LT3508
APPLICATIONS INFORMATION
inductor size by allowing an increase in frequency. A dual
step-down application (Figure 9) steps down the input
RatiometrictrackingisachievedinFigure8bbyconnecting
both the TRACK/SS pins together. In this configuration the
TRACK/SS pin source current is doubled (2.4μA) which
must be taken into account when calculating the output
rise time.
voltage (V ) to the highest output voltage, then uses that
IN1
voltage to power the second output (V ). V
must be
IN2
OUT1
able toprovide enoughcurrentforitsoutputplus the input
current at V when V is at its maximum load.
IN2
OUT2
By connecting a feedback network from V
to the
OUT2
OUT1
For applications with multiple input voltages, the LT3508
can accommodate input voltages as low as 3V on V
TRACK/SS2 pin with the same ratio that set the V
.
voltage, absolute tracking shown in Figure 8c is imple-
mented. A small V voltage offset will be present due
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
OUT2
to the TRACK/SS2 1.2μA source current. This offset can
be corrected for by slightly reducing the value of R2. Use
outputs. In this case, tie the 12V input to V and the
a resistor divider such that when V
is in regulation,
IN1
OUT1
3.3V input to V . See the Typical Application section for
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.
IN2
an example circuit.
Do not tie TRACK/SS1 and TRACK/SS2 together if using
multiple inputs. If V is below 3V, TRACK/SS2 pulls low
Figure 8d illustrates output sequencing. When V
is within 10% of its regulated voltage, PG1 releases
IN2
OUT1
and would hold TRACK/SS1 low as well if the two pins
are tied together, which would prevent channel 1 from
operating.
the TRACK/SS2 soft-start pin allowing V
to soft-
OUT2
start. In this case PG1 will be pulled up to 1.3V by the
TRACK/SS pin.
Shorted and Reverse Input Protection
If precise output ramp up and down is required, drive the
TRACK/SS pins as shown in Figure 8e.
If the inductor is chosen so that it won’t saturate exces-
sively,anLT3508step-downregulatorwilltolerateashorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3508 is absent. This may occur in battery charging
Multiple Inputs
For applications requiring large inductors due to high V
IN
to V
ratios, a 2-stage step down approach may reduce
OUT
V
IN
OUT1
5.7V TO 36V
C1
4.7ꢀF
V
IN1
V
IN2
D2
ON OFF
C2
SHDN
D1
BOOST1
BOOST2
SW2
L2
C3
L1 6.8ꢀH
3.3ꢀH
OUT1
5V
0.9A
0.1ꢀF
0.1ꢀF
OUT2
1.8V
1A
SW1
D3
D4
LT3508
R1
56.2k
R2
18.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
15.0k
R6
47k
C5
47ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
10.7k
39k
R7
100k
C6
100pF
C4
10ꢀF
GND R /SYNC
T
C7
330pF
R8
33.2k
C8
1nF
C9
3.3nF
POWER
GOOD
f
= 1MHz
SW
3508 F09
Figure 9. 1MHz, Wide Input Range 5V and 1.8V Outputs
3508fb
17
Linear [ Linear ]
