Datasheet

LT3507
15
3507fb
For more information www.linear.com/LT3507
Converter with Backup Output Regulator
There is another situation to consider in systems where
the output will be held high when the input to the LT3507
is absent. If the V
IN
and one of the RUN pins are allowed
to float, then the LT3507’s internal circuitry will pull its
quiescent current through its SW pin. This is acceptable if
the system can tolerate a few mA of load in this state. With
all three RUN pins grounded, the LT3507 enters shutdown
mode and the SW pin current drops to <50µA. However, if
the V
IN
pin is grounded while the output is held high, then
parasitic diodes inside the LT3507 can pull large currents
from the output through the SW pin and the V
IN
pin. A
Schottky diode in series with the input to the LT3507, as
shown in Figure 5, will protect the LT3507 and the system
from a shorted or reversed input.
Input Capacitor Selection
Bypass the input of the LT3507 circuit with a 10µF or higher
ceramic capacitor of X7R or X5R type. A lower value or
a less expensive Y5V type will work if there is additional
bypassing provided by bulk electrolytic capacitors, or if the
input source impedance is low. The following paragraphs
describe the input capacitor considerations in more detail.
Step-down regulators draw current from the input sup
-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3507 input and to force this switching current
into a tight local loop, minimizing EMI. The input capaci
-
tor must have low impedance at the switching frequency
to do this effectively and it must have an adequate ripple
current rating.
With three switchers operating at the same
frequency but with different phases and duty cycles, cal
-
culating the input capacitor RMS current is not simple;
however,
a conservative value is the RMS input current
for the phase delivering the most power (V
OUT
I
OUT
):
I
IN(RMS)
=I
OUT
V
OUT
V
IN
V
OUT
( )
V
IN
<
I
OUT
2
applications inForMation
Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
5.5
5.0
4.0
3.0
4.5
3.5
2.5
3507 F04b
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 3.3V
TO START
TO RUN
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
8.0
7.5
6.5
5.5
4.5
7.0
6.0
5.0
4.0
3507 F04a
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 5V
TO START
TO RUN
Figure 5. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output
V
IN
V
IN
V
OUT
SW
LT3507
D4
PARASITIC DIODE
3507 F05
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