Block Diagram

10133330

PIN FUNCTIONS

+V

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

mately −0.5V, with a duty cycle of V

Error Flag (Pin 3) — Open collector output that goes active

low (

≤ 1.0V) when the output of the switching regulator is out

of regulation (less than 95% of its nominal value). In this

state it can sink maximum 3mA. When not low, it can be

pulled high to signal that the output of the regulator is in

regulation (power good). During power-up, it can be pro-

grammed to go high after a certain delay as set by the Delay

pin (Pin 5). The maximum rating of this pin should not be

exceeded, so if the rail to which it will be pulled-up to is

higher than 45V, a resistive divider must be used instead of

a single pull-up resistor, as indicated in

Figure 1.

Ground (Pin 4) — Circuit ground.

Delay (Pin 5) — This sets a programmable power-up delay

from the moment that the output reaches regulation, to the

high signal output (power good) on Pin 3. A capacitor on this

pin starts charging up by means on an internal () 3 µA)

current source when the regulated output rises to within 5%

of its nominal value. Pin 3 goes high (with an external

pull-up) when the voltage on the capacitor on Pin 5 exceeds

1.3V. The voltage on this pin is clamped internally to about

1.7V. If the regulated output drops out of regulation (less

than 95% of its nominal value), the capacitor on Pin 5 is

rapidly discharged internally and Pin 3 will be forced low in

about 1/1000

Feedback (Pin 6) — 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.

Shutdown /Soft-start (Pin 7) — The regulator is in shut-

down mode, drawing about 90 µA, when this pin is driven to

a low level (

≤ 0.6V), and is in normal operation when this Pin

is left floating (internal pull-up) or driven to a high level (

≥

2.0V). The typical value of the threshold is 1.3V and the pin

is internally clamped to a maximum of about 7V. If it is driven

higher than the clamp voltage, it must be ensured by means

of an external resistor that the current into the pin does not

exceed 1mA. The duty cycle is minimum (0%) if this Pin is

below 1.8V, and increases as the voltage on the pin is

increased. The maximum duty cycle (100%) occurs when

this pin is at 2.8V or higher. So adding a capacitor to this pin

produces a softstart feature. An internal current source will

charge the capacitor from zero to its internally clamped

value. The charging current is about 5 µA when the pin is

below 1.3V but is reduced to only 1.6 µA above 1.3V, so as

to allow the use of smaller softstart capacitors.

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