LM2591HV datasheet(9/20 Pages) NSC

9 / 20 page

Block Diagram

10129383

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.

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.

Ground (Pin 3) — Circuit ground.

Feedback (Pin 4) — Senses the regulated output voltage to

complete the feedback loop. This pin is directly connected to

the Output for the fixed voltage versions, but is set to 1.23V

by means of a resistive divider from the output for the

Adjustable version. If a feedforward capacitor is used (Ad-

justable version), then a negative voltage spike is generated

on this pin whenever the output is shorted. This happens

because the feedforward capacitor cannot discharge fast

enough, and since one end of it is dragged to Ground, the

other end goes momentarily negative. To prevent the energy

rating of this pin from being exceeded, a small-signal Schot-

tky diode to Ground is recommended for DC input voltages

above 40V whenever a feedforward capacitor is present

(See Figure 1). Feedforward capacitor values larger than 0.1

µF are not recommended for the same reason, whatever be

the DC input voltage.

ON /OFF (Pin 5) — The regulator is in shutdown mode,

drawing about 90 µA, when this pin is driven to a high level

(

≥ 2.0V), and is in normal operation when this Pin is left

floating or driven to a low level (

≤ 0.6V). The typical value of

the threshold is 1.3V and the voltage on this pin must not

exceed 25V.

LM2591HV Datasheet(PDF) 9 Page - National Semiconductor (TI)