Datasheet

LT1941
17
1941fb
APPLICATIONS INFORMATION
Table 5. Converter Equivalent Model Parameters
STEP-DOWN1 STEP-DOWN2 BOOST INVERTER
V
FB
0.628V 0.628V 1.25 1.24
R
O
500kΩ 500kΩ 500kΩ 500kΩ
g
ma
1700μmho 1700μmho 800μmho 800μmho
g
mp
5mho 3.6mho V
IN
• 5mho
V
OUT
V
IN
• 5mho
–|V
OUT
|
single capacitor providing soft-start. The internal current
sources will charge these pins to ~2V.
The RUN/SS pins provide a soft-start function that limits
peak input current to the circuit during start-up. This helps
to avoid drawing more current than the input source can
supply or glitching the input supply when the LT1941 is
enabled. The RUN/SS pins do not provide an accurate
delay to start or an accurately controlled ramp at the
output voltage, both of which depend on the output ca-
pacitance and the load current. However, the power good
indicators can be used to sequence the three outputs, as
described below.
POWER GOOD INDICATORS
The PGOOD pin is the open-collector output of an internal
comparator. PGOOD remains low until the FB pin is within
10% of the fi nal regulation voltage. Tie the PGOOD to any
supply with a pull-up resistor that will supply less than
200μA. Note that this pin will be open when the LT1941
is in shutdown mode (all three RUN/SS pins at ground)
regardless of the voltage at the FB pin. PGOOD is valid
when the LT1941 is enabled (any RUN/SS pin is high) and
V
IN
is greater than ~3.5V.
The 5GOOD and 12GOOD pins are also open-collector
outputs of internal comparators. The 5GOOD pin remains
low until the input is within 10% of 5V. Tie the 5GOOD and
12GOOD pins to any supply with a pull-up resistor that will
supply less than 100μA. The 12GOOD pin remains low until
the input is within 10% of 12V. The 5GOOD and 12GOOD
pins are valid as long as V
IN
is greater than 1.1V. Both the
5GOOD and 12GOOD pins will sink current when the part
is in shutdown, independent of the voltage at V
IN
.
Output Sequencing
The PG and RUN/SS pins can be used to sequence the
three outputs. Figure 6 shows several circuits to do this.
The techniques shown to sequence two channels can be
extended to sequence the third. In each case channel 1
starts fi rst. Note that these circuits sequence the outputs
during start-up. When shut down the three channels turn
off simultaneously.
Figure 5. Model for Loop Response
SOFT-START AND SHUTDOWN
The RUN/SS (Run/Soft-Start) pins are used to place the
individual switching regulators and the internal bias cir-
cuits in shutdown mode. They also provide a soft-start
function. To shut down a regulator, pull its RUN/SS pin to
ground with an open drain or collector. If all three RUN/SS
pins are pulled to ground, the LT1941 enters its shutdown
mode with all regulators off and quiescent current reduced
to ~50mA. Internal 2μA current sources pull up on each
pin. If any RUN/SS pin reaches ~0.6V, the internal bias
circuits start and the quiescent currents increase to their
nominal levels.
If a capacitor is tied from the RUN/SS pin to ground, then
the internal pull-up current will generate a voltage ramp on
this pin. This voltage clamps the V
C
pin, limiting the peak
switch current and therefore input current during start-up.
A good value for the soft-start capacitor is C
OUT
/10,000,
where C
OUT
is the value of the output capacitor.
The RUN/SS pins can be left fl oating if the shutdown
feature is not used. They can also be tied together with a
–
+
V
FB
V
SW
V
C
LT1941
GND
1941 F05
R1
OUTPUT
ESR
C
F
C
C
R
C
500k
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
mp
g
ma
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL