LT3508
17
3508fd
Ratiometric tracking is achieved in Figure 8b by connecting
both the TRACK/SS pins together. In this confi guration the
TRACK/SS pin source current is doubled (2.4A) which
must be taken into account when calculating the output
rise time.
Do not tie TRACK/SS1 and TRACK/SS2 together if using
multiple inputs. If V
IN2
is below 3V, TRACK/SS2 pulls low
and would hold TRACK/SS1 low as well if the two pins
are tied together, which would prevent channel 1 from
operating.
By connecting a feedback network from V
OUT1
to the
TRACK/SS2 pin with the same ratio that set the V
OUT2
voltage, absolute tracking shown in Figure 8c is imple-
mented. A small V
OUT2
voltage offset will be present due
to the TRACK/SS2 1.2A source current. This offset can
be corrected for by slightly reducing the value of R2. Use
a resistor divider such that when V
OUT1
is in regulation,
TRACK/SS2 is pulled up to 1V or greater. If TRACK/SS is
below 1V, the output may regulate FB to a voltage lower
than the 800mV reference voltage.
Figure 8d illustrates output sequencing. When V
OUT1
is
within 10% of its regulated voltage, PG1 releases the
TRACK/SS2 soft-start pin allowing V
OUT2
to soft-start.
In this case PG1 will be pulled up to 1.3V by the TRACK/
SS pin.
If precise output ramp up and down is required, drive the
TRACK/SS pins as shown in Figure 8e.
Multiple Inputs
For applications requiring large inductors due to high V
IN
to V
OUT
ratios, a 2-stage step down approach may reduce
inductor size by allowing an increase in frequency. A dual
step-down application (Figure 9) steps down the input
voltage (V
IN1
) to the highest output voltage, then uses that
voltage to power the second output (V
IN2
). V
OUT1
must
be able to provide enough current for its output plus the
input current at V
IN2
when V
OUT2
is at its maximum load.
For applications with multiple input voltages, the LT3508
can accommodate input voltages as low as 3V on V
IN2
.
This can be useful in applications regulating outputs from
a PCI Express bus, where the 12V input is power limited
and the 3.3V input has power available to drive other
outputs. In this case, tie the 12V input to V
IN1
and the
3.3V input to V
IN2
. See the Typical Application section for
an example circuit.
Shorted and Reverse Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3508 step-down regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
APPLICATIONS INFORMATION
Figure 9. 1MHz, Wide Input Range 5V and 1.8V Outputs
V
IN
5.7V TO 36V
BOOST2
LT3508
V
IN1
V
IN2
GND
3508 F09
RT/SYNC
C7
330pF
C5
47µF
C4
10µF
C1
4.7µF
C2
0.1µF
C3
0.1µF
R6
47k
C6
100pF
R5
39k
R4
15.0k
R8
33.2k
f
SW
= 1MHz
R1
56.2k
R7
100k
R2
18.7k
R3
10.7k
L2
3.3µH
L1 6.8µH
D1
D2
OUT1
D3 D4
POWER
GOOD
OUT2
1.8V
1A
OUT1
5V
0.9A
C8
1nF
C9
3.3nF
BOOST1
SHDN
SW2
SW1
FB2FB1
V
C2
PG1
V
C1
TRACK/SS1
PG2TRACK/SS2
ON OFF
