Datasheet

LT3582/LT3582-5/LT3582-12
20
3582512fb
Output Disconnect Operating Limits
The LT3582 series has a PMOS output disconnect switch
connected between CAPP and V
OUTP
. 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 signifi cant rise in PMOS
power consumption which may damage the device.
During shutdown, the PMOS switch is open and CAPP is
isolated from V
OUTP
up to a voltage difference of 5-5.5V.
In most cases this allows V
OUTP
to discharge to ground.
However, when the Boost inductor input exceeds 5.5V, the
CAPP-V
OUTP
voltage may exceed 5V allowing some current
fl ow through the PMOS switch. In addition, applying CAPP-
V
OUTP
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-V
OUTP
voltages and currents.
Improving Boost Converter Effi ciency
The effi ciency of the Boost converter can be improved by
shorting the CAPP pin to the V
OUTP
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 V
OUTP
capacitor
can adequately fi lter the output voltage.
APPLICATIONS INFORMATION
LT3582
V
IN
CAPP
CAPP
V
OUTP
SDA
V
PP
CASHDN
GND
SWN
SWN
V
OUTN
3582512 F12
SCL
SWP
81
16
15
14
9
10
11
5
12
76
2
13
3
4
RAMPNRAMPP
C1
I
LOAD
Figure 10. PMOS Current vs Voltage During Shutdown
Figure 11. Improved Effi ciency
I
CAPP-VOUTP
20µA/DIV
V
CAPP-VOUTP
1V/DIV
3582512 F11
Figure 9. PMOS Current vs Voltage During Normal Operation
CAPP-V
OUTP
(mV)
0 100 200 400300 500
0
20
40
100
80
120
140
60
160
180
PMOS CURRENT (mA)
3582512 F10