lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 17

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Table 5. Converter Equivalent Model Parameters
V
R
g
g
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
switch current and therefore input current during start-up.
A good value for the soft-start capacitor is C
where C
The RUN/SS pins can be left floating if the shutdown
feature is not used. They can also be tied together with a
ma
mp
FB
O
GND
CURRENT MODE
POWER STAGE
STEP-DOWN1
g
OUT
1700µmho
mp
V
0.628V
R
500kΩ
5mho
C
C
C
C
500k
is the value of the output capacitor.
Figure 5. Model for Loop Response
C
AMPLIFIER
g
F
ma
ERROR
U
STEP-DOWN2
–
+
1700µmho
3.6mho
LT1941
0.628V
500kΩ
V
V
U
SW
FB
FB
R1
R2
C
V
W
800µmho
TANTALUM
IN
pin, limiting the peak
BOOST
POLYMER
500kΩ
V
1.25
• 5mho
OUT
C
ESR
C1
PL
OR
+
1941 F05
OUT
OUTPUT
U
V
INVERTER
800µmho
–
IN
500kΩ
/10,000,
⏐
C1
CERAMIC
1.24
V
• 5mho
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
capacitance and the load current. However, the power
good indicators can be used to sequence the three out-
puts, 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 final 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
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
Both the 5GOOD and 12GOOD pins will sink current when
the part is in shutdown, independent of the voltage at V
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 first. Note that these circuits sequence the outputs
during start-up. When shut down the three channels turn
off simultaneously.
IN
is greater than ~3.5V.
IN
is greater than 1.1V.
LT1941
17
1941fa
IN.