LM2598 datasheet(17/47 Pages) TI1

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LM2598

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SNVS125D – MARCH 1998 – REVISED MAY 2016

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

9.1.1 Soft-Start Capacitor (CSS)

A capacitor on this pin provides the regulator with a Soft-start feature (slow start-up). When the DC input voltage

is first applied to the regulator, or when the Shutdown/Soft-start pin is allowed to go high, a constant current

(approximately 5 μA begins charging this capacitor). As the capacitor voltage rises, the regulator goes through

four operating regions (See the bottom curve in Figure 28).

1. Regulator in shutdown: When the SD/SS pin voltage is between 0 V and 1.3 V, the regulator is in shutdown,

the output voltage is zero, and the IC quiescent current is approximately 85 μA.

2. Regulator ON, but the output voltage is zero: With the SD/SS pin voltage between approximately 1.3 V and

1.8 V, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 mA, but the

output voltage is still zero. Also, as the 1.3-V threshold is exceeded, the Soft-start capacitor charging current

decreases from 5 μA down to approximately 1.6 μA. This decreases the slope of capacitor voltage ramp.

3. Soft-start region: When the SD/SS pin voltage is between 1.8 V and 2.8 V at 25°C, the regulator is in a Soft-

start condition. The switch (Pin 1) duty cycle initially starts out very low, with narrow pulses and gradually get

wider as the capacitor SD/SS pin ramps up towards 2.8 V. As the duty cycle increases, the output voltage

also increases at a controlled ramp up. See the center curve in Figure 28. The input supply current

requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. This is a

very useful feature in some switcher topologies that require large start-up currents (such as the inverting

configuration) which can load down the input power supply.

Note: The lower curve shown in Figure 28 shows the Soft-start region from 0% to 100%. This is not the duty

cycle percentage, but the output voltage percentage. Also, the Soft-start voltage range has a negative

temperature coefficient associated with it.

4. Normal operation: Above 2.8 V, the circuit operates as a standard pulse width modulated switching regulator.

The capacitor continues to charge up until it reaches the internal clamp voltage of approximately 7 V. If this

pin is driven from a voltage source, the current must be limited to about 1 mA.

If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately 7 V,

the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To maintain

proper function under these conditions, it is strongly recommended that the SD/SS pin be clamped externally

between the 3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 30 is an example of

an external approximately 3.7-V clamp that prevents a line-step related glitch but does not interfere with the soft-

start behavior of the device.

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