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LT3470ETS8 Datasheet

  • LT3470ETS8

  • Micropower Buck Regulator with Integrated Boost and Catch Di...

  • 16頁

  • Linear

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LT3470
APPLICATIO S I FOR ATIO
(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 Input Voltage, V
OUT
= 3.3V
6.0
5.5
T
A
= 25擄C
V
IN
TO START
INPUT VOLTAGE (V)
5.0
4.5
4.0
3.5
3.0
0
50
100
150
LOAD CURRENT (mA)
200
3470 G18
V
IN
TO RUN
Minimum Input Voltage, V
OUT
= 5V
8
T
A
= 25擄C
V
IN
TO START
7
INPUT VOLTAGE (V)
6
V
IN
TO RUN
5
4
0
100
150
50
LOAD CURRENT (mA)
200
3470 G19
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
U
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鈥檛 saturate exces-
sively at the top switch current limit maximum of 450碌A,
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 charg-
ing applications or in battery backup systems where a
battery or some other supply is diode OR-ed with the
LT3470鈥檚 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鈥檚 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
essentially 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.
D1
V
IN
3
V
IN
100k
1
SHDN
6
BOOST
SW
BIAS
1M
GND
4
3470 F04
W
U U
LT3470 SOT-23
5
7
8
V
OUT
FB
BACKUP
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
3470f
11

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