LM5574QMTSLASHNOPB datasheet(11/33 Pages) TI1

11 / 33 page

RT =

- 580 x 10

-9

135 x 10

-12

VIN

VCC

Internal Enable Signal

5.25V

LM5574-Q1

www.ti.com

SNOSB22C – OCTOBER 2008 – REVISED OCTOBER 2014

Device Functional Modes (continued)

An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary

voltage is greater than 7.3 V, 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 76 V. 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 8. VIN and VCCSequencing

7.4.2 Oscillator and Sync Capability

The LM5574-Q1 oscillator frequency is set by a single external resistor connected between the RT pin and the

AGND pin. The RT resistor should be located very close to the device and connected directly to the pins of the IC

(RT and AGND).To set a desired oscillator frequency (F), the necessary value for the RT resistor can be

calculated from the following equation:

(1)

The SYNC pin can be used to synchronize the internal oscillator to an external clock. The external clock must be

of higher frequency than the free-running frequency set by the RT resistor. A clock circuit with an open drain

output is the recommended interface from the external clock to the SYNC pin. The clock pulse duration should

be greater than 15 ns.

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LM5574QMT/NOPB Datasheet(PDF) 11 Page - Texas Instruments