Block Diagram
10129483
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.
PIN FUNCTIONS
+VIN (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 (+VIN − VSAT) and approxi-
mately −0.5V, with a duty cycle of VOUT/VIN
.
Ground (Pin 3)—Circuit ground.
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.
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
9
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