LM2598 datasheet(21/45 Pages) TI1

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LM2598

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SNVS125C – MARCH 1998 – REVISED APRIL 2013

APPLICATION INFORMATION

Table 9. PIN DESCRIPTIONS

Name

Description

This is the positive input supply for the IC switching regulator. A suitable input bypass

+VIN (Pin 2)

capacitor must be present at this pin to minimize voltage transients and to supply the

switching currents needed by the regulator.

Ground (Pin 4)

Circuit ground.

Internal switch. The voltage at this pin switches between approximately (+VIN − VSAT) and

Output (Pin 1)

approximately

−0.5V, with a duty cycle of VOUT/VIN. To minimize coupling to sensitive

circuitry, the PC board copper area connected to this pin should be kept to a minimum.

Feedback (Pin 6)

Senses the regulated output voltage to complete the feedback loop.

This dual function pin provides the following features: (a) Allows the switching regulator

circuit to be shut down using logic level signals thus dropping the total input supply current to

Shutdown /Soft-start (Pin 7)

approximately 85

μA. (b) Adding a capacitor to this pin provides a soft-start feature which

minimizes startup current and provides a controlled ramp up of the output voltage.

Open collector output that provides a low signal (flag transistor ON) when the regulated

output voltage drops more than 5% from the nominal output voltage. On start up, Error Flag

Error Flag (Pin 3)

is low until VOUT reaches 95% of the nominal output voltage and a delay time determined by

the Delay pin capacitor. This signal can be used as a reset to a microprocessor on power-up.

At power-up, this pin can be used to provide a time delay between the time the regulated

Delay (Pin 5)

output voltage reaches 95% of the nominal output voltage, and the time the error flag output

goes high.

If any of the above three features (Shutdown /Soft-start, Error Flag, or Delay) are not used,

Special Note

the respective pins should be left open.

EXTERNAL COMPONENTS

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 30).

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

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

the output voltage is still zero. Also, as the 1.3V 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 1.8V and 2.8V (@ 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 2.8V. As the duty cycle increases, the

output voltage also increases at a controlled ramp up. See the center curve in Figure 30. 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 startup currents (such as the

inverting configuration) which can load down the input power supply.

Note: The lower curve shown in Figure 30 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. See the Soft-start curve in the Electrical Characteristics section.

4. Normal operation. Above 2.8V, the circuit operates as a standard Pulse Width Modulated switching

regulator. The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately

7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.

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