOSB24B – OCTOBER 2008 – REVISED APRIL 2013

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DETAILED OPERATING DESCRIPTION

The LM5576Q0 switching regulator features all of the functions necessary to implement an efficient high voltage

buck regulator using a minimum of external components. This easy to use regulator integrates a 75V N-Channel

buck switch with an output current capability of 3 Amps. The regulator control method is based on current mode

control utilizing an emulated current ramp. Peak current mode control provides inherent line voltage feed-forward,

cycle-by-cycle current limiting, and ease of loop compensation. The use of an emulated control ramp reduces

noise sensitivity of the pulse-width modulation circuit, allowing reliable processing of very small duty cycles

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

500kHz. An oscillator synchronization pin allows multiple LM5576Q0 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-20 package featuring an exposed pad to aid thermal dissipation.

The functional block diagram and typical application of the LM5576Q0 are shown in Figure 9. The LM5576Q0

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 LM5576Q0 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 10. Vin and Vcc Sequencing

8

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