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LT3503EDCB#TRMPBF

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13
LT3503
3503f
The minimum operating voltage of an LT3503 application
is limited by the undervoltage lockout (3.6V) and by the
maximum duty cycle as outlined above. For proper start-
up, the minimum input voltage is also limited by the boost
circuit. If the input voltage is ramped slowly, or the LT3503
is turned on with its SHDN pin when the output is already
in regulation, then the boost capacitor may not be fully
charged. Because the boost capacitor is charged with the
energy stored in the inductor, the circuit will rely on some
minimum load current to get the boost circuit running
properly. This minimum load will depend on the input and
output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 7 shows a plot of minimum load
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to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
IN
is ramping verly slowly.
For lower start-up voltage, the boost diode can be tied to
V
IN
; however this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
400mV 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 LT3503, requiring a higher
input voltage to maintain regulation.
(7a) Typical Minimum Input Voltage,
V
OUT
= 5V
(7b) Typical Minimum Input Voltage,
V
OUT
= 3.3V
Figure 7
Figure 6. Two Circuits for Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT3503
(6a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
3503 F06a
V
IN
BOOST
GND
SW
V
IN
LT3503
(6b)
D2
3503 F06b
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
LOAD CURRENT (mA)
1
5.2
INPUT VOLTAGE (V)
6.7
7.2
7.7
10 100 1000
3503 F07a
6.2
5.7
TO START
TO RUN
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
5.0
5.5
TO START
TO RUN
10 1OO 1000
3503 F07b
4.5
4.0
3.5
14
LT3503
3503f
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Figure 8. To Soft-Start the LT3503, Add a Resistor and Capacitor to the SHDN Pin. V
IN
= 12V, V
OUT
= 3.3V, C
OUT
= 10µF, R
LOAD
= 5Ω
Soft-Start
The SHDN pin can be used to soft-start the LT3503,
reducing the maximum input current during start-up. The
SHDN pin is driven through an external RC filter to create
a voltage ramp at this pin. Figure 8 shows the start-up
waveforms with and without the soft-start circuit. By
choosing a large RC time constant, the peak start up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20µA when the SHDN
pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3503 buck 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
LT3503 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 LT3503’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 LT3503’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 essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3503 can
pull large currents from the output through the SW pin and
the V
IN
pin. Figure 9 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
RUN
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 3.3µH
C
OUT
= 10µF
V
OUT
2V/DIV
10µs/DIV
I
L
1A/DIV
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 3.3µH
C
OUT
= 10µF
V
OUT
2V/DIV
10µs/DIV
I
L
1A/DIV
SHDN
GND
RUN
15k
0.068µF
SHDN
GND
3503 F08b
3503 F08a
15
LT3503
3503f
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Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3503 is Connected to a Live Supply
a live supply (see Linear Technology Application Note 88
for a complete discussion). The low loss ceramic capaci-
tor combined with stray inductance in series with the
power source forms an underdamped tank circuit, and the
voltage at the V
IN
pin of the LT3503 can ring to twice the
nominal input voltage, possibly exceeding the LT3503’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3503 into an
energized supply, the input network should be designed to
prevent this overshoot.
Figure 10 shows the waveforms that result when an
LT3503 circuit is connected to a 20V supply through six
feet of 24-gauge twisted pair. The first plot is the response
with a 2.2µF ceramic capacitor at the input. The input
voltage rings as high as 35V and the input current peaks
at 20A. One method of damping the tank circuit is to add
another capacitor with a series resistor to the circuit. In
Figure 10b an aluminum electrolytic capacitor has been
added. This capacitor’s high equivalent series resistance
Figure 9. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3503 Runs Only When the Input
is Present
V
IN
BOOST
GND FB
SHDN SW
D4
V
IN
LT3503
3503 F09
V
OUT
BACKUP
+
+
LT3503
2.2µF
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(10a)
(10b)
(10c)
LOW
IMPEDANCE
ENERGIZED
20V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT3503
2.2µF
10µF
35V
AI.EI.
LT3503
2.2µF0.1µF
1Ω
3503 F10
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
DANGER!
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3503
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3503 circuits. However, these ca-
pacitors can cause problems if the LT3503 is plugged into
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LT3503EDCB#TRMPBF

Mfr. #:
Buy LT3503EDCB#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1A, 2.2MHz Buck Sw Reg in 2 x 3 DFN
Lifecycle:
New from this manufacturer.
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