Datasheet

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135 x 10

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Internal Enable Signal

5.25V

LM25574, LM25574-Q1

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SNVS483G –JANUARY 2007–REVISED APRIL 2013

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 45V. 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 9. Vin and Vcc Sequencing

Shutdown / Standby

The LM25574 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 SD

pin is internally clamped with a 1kΩ resistor and an 8V zener clamp. The voltage at the SD pin should never

exceed 14V. If the voltage at the SD pin exceeds 8V, the bias current will increase at a rate of 1 mA/V.

The SD pin can also be used to implement various remote enable / disable functions. Pulling the SD pin below

the 0.7V threshold totally disables the controller. If the SD pin voltage is above 1.225V the regulator will be

operational.

Oscillator and Sync Capability

The LM25574 oscillator frequency is set by a single external resistor connected between the RT pin and the

AGND pin. The R

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 R

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 R

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 15ns.

Product Folder Links: LM25574 LM25574-Q1