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LT3582/LT3582-5/LT3582-12

3582512f

Output Disconnect Operating Limits

The LT3582 series has a PMOS output disconnect switch

connected between CAPP and VOUTP. During normal

operation, the switch is closed and current is internally

limited to about 155mA (see Figure 9). Make sure that

the output load current doesn’t exceed the PMOS current

limit. Exceeding the current limit causes a signiﬁcant

rise in PMOS power consumption which may damage

the device.

During shutdown, the PMOS switch is open and CAPP is

isolated from VOUTP up to a voltage difference of 5-5.5V.

In most cases this allows VOUTP to discharge to ground.

However, when the Boost inductor input exceeds 5.5V,

the CAPP-VOUTP voltage may exceed 5V allowing some

current ﬂow through the PMOS switch. In addition, apply-

ing CAPP-VOUTP voltages in excess of 5.7V(typical) may

activate internal protection circuitry which turns the PMOS

“on” (see Figure 10). If the current is not limited, this can

lead to a sharp increase in the PMOS power consumption

and may damage the device. If this situation cannot be

avoided, limit PMOS power consumption to less than 1/3

Watt (about 50mA at 7V) to avoid damaging the device.

Refer to the Absolute Maximum Ratings table for maximum

limits on CAPP-VOUTP voltages and currents.

Improving Boost Converter Efﬁciency

The efﬁciency of the Boost converter can be improved by

shorting the CAPP pin to the VOUTP pin (see Figure 11). The

power loss in the PMOS disconnect circuit is then made

negligible. In most applications, the associated CAPP pin

capacitor can be removed and the larger VOUTP capacitor

can adequately ﬁlter the output voltage.

Note that the ripple voltage on VOUTP will typically in-

crease in this conﬁguration since the output disconnect

PMOS, when not shorted, helps to create an RC ﬁlter at

the output. Also, if the VOUTP pin is shorted to CAPP,

the power-down discharge should not be enabled. VOUTP

cannot be discharged to ground during shutdown due to

the path from VIN to VOUTP through the external inductor

APPLICATIONS INFORMATION

LT3582

VIN

CAPP

VOUTP

SDA

VPP

SHDN

GND

SWN

VOUTN

3582512 F12

SCL

SWP

RAMPN

RAMPP

ILOAD

Figure 10: PMOS Current vs. Voltage During Shutdown

Figure 11: Improved Efﬁciency

ICAPP-VOUTP

20μA/DIV

VCAPP-VOUTP 1V/DIV

3582512 F11

Figure 9: PMOS Current vs. Voltage During Normal Operation

CAPP-VOUTP (mV)

100

200

400

300

500

100

120

140

160

180

3582512 F10

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