LM2599 datasheet(18/31 Pages) NSC

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Block Diagram

Application Information

PIN FUNCTIONS

IN (Pin 1) — This is the positive input supply for the IC

switching regulator. A suitable input bypass 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.

Output (Pin 2) — Internal switch. The voltage at this pin

switches between approximately (+V

IN −VSAT) and approxi-

mately −0.5V, with a duty cycle of V

OUT/VIN. To minimize

coupling to sensitive circuitry, the PC board copper area con-

nected to this pin should be kept to a minimum.

Feedback (Pin 6) — Senses the regulated output voltage to

complete the feedback loop.

Shutdown /Soft-start (Pin 7) — This dual function pin pro-

vides the following features: (a) Allows the switching regula-

tor circuit to be shut down using logic level signals thus drop-

ping the total input supply current to approximately 80 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.

Error Flag (Pin 3) — Open collector output that provides a

low signal (flag transistor ON) when the regulated output

voltage drops more than 5% from the nominal output volt-

age. On start up, Error Flag is low until V

OUT 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.

Delay (Pin 5) — At power-up, this pin can be used to provide

a time delay between the time the regulated output voltage

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

error flag output goes high.

Special Note If any of the above three features (Shutdown

/Soft-start, Error Flag, or Delay) are not used, the respective

pins should be left open.

EXTERNAL COMPONENTS

SOFT-START CAPACITOR

SS — 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 (ap-

proximately 5 µA begins charging this capacitor). As the ca-

pacitor voltage rises, the regulator goes through four operat-

ing regions (See the bottom curve in

Figure 13).

1. Regulator in Shutdown. When the SD /SS pin voltage is

between 0V and 1.3V, the regulator is in shutdown, the out-

put voltage is zero, and the IC quiescent current is approxi-

mately 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 cur-

rent 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 be-

tween 1.8V and 2.8V (@ 25˚C), the regulator is in a Soft-start

condition. The switch (Pin 2) 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 con-

trolled ramp up. See the center curve in

Figure 13. The input

supply current requirement also starts out at a low level for

DS012582-30

FIGURE 12.

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