Datasheet

LT3470
12
3470fd
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
Minimum Input Voltage, V
OUT
= 3.3V
Minimum Input Voltage, V
OUT
= 5V
Figure 4. Diode D1 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3470 Runs Only When the Input Is
Present Hot-Plugging Safely
applicaTions inForMaTion
(Figure 2b). The circuit in Figure 2a is more efficient
because the BOOST pin current and BIAS pin quiescent
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BIAS pins are not exceeded.
The minimum operating voltage of an LT3470 application
is limited by the undervoltage lockout (4V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3470 is turned on with its SHDN pin when the
output is already in regulation, then the boost capacitor may
not be fully charged. The plots in Figure 3 show minimum
V
IN
to start and to run. At light loads, the inductor current
becomes discontinuous and the effective duty cycle can
be very high. This reduces the minimum input voltage to
approximately 300mV above V
OUT
. At higher load currents,
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3470, requiring
a higher input voltage to maintain regulation.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively at the top switch current limit maximum of 450mA,
an LT3470 buck regulator will tolerate a shorted output
even if V
IN
= 40V. There is another situation to consider
in systems where the output will be held high when the
input to the LT3470 is absent. This may occur in battery
charging applications or in battery backup systems where
a battery or some other supply is diode OR-ed with the
LT3470’s output. If the V
IN
pin is allowed to float and the
SHDN pin is held high (either by a logic signal or because
it is tied to V
IN
), then the LT3470’s internal circuitry will
pull its quiescent current through its SW pin. This is fine
if your system can tolerate a few mA in this state. If you
ground the SHDN pin, the SW pin current will drop to es-
sentially zero. However, if the V
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT3470 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 4 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200
3470 G18
T
A
= 25°C
V
IN
TO START
V
IN
TO RUN
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
200
3470 G19
5
4
50
100
150
8
T
A
= 25°C
V
IN
TO START
V
IN
TO RUN
V
IN
BOOST
LT3470 SOT-23
SWSHDN
3470 F04
V
IN
100k
D1
1M
V
OUT
BACKUP
BIAS
FB
GND