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LT3032 Series

3032fd

APPLICATIONS INFORMATION

In situations where the ADJP pin is connected to a resistor

divider that would pull the ADJP pin above its 7V clamp

voltage if the output is pulled high, the ADJP pin input

current must be limited to less than 5mA. For example, a

resistor divider is used to provide a 1.5V output from the

1.22V reference and the output is forced to 20V. The top

resistor of the divider must be chosen to limit the current

into the ADJP pin to less than 5mA when the ADJP pin is

at 7V. The 13V difference between OUTP and ADJP divided

by the 5mA maximum current into the ADJP pin yields a

minimum top resistor value of 2.6k.

In circuits where a backup battery is required on the posi-

tive output, several different input/output conditions can

occur. The output voltage may be held up while the input

is either pulled to ground, pulled to some intermediate

voltage or is left open circuit. Current ﬂow back into OUTP

follows the curve shown in Figure 6.

If the INP pin is forced below the OUTP pin or the OUTP

pin is pulled above the INP pin, input current typically

drops to less than 2μA. This can happen if the device is

connected to a discharged (low voltage) battery and the

output is held up by a backup battery or a second regula-

tor circuit. The state of the SHDNP pin has no effect on

the reverse output current if OUTP is pulled above INP.

Figure 6. Reverse Output Current

OUTP PIN VOLTAGE (V)

100

3032 F06

0246

10 12 14 16 18 20

TJ = 25°C, VINP = 0V

CURRENT FLOWS

INTO OUTP PIN

VOUTP = VADJP (LT3032)

LT3032

LT3032-5

LT3032-15

LT3032-12

Like many IC power regulators, the negative side of the

LT3032 has safe operating area (SOA) protection. The safe

operating area protection activates when the differential

voltage between INN and OUTN is greater than -7V. The

SOA protection decreases current limit as a function of

the voltage differential between INN and OUTN and keeps

the power transistor inside a safe operating region for all

values of forward input-to-output voltage. The protection

is designed to provide some output current at all values

of INN to OUTN differential voltage up to the Absolute

Maximum Rating. A 50μA load is required to maintain

regulation for INN to OUTN differential voltages greater

than –7V. When in shutdown, protection circuitry remains

active and will cause the output to rise slightly at zero load.

A small pre-load is needed for zero output, if desired (see

graph of Quiescent Current vs Input Voltage in Typical

Performance Characteristics).

When power to the negative side is ﬁrst turned on, as the

input voltage rises, OUTN follows INN, allowing the regula-

tor to start into very heavy loads. During start-up, as the

INN voltage is rising, the differential voltage between INN

and OUTN is small, allowing the negative side to supply

large output currents. With a high INN voltage, a problem

can occur wherein removal of an output short will not al-

low the output voltage to fully recover. Other regulators,

such as the LT1175, LT1964, and LT3080 also exhibit this

phenomenon, so it is not unique to the LT3032.

The problem occurs with a heavy output load when the INN

voltage is high and the OUTN voltage is low. Common situ-

ations are immediately after the removal of a short-circuit

or when the SHDNN pin is pulled high after the INN pin

has already been turned on. The load line for such a load

may intersect the output current curve at two points. If

this happens, there are two stable operating points for the

negative side of the LT3032. With this double intersection,

the INN supply may need to be cycled down to zero and

brought up again to make OUTN recover.

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