10
LT1579
PIN FUNCTIONS
UUU
nally clamped to 7V and –0.6V (one V
BE
). If unused, this
pin can be left open circuit. Device operation is unaffected
if this pin is not connected.
DROPOUT: The dropout flag is an open collector output
which pulls low when both input voltages drop suffi-
ciently for the LT1579 to enter the dropout region. This
signals that the output is beginning to go unregulated.
The DROPOUT output voltage is 1V when sinking 5mA,
dropping to under 200mV at 20µA (see curve of Logic
Flag Voltage vs Current in the Typical Performance Char-
acteristics). This makes the DROPOUT pin equally useful
in driving both bipolar and CMOS logic inputs with the
addition of an external pull-up resistor. It is also capable
of driving higher current devices, such as LEDs. This pin
is internally clamped to 7V and –0.6V (one V
BE
). If
unused, this pin can be left open circuit. Device operation
is unaffected if this pin is not connected.
BIASCOMP: This is a compensation point for the internal
bias circuitry. It must be bypassed with a 0.01µF capaci-
tor for stability during the switch from V
IN1
to V
IN2
.
LBI1: This is the noninvering input to low-battery com-
parator LB1 which is used to detect a low input/battery
condition. The inverting input is connected to a 1.5V
reference. The low-battery comparator input has 18mV of
hysteresis with more than 20µA of sink current on the
output (see Applications Information section). This pin is
internally clamped to 7V and –0.6 (one V
BE
). If not used,
this pin can be left open circuit, with no effect on normal
circuit operation. If unconnected, the pin will float to 1.5V
and the logic output of LB1 will be high impedance.
LBI2: This is the noninverting input to low-battery com-
parator LB2 which is used to detect a low input/battery
condition. The inverting input is connected to a 1.5V
reference. The low-battery comparator input has 18mV of
hysteresis with more than 20µA of sink current on the
output (see Applications Information section). This pin is
internally clamped to 7V and – 0.6V (one V
BE
). If not used,
this pin can be left open circuit, with no effect on normal
circuit operation. If unconnected, the pin will float to 1.5V
and the logic output of LB2 will be high impedance.
out of the pin (see curve of Adjust Pin Bias Current vs
Temperature in the Typical Performance Characteristics).
A DC load of 3µA is needed on the output of the adjustable
part to maintain regulation. The adjust pin voltage is 1.5V
referenced to ground and the output voltage range is 1.5V
to 20V.
SHDN: The shutdown pin is used to put the LT1579 into
a low power shutdown state. All functions are disabled if
the shutdown pin is pulled low. The output will be off, all
logic outputs will be high impedance and the voltage
comparators will be off when the shutdown pin is pulled
low. The shutdown pin is internally clamped to 7V and
– 0.6V (one V
BE
), allowing the shutdown pin to be driven
either by 5V logic or open collector logic with a pull-up
resistor. The pull-up resistor is only required to supply
the pull-up current of the open collector gate, normally
several microamperes. If unused, the shutdown pin can
be left open circuit. The device is active if the shutdown
pin is not connected.
SS: The secondary select pin forces the LT1579 to switch
power draw to the secondary input (V
IN2
). This pin is
active low. The current drawn out of V
IN1
is reduced to
3µA when this pin is pulled low. The secondary select pin
is internally clamped to 7V and –0.6V (one V
BE
), allowing
the pin to be driven directly by either 5V logic or open
collector logic with a pull-up resistor. The pull-up resistor
is required only to supply the leakage current of the open
collector gate, normally several microamperes. If sec-
ondary select is not used, it can be left open circuit. The
LT1579 draws power from the primary first if the second-
ary select pin is not connected.
BACKUP: The backup flag is an open collector output
which pulls low when the LT1579 starts drawing power
from the secondary input (V
IN2
). The BACKUP output
voltage is 1V when sinking 5mA, dropping to under
200mV at 20µA (see curve of Logic Flag Voltage vs
Current in the Typical Performance Characteristics). This
makes the BACKUP pin equally useful in driving both
bipolar and CMOS logic inputs with the addition of an
external pull-up resistor. It is also capable of driving
higher current devices, such as LEDs. This pin is inter-
